One page Swift, updated at 2015-07-23 on developer.apple

Swift is a new programming language for iOS, OS X, and watchOS apps that builds on the best of C and Objective-C, without the constraints of C compatibility. Swift adopts safe programming patterns and adds modern features to make programming easier, more flexible, and more fun. Swift’s clean slate, backed by the mature and much-loved Cocoa and Cocoa Touch frameworks, is an opportunity to reimagine how software development works.

Swift has been years in the making. Apple laid the foundation for Swift by advancing our existing compiler, debugger, and framework infrastructure. We simplified memory management with Automatic Reference Counting (ARC). Our framework stack, built on the solid base of Foundation and Cocoa, has been modernized and standardized throughout. Objective-C itself has evolved to support blocks, collection literals, and modules, enabling framework adoption of modern language technologies without disruption. Thanks to this groundwork, we can now introduce a new language for the future of Apple software development.

Swift feels familiar to Objective-C developers. It adopts the readability of Objective-C’s named parameters and the power of Objective-C’s dynamic object model. It provides seamless access to existing Cocoa frameworks and mix-and-match interoperability with Objective-C code. Building from this common ground, Swift introduces many new features and unifies the procedural and object-oriented portions of the language.

Swift is friendly to new programmers. It is the first industrial-quality systems programming language that is as expressive and enjoyable as a scripting language. It supports playgrounds, an innovative feature that allows programmers to experiment with Swift code and see the results immediately, without the overhead of building and running an app.

Swift combines the best in modern language thinking with wisdom from the wider Apple engineering culture. The compiler is optimized for performance, and the language is optimized for development, without compromising on either. It’s designed to scale from “hello, world” to an entire operating system. All this makes Swift a sound future investment for developers and for Apple.

Swift is a fantastic way to write iOS, OS X, and watchOS apps, and will continue to evolve with new features and capabilities. Our goals for Swift are ambitious. We can’t wait to see what you create with it.

# A Swift Tour

Tradition suggests that the first program in a new language should print the words “Hello, world!” on the screen. In Swift, this can be done in a single line:

1. print("Hello, world!")

If you have written code in C or Objective-C, this syntax looks familiar to you—in Swift, this line of code is a complete program. You don’t need to import a separate library for functionality like input/output or string handling. Code written at global scope is used as the entry point for the program, so you don’t need a main() function. You also don’t need to write semicolons at the end of every statement.

This tour gives you enough information to start writing code in Swift by showing you how to accomplish a variety of programming tasks. Don’t worry if you don’t understand something—everything introduced in this tour is explained in detail in the rest of this book.

## Simple Values

Use let to make a constant and var to make a variable. The value of a constant doesn’t need to be known at compile time, but you must assign it a value exactly once. This means you can use constants to name a value that you determine once but use in many places.

1. var myVariable = 42
2. myVariable = 50
3. let myConstant = 42

A constant or variable must have the same type as the value you want to assign to it. However, you don’t always have to write the type explicitly. Providing a value when you create a constant or variable lets the compiler infer its type. In the example above, the compiler infers that myVariable is an integer because its initial value is an integer.

If the initial value doesn’t provide enough information (or if there is no initial value), specify the type by writing it after the variable, separated by a colon.

1. let implicitInteger = 70
2. let implicitDouble = 70.0
3. let explicitDouble: Double = 70

Values are never implicitly converted to another type. If you need to convert a value to a different type, explicitly make an instance of the desired type.

1. let label = "The width is "
2. let width = 94
3. let widthLabel = label + String(width)

There’s an even simpler way to include values in strings: Write the value in parentheses, and write a backslash (\) before the parentheses. For example:

1. let apples = 3
2. let oranges = 5
3. let appleSummary = "I have \(apples) apples."
4. let fruitSummary = "I have \(apples + oranges) pieces of fruit."

Create arrays and dictionaries using brackets ([]), and access their elements by writing the index or key in brackets. A comma is allowed after the last element.

1. var shoppingList = ["catfish", "water", "tulips", "blue paint"]
2. shoppingList[1] = "bottle of water"
3. 
4. var occupations = [
5.  "Malcolm": "Captain",
6.  "Kaylee": "Mechanic",
7. ]
8. occupations["Jayne"] = "Public Relations"

To create an empty array or dictionary, use the initializer syntax.

1. let emptyArray = [String]()
2. let emptyDictionary = [String: Float]()

If type information can be inferred, you can write an empty array as [] and an empty dictionary as [:]—for example, when you set a new value for a variable or pass an argument to a function.

1. shoppingList = []
2. occupations = [:]

## Control Flow

Use if and switch to make conditionals, and use for-in, for, while, and repeat-while to make loops. Parentheses around the condition or loop variable are optional. Braces around the body are required.

1. let individualScores = [75, 43, 103, 87, 12]
2. var teamScore = 0
3. for score in individualScores {
4.  if score > 50 {
5.  teamScore += 3
6.  } else {
7.  teamScore += 1
8.  }
9. }
10. print(teamScore)

In an if statement, the conditional must be a Boolean expression—this means that code such as if score { ... } is an error, not an implicit comparison to zero.

You can use if and let together to work with values that might be missing. These values are represented as optionals. An optional value either contains a value or contains nil to indicate that a value is missing. Write a question mark (?) after the type of a value to mark the value as optional.

1. var optionalString: String? = "Hello"
2. print(optionalString == nil)
3. 
4. var optionalName: String? = "John Appleseed"
5. var greeting = "Hello!"
6. if let name = optionalName {
7.  greeting = "Hello, \(name)"
8. }

If the optional value is nil, the conditional is false and the code in braces is skipped. Otherwise, the optional value is unwrapped and assigned to the constant after let, which makes the unwrapped value available inside the block of code.

Switches support any kind of data and a wide variety of comparison operations—they aren’t limited to integers and tests for equality.

1. let vegetable = "red pepper"
2. switch vegetable {
3. case "celery":
4.  let vegetableComment = "Add some raisins and make ants on a log."
5. case "cucumber", "watercress":
6.  let vegetableComment = "That would make a good tea sandwich."
7. case let x where x.hasSuffix("pepper"):
8.  let vegetableComment = "Is it a spicy \(x)?"
9. default:
10.  let vegetableComment = "Everything tastes good in soup."
11. }

Notice how let can be used in a pattern to assign the value that matched that part of a pattern to a constant.

After executing the code inside the switch case that matched, the program exits from the switch statement. Execution doesn’t continue to the next case, so there is no need to explicitly break out of the switch at the end of each case’s code.

You use for-in to iterate over items in a dictionary by providing a pair of names to use for each key-value pair. Dictionaries are an unordered collection, so their keys and values are iterated over in an arbitrary order.

1. let interestingNumbers = [
2.  "Prime": [2, 3, 5, 7, 11, 13],
3.  "Fibonacci": [1, 1, 2, 3, 5, 8],
4.  "Square": [1, 4, 9, 16, 25],
5. ]
6. var largest = 0
7. for (kind, numbers) in interestingNumbers {
8.  for number in numbers {
9.  if number > largest {
10.  largest = number
11.  }
12.  }
13. }
14. print(largest)

Use while to repeat a block of code until a condition changes. The condition of a loop can be at the end instead, ensuring that the loop is run at least once.

1. var n = 2
2. while n < 100 {
3.  n = n * 2
4. }
5. print(n)
6. 
7. var m = 2
8. repeat {
9.  m = m * 2
10. } while m < 100
11. print(m)

You can keep an index in a loop—either by using ..< to make a range of indexes or by writing an explicit initialization, condition, and increment. These two loops do the same thing:

1. var firstForLoop = 0
2. for i in 0..<4 {
3.  firstForLoop += i
4. }
5. print(firstForLoop)
6. 
7. var secondForLoop = 0
8. for var i = 0; i < 4; ++i {
9.  secondForLoop += i
10. }
11. print(secondForLoop)

Use ..< to make a range that omits its upper value, and use ... to make a range that includes both values.

## Functions and Closures

Use func to declare a function. Call a function by following its name with a list of arguments in parentheses. Use -> to separate the parameter names and types from the function’s return type.

1. func greet(name: String, day: String) -> String {
2.  return "Hello \(name), today is \(day)."
3. }
4. greet("Bob", day: "Tuesday")

Use a tuple to make a compound value—for example, to return multiple values from a function. The elements of a tuple can be referred to either by name or by number.

1. func calculateStatistics(scores: [Int]) -> (min: Int, max: Int, sum: Int) {
2.  var min = scores[0]
3.  var max = scores[0]
4.  var sum = 0
5. 
6.  for score in scores {
7.  if score > max {
8.  max = score
9.  } else if score < min {
10.  min = score
11.  }
12.  sum += score
13.  }
14. 
15.  return (min, max, sum)
16. }
17. let statistics = calculateStatistics([5, 3, 100, 3, 9])
18. print(statistics.sum)
19. print(statistics.2)

Functions can also take a variable number of arguments, collecting them into an array.

1. func sumOf(numbers: Int...) -> Int {
2.  var sum = 0
3.  for number in numbers {
4.  sum += number
5.  }
6.  return sum
7. }
8. sumOf()
9. sumOf(42, 597, 12)

Functions can be nested. Nested functions have access to variables that were declared in the outer function. You can use nested functions to organize the code in a function that is long or complex.

1. func returnFifteen() -> Int {
2.  var y = 10
3.  func add() {
4.  y += 5
5.  }
6.  add()
7.  return y
8. }
9. returnFifteen()

Functions are a first-class type. This means that a function can return another function as its value.

1. func makeIncrementer() -> (Int -> Int) {
2.  func addOne(number: Int) -> Int {
3.  return 1 + number
4.  }
5.  return addOne
6. }
7. var increment = makeIncrementer()
8. increment(7)

A function can take another function as one of its arguments.

1. func hasAnyMatches(list: [Int], condition: Int -> Bool) -> Bool {
2.  for item in list {
3.  if condition(item) {
4.  return true
5.  }
6.  }
7.  return false
8. }
9. func lessThanTen(number: Int) -> Bool {
10.  return number < 10
11. }
12. var numbers = [20, 19, 7, 12]
13. hasAnyMatches(numbers, condition: lessThanTen)

Functions are actually a special case of closures: blocks of code that can be called later. The code in a closure has access to things like variables and functions that were available in the scope where the closure was created, even if the closure is in a different scope when it is executed—you saw an example of this already with nested functions. You can write a closure without a name by surrounding code with braces ({}). Use in to separate the arguments and return type from the body.

1. numbers.map({
2.  (number: Int) -> Int in
3.  let result = 3 * number
4.  return result
5. })

You have several options for writing closures more concisely. When a closure’s type is already known, such as the callback for a delegate, you can omit the type of its parameters, its return type, or both. Single statement closures implicitly return the value of their only statement.

1. let mappedNumbers = numbers.map({ number in 3 * number })
2. print(mappedNumbers)

You can refer to parameters by number instead of by name—this approach is especially useful in very short closures. A closure passed as the last argument to a function can appear immediately after the parentheses. When a closure is the only argument to a function, you can omit the parentheses entirely.

1. let sortedNumbers = numbers.sort { $0 >$1 }
2. print(sortedNumbers)

## Objects and Classes

Use class followed by the class’s name to create a class. A property declaration in a class is written the same way as a constant or variable declaration, except that it is in the context of a class. Likewise, method and function declarations are written the same way.

1. class Shape {
2.  var numberOfSides = 0
3.  func simpleDescription() -> String {
4.  return "A shape with \(numberOfSides) sides."
5.  }
6. }

Create an instance of a class by putting parentheses after the class name. Use dot syntax to access the properties and methods of the instance.

1. var shape = Shape()
2. shape.numberOfSides = 7
3. var shapeDescription = shape.simpleDescription()

This version of the Shape class is missing something important: an initializer to set up the class when an instance is created. Use init to create one.

1. class NamedShape {
2.  var numberOfSides: Int = 0
3.  var name: String
4. 
5.  init(name: String) {
6.  self.name = name
7.  }
8. 
9.  func simpleDescription() -> String {
10.  return "A shape with \(numberOfSides) sides."
11.  }
12. }

Notice how self is used to distinguish the name property from the name argument to the initializer. The arguments to the initializer are passed like a function call when you create an instance of the class. Every property needs a value assigned—either in its declaration (as with numberOfSides) or in the initializer (as with name).

Use deinit to create a deinitializer if you need to perform some cleanup before the object is deallocated.

Subclasses include their superclass name after their class name, separated by a colon. There is no requirement for classes to subclass any standard root class, so you can include or omit a superclass as needed.

Methods on a subclass that override the superclass’s implementation are marked with override—overriding a method by accident, without override, is detected by the compiler as an error. The compiler also detects methods with override that don’t actually override any method in the superclass.

1. class Square: NamedShape {
2.  var sideLength: Double
3. 
4.  init(sideLength: Double, name: String) {
5.  self.sideLength = sideLength
6.  super.init(name: name)
7.  numberOfSides = 4
8.  }
9. 
10.  func area() -> Double {
11.  return sideLength * sideLength
12.  }
13. 
14.  override func simpleDescription() -> String {
15.  return "A square with sides of length \(sideLength)."
16.  }
17. }
18. let test = Square(sideLength: 5.2, name: "my test square")
19. test.area()
20. test.simpleDescription()

In addition to simple properties that are stored, properties can have a getter and a setter.

1. class EquilateralTriangle: NamedShape {
2.  var sideLength: Double = 0.0
3. 
4.  init(sideLength: Double, name: String) {
5.  self.sideLength = sideLength
6.  super.init(name: name)
7.  numberOfSides = 3
8.  }
9. 
10.  var perimeter: Double {
11.  get {
12.  return 3.0 * sideLength
13.  }
14.  set {
15.  sideLength = newValue / 3.0
16.  }
17.  }
18. 
19.  override func simpleDescription() -> String {
20.  return "An equilateral triangle with sides of length \(sideLength)."
21.  }
22. }
23. var triangle = EquilateralTriangle(sideLength: 3.1, name: "a triangle")
24. print(triangle.perimeter)
25. triangle.perimeter = 9.9
26. print(triangle.sideLength)

In the setter for perimeter, the new value has the implicit name newValue. You can provide an explicit name in parentheses after set.

Notice that the initializer for the EquilateralTriangle class has three different steps:

1. Setting the value of properties that the subclass declares.

2. Calling the superclass’s initializer.

3. Changing the value of properties defined by the superclass. Any additional setup work that uses methods, getters, or setters can also be done at this point.

If you don’t need to compute the property but still need to provide code that is run before and after setting a new value, use willSet and didSet. For example, the class below ensures that the side length of its triangle is always the same as the side length of its square.

1. class TriangleAndSquare {
2.  var triangle: EquilateralTriangle {
3.  willSet {
4.  square.sideLength = newValue.sideLength
5.  }
6.  }
7.  var square: Square {
8.  willSet {
9.  triangle.sideLength = newValue.sideLength
10.  }
11.  }
12.  init(size: Double, name: String) {
13.  square = Square(sideLength: size, name: name)
14.  triangle = EquilateralTriangle(sideLength: size, name: name)
15.  }
16. }
17. var triangleAndSquare = TriangleAndSquare(size: 10, name: "another test shape")
18. print(triangleAndSquare.square.sideLength)
19. print(triangleAndSquare.triangle.sideLength)
20. triangleAndSquare.square = Square(sideLength: 50, name: "larger square")
21. print(triangleAndSquare.triangle.sideLength)

When working with optional values, you can write ? before operations like methods, properties, and subscripting. If the value before the ? is nil, everything after the ? is ignored and the value of the whole expression is nil. Otherwise, the optional value is unwrapped, and everything after the ? acts on the unwrapped value. In both cases, the value of the whole expression is an optional value.

1. let optionalSquare: Square? = Square(sideLength: 2.5, name: "optional square")
2. let sideLength = optionalSquare?.sideLength

## Enumerations and Structures

Use enum to create an enumeration. Like classes and all other named types, enumerations can have methods associated with them.

1. enum Rank: Int {
2.  case Ace = 1
3.  case Two, Three, Four, Five, Six, Seven, Eight, Nine, Ten
4.  case Jack, Queen, King
5.  func simpleDescription() -> String {
6.  switch self {
7.  case .Ace:
8.  return "ace"
9.  case .Jack:
10.  return "jack"
11.  case .Queen:
12.  return "queen"
13.  case .King:
14.  return "king"
15.  default:
16.  return String(self.rawValue)
17.  }
18.  }
19. }
20. let ace = Rank.Ace
21. let aceRawValue = ace.rawValue

In the example above, the raw-value type of the enumeration is Int, so you only have to specify the first raw value. The rest of the raw values are assigned in order. You can also use strings or floating-point numbers as the raw type of an enumeration. Use the rawValue property to access the raw value of an enumeration member.

Use the init?(rawValue:) initializer to make an instance of an enumeration from a raw value.

1. if let convertedRank = Rank(rawValue: 3) {
2.  let threeDescription = convertedRank.simpleDescription()
3. }

The member values of an enumeration are actual values, not just another way of writing their raw values. In fact, in cases where there isn’t a meaningful raw value, you don’t have to provide one.

1. enum Suit {
2.  case Spades, Hearts, Diamonds, Clubs
3.  func simpleDescription() -> String {
4.  switch self {
5.  case .Spades:
6.  return "spades"
7.  case .Hearts:
8.  return "hearts"
9.  case .Diamonds:
10.  return "diamonds"
11.  case .Clubs:
12.  return "clubs"
13.  }
14.  }
15. }
16. let hearts = Suit.Hearts
17. let heartsDescription = hearts.simpleDescription()

Notice the two ways that the Hearts member of the enumeration is referred to above: When assigning a value to the hearts constant, the enumeration member Suit.Hearts is referred to by its full name because the constant doesn’t have an explicit type specified. Inside the switch, the enumeration member is referred to by the abbreviated form .Hearts because the value of self is already known to be a suit. You can use the abbreviated form anytime the value’s type is already known.

Use struct to create a structure. Structures support many of the same behaviors as classes, including methods and initializers. One of the most important differences between structures and classes is that structures are always copied when they are passed around in your code, but classes are passed by reference.

1. struct Card {
2.  var rank: Rank
3.  var suit: Suit
4.  func simpleDescription() -> String {
5.  return "The \(rank.simpleDescription()) of \(suit.simpleDescription())"
6.  }
7. }
8. let threeOfSpades = Card(rank: .Three, suit: .Spades)
9. let threeOfSpadesDescription = threeOfSpades.simpleDescription()

An instance of an enumeration member can have values associated with the instance. Instances of the same enumeration member can have different values associated with them. You provide the associated values when you create the instance. Associated values and raw values are different: The raw value of an enumeration member is the same for all of its instances, and you provide the raw value when you define the enumeration.

For example, consider the case of requesting the sunrise and sunset time from a server. The server either responds with the information or it responds with some error information.

1. enum ServerResponse {
2.  case Result(String, String)
3.  case Error(String)
4. }
5. 
6. let success = ServerResponse.Result("6:00 am", "8:09 pm")
7. let failure = ServerResponse.Error("Out of cheese.")
8. 
9. switch success {
10. case let .Result(sunrise, sunset):
11.  let serverResponse = "Sunrise is at \(sunrise) and sunset is at \(sunset)."
12. case let .Error(error):
13.  let serverResponse = "Failure... \(error)"
14. }

Notice how the sunrise and sunset times are extracted from the ServerResponse value as part of matching the value against the switch cases.

## Protocols and Extensions

Use protocol to declare a protocol.

1. protocol ExampleProtocol {
2.  var simpleDescription: String { get }
3.  mutating func adjust()
4. }

Classes, enumerations, and structs can all adopt protocols.

1. class SimpleClass: ExampleProtocol {
2.  var simpleDescription: String = "A very simple class."
3.  var anotherProperty: Int = 69105
4.  func adjust() {
5.  simpleDescription += " Now 100% adjusted."
6.  }
7. }
8. var a = SimpleClass()
9. a.adjust()
10. let aDescription = a.simpleDescription
11. 
12. struct SimpleStructure: ExampleProtocol {
13.  var simpleDescription: String = "A simple structure"
14.  mutating func adjust() {
15.  simpleDescription += " (adjusted)"
16.  }
17. }
18. var b = SimpleStructure()
19. b.adjust()
20. let bDescription = b.simpleDescription

Notice the use of the mutating keyword in the declaration of SimpleStructure to mark a method that modifies the structure. The declaration of SimpleClass doesn’t need any of its methods marked as mutating because methods on a class can always modify the class.

Use extension to add functionality to an existing type, such as new methods and computed properties. You can use an extension to add protocol conformance to a type that is declared elsewhere, or even to a type that you imported from a library or framework.

1. extension Int: ExampleProtocol {
2.  var simpleDescription: String {
3.  return "The number \(self)"
4.  }
5.  mutating func adjust() {
6.  self += 42
7.  }
8. }
9. print(7.simpleDescription)

You can use a protocol name just like any other named type—for example, to create a collection of objects that have different types but that all conform to a single protocol. When you work with values whose type is a protocol type, methods outside the protocol definition are not available.

1. let protocolValue: ExampleProtocol = a
2. print(protocolValue.simpleDescription)
3. // print(protocolValue.anotherProperty) // Uncomment to see the error

Even though the variable protocolValue has a runtime type of SimpleClass, the compiler treats it as the given type of ExampleProtocol. This means that you can’t accidentally access methods or properties that the class implements in addition to its protocol conformance.

## Generics

Write a name inside angle brackets to make a generic function or type.

1. func repeatItem<Item>(item: Item, numberOfTimes: Int) -> [Item] {
2.  var result = [Item]()
3.  for _ in 0..<numberOfTimes {
4.  result.append(item)
5.  }
6.  return result
7. }
8. repeatItem("knock", numberOfTimes:4)

You can make generic forms of functions and methods, as well as classes, enumerations, and structures.

1. // Reimplement the Swift standard library's optional type
2. enum OptionalValue<T> {
3.  case None
4.  case Some(T)
5. }
6. var possibleInteger: OptionalValue<Int> = .None
7. possibleInteger = .Some(100)

Use where after the type name to specify a list of requirements—for example, to require the type to implement a protocol, to require two types to be the same, or to require a class to have a particular superclass.

1. func anyCommonElements <T, U where T: SequenceType, U: SequenceType, T.Generator.Element: Equatable, T.Generator.Element == U.Generator.Element> (lhs: T, _ rhs: U) -> Bool {
2.  for lhsItem in lhs {
3.  for rhsItem in rhs {
4.  if lhsItem == rhsItem {
5.  return true
6.  }
7.  }
8.  }
9.  return false
10. }
11. anyCommonElements([1, 2, 3], [3])

In the simple cases, you can omit where and simply write the protocol or class name after a colon. Writing <T: Equatable> is the same as writing <T where T: Equatable>.

# The Basics

Swift is a new programming language for iOS, OS X, and watchOS app development. Nonetheless, many parts of Swift will be familiar from your experience of developing in C and Objective-C.

Swift provides its own versions of all fundamental C and Objective-C types, including Int for integers, Double and Float for floating-point values, Bool for Boolean values, and String for textual data. Swift also provides powerful versions of the three primary collection types, Array, Set, and Dictionary, as described in Collection Types.

Like C, Swift uses variables to store and refer to values by an identifying name. Swift also makes extensive use of variables whose values cannot be changed. These are known as constants, and are much more powerful than constants in C. Constants are used throughout Swift to make code safer and clearer in intent when you work with values that do not need to change.

In addition to familiar types, Swift introduces advanced types not found in Objective-C, such as tuples. Tuples enable you to create and pass around groupings of values. You can use a tuple to return multiple values from a function as a single compound value.

Swift also introduces optional types, which handle the absence of a value. Optionals say either “there is a value, and it equals x” or “there isn’t a value at all”. Optionals are similar to using nil with pointers in Objective-C, but they work for any type, not just classes. Optionals are safer and more expressive than nil pointers in Objective-C and are at the heart of many of Swift’s most powerful features.

Optionals are an example of the fact that Swift is a type safe language. Swift helps you to be clear about the types of values your code can work with. If part of your code expects a String, type safety prevents you from passing it an Int by mistake. This restriction enables you to catch and fix errors as early as possible in the development process.

## Constants and Variables

Constants and variables associate a name (such as maximumNumberOfLoginAttempts or welcomeMessage) with a value of a particular type (such as the number 10 or the string "Hello"). The value of a constant cannot be changed once it is set, whereas a variable can be set to a different value in the future.

### Declaring Constants and Variables

Constants and variables must be declared before they are used. You declare constants with the let keyword and variables with the var keyword. Here’s an example of how constants and variables can be used to track the number of login attempts a user has made:

1. let maximumNumberOfLoginAttempts = 10
2. var currentLoginAttempt = 0

This code can be read as:

“Declare a new constant called maximumNumberOfLoginAttempts, and give it a value of 10. Then, declare a new variable called currentLoginAttempt, and give it an initial value of 0.”

In this example, the maximum number of allowed login attempts is declared as a constant, because the maximum value never changes. The current login attempt counter is declared as a variable, because this value must be incremented after each failed login attempt.

You can declare multiple constants or multiple variables on a single line, separated by commas:

1. var x = 0.0, y = 0.0, z = 0.0

### Type Annotations

You can provide a type annotation when you declare a constant or variable, to be clear about the kind of values the constant or variable can store. Write a type annotation by placing a colon after the constant or variable name, followed by a space, followed by the name of the type to use.

This example provides a type annotation for a variable called welcomeMessage, to indicate that the variable can store String values:

1. var welcomeMessage: String

The colon in the declaration means “…of type…,” so the code above can be read as:

“Declare a variable called welcomeMessage that is of type String.”

The phrase “of type String” means “can store any String value.” Think of it as meaning “the type of thing” (or “the kind of thing”) that can be stored.

The welcomeMessage variable can now be set to any string value without error:

1. welcomeMessage = "Hello"

You can define multiple related variables of the same type on a single line, separated by commas, with a single type annotation after the final variable name:

1. var red, green, blue: Double

### Naming Constants and Variables

Constant and variable names can contain almost any character, including Unicode characters:

1. let π = 3.14159
2. let 你好 = "你好世界"
3. let 🐶🐮 = "dogcow"

Constant and variable names cannot contain whitespace characters, mathematical symbols, arrows, private-use (or invalid) Unicode code points, or line- and box-drawing characters. Nor can they begin with a number, although numbers may be included elsewhere within the name.

Once you’ve declared a constant or variable of a certain type, you can’t redeclare it again with the same name, or change it to store values of a different type. Nor can you change a constant into a variable or a variable into a constant.

You can change the value of an existing variable to another value of a compatible type. In this example, the value of friendlyWelcome is changed from "Hello!" to "Bonjour!":

1. var friendlyWelcome = "Hello!"
2. friendlyWelcome = "Bonjour!"
3. // friendlyWelcome is now "Bonjour!"

Unlike a variable, the value of a constant cannot be changed once it is set. Attempting to do so is reported as an error when your code is compiled:

1. let languageName = "Swift"
2. languageName = "Swift++"
3. // this is a compile-time error - languageName cannot be changed

### Printing Constants and Variables

You can print the current value of a constant or variable with the print(_:) function:

1. print(friendlyWelcome)
2. // prints "Bonjour!"

print(_:) is a global function that prints a value, followed by a line break, to an appropriate output. In Xcode, for example, the print(_:) function prints its output in Xcode’s “console” pane. (A related function, print(_:appendNewline:), performs the same task while optionally appending a line break to the end of the value to be printed.)

The print(_:) function prints any String value you pass to it:

1. print("This is a string")
2. // prints "This is a string"

The print(_:) function can print more complex logging messages. These messages can include the current values of constants and variables.

Swift uses string interpolation to include the name of a constant or variable as a placeholder in a longer string, and to prompt Swift to replace it with the current value of that constant or variable. Wrap the name in parentheses and escape it with a backslash before the opening parenthesis:

1. print("The current value of friendlyWelcome is \(friendlyWelcome)")
2. // prints "The current value of friendlyWelcome is Bonjour!"

Use comments to include non-executable text in your code, as a note or reminder to yourself. Comments are ignored by the Swift compiler when your code is compiled.

Comments in Swift are very similar to comments in C. Single-line comments begin with two forward-slashes (//):

1. // this is a comment

Multiline comments start with a forward-slash followed by an asterisk (/*) and end with an asterisk followed by a forward-slash (*/):

1. /* this is also a comment,
2. but written over multiple lines */

Unlike multiline comments in C, multiline comments in Swift can be nested inside other multiline comments. You write nested comments by starting a multiline comment block and then starting a second multiline comment within the first block. The second block is then closed, followed by the first block:

1. /* this is the start of the first multiline comment
2. /* this is the second, nested multiline comment */
3. this is the end of the first multiline comment */

Nested multiline comments enable you to comment out large blocks of code quickly and easily, even if the code already contains multiline comments.

## Semicolons

Unlike many other languages, Swift does not require you to write a semicolon (;) after each statement in your code, although you can do so if you wish. Semicolons are required, however, if you want to write multiple separate statements on a single line:

1. let cat = "🐱"; print(cat)
2. // prints "🐱"

## Integers

Integers are whole numbers with no fractional component, such as 42 and -23. Integers are either signed (positive, zero, or negative) or unsigned (positive or zero).

Swift provides signed and unsigned integers in 8, 16, 32, and 64 bit forms. These integers follow a naming convention similar to C, in that an 8-bit unsigned integer is of type UInt8, and a 32-bit signed integer is of type Int32. Like all types in Swift, these integer types have capitalized names.

### Integer Bounds

You can access the minimum and maximum values of each integer type with its min and max properties:

1. let minValue = UInt8.min // minValue is equal to 0, and is of type UInt8
2. let maxValue = UInt8.max // maxValue is equal to 255, and is of type UInt8

The values of these properties are of the appropriate-sized number type (such as UInt8 in the example above) and can therefore be used in expressions alongside other values of the same type.

### Int

In most cases, you don’t need to pick a specific size of integer to use in your code. Swift provides an additional integer type, Int, which has the same size as the current platform’s native word size:

1. On a 32-bit platform, Int is the same size as Int32.

2. On a 64-bit platform, Int is the same size as Int64.

Unless you need to work with a specific size of integer, always use Int for integer values in your code. This aids code consistency and interoperability. Even on 32-bit platforms, Int can store any value between -2,147,483,648 and 2,147,483,647, and is large enough for many integer ranges.

### UInt

Swift also provides an unsigned integer type, UInt, which has the same size as the current platform’s native word size:

1. On a 32-bit platform, UInt is the same size as UInt32.

2. On a 64-bit platform, UInt is the same size as UInt64.

## Floating-Point Numbers

Floating-point numbers are numbers with a fractional component, such as 3.14159, 0.1, and -273.15.

Floating-point types can represent a much wider range of values than integer types, and can store numbers that are much larger or smaller than can be stored in an Int. Swift provides two signed floating-point number types:

1. Double represents a 64-bit floating-point number.

2. Float represents a 32-bit floating-point number.

## Type Safety and Type Inference

Swift is a type safe language. A type safe language encourages you to be clear about the types of values your code can work with. If part of your code expects a String, you can’t pass it an Int by mistake.

Because Swift is type safe, it performs type checks when compiling your code and flags any mismatched types as errors. This enables you to catch and fix errors as early as possible in the development process.

Type-checking helps you avoid errors when you’re working with different types of values. However, this doesn’t mean that you have to specify the type of every constant and variable that you declare. If you don’t specify the type of value you need, Swift uses type inference to work out the appropriate type. Type inference enables a compiler to deduce the type of a particular expression automatically when it compiles your code, simply by examining the values you provide.

Because of type inference, Swift requires far fewer type declarations than languages such as C or Objective-C. Constants and variables are still explicitly typed, but much of the work of specifying their type is done for you.

Type inference is particularly useful when you declare a constant or variable with an initial value. This is often done by assigning a literal value (or literal) to the constant or variable at the point that you declare it. (A literal value is a value that appears directly in your source code, such as 42 and 3.14159 in the examples below.)

For example, if you assign a literal value of 42 to a new constant without saying what type it is, Swift infers that you want the constant to be an Int, because you have initialized it with a number that looks like an integer:

1. let meaningOfLife = 42
2. // meaningOfLife is inferred to be of type Int

Likewise, if you don’t specify a type for a floating-point literal, Swift infers that you want to create a Double:

1. let pi = 3.14159
2. // pi is inferred to be of type Double

Swift always chooses Double (rather than Float) when inferring the type of floating-point numbers.

If you combine integer and floating-point literals in an expression, a type of Double will be inferred from the context:

1. let anotherPi = 3 + 0.14159
2. // anotherPi is also inferred to be of type Double

The literal value of 3 has no explicit type in and of itself, and so an appropriate output type of Double is inferred from the presence of a floating-point literal as part of the addition.

## Numeric Literals

Integer literals can be written as:

1. A decimal number, with no prefix

2. A binary number, with a 0b prefix

3. An octal number, with a 0o prefix

4. A hexadecimal number, with a 0x prefix

All of these integer literals have a decimal value of 17:

1. let decimalInteger = 17
2. let binaryInteger = 0b10001 // 17 in binary notation
3. let octalInteger = 0o21 // 17 in octal notation
4. let hexadecimalInteger = 0x11 // 17 in hexadecimal notation

Floating-point literals can be decimal (with no prefix), or hexadecimal (with a 0x prefix). They must always have a number (or hexadecimal number) on both sides of the decimal point. They can also have an optional exponent, indicated by an uppercase or lowercase e for decimal floats, or an uppercase or lowercase p for hexadecimal floats.

For decimal numbers with an exponent of exp, the base number is multiplied by 10exp:

1. 1.25e2 means 1.25 x 102, or 125.0.

2. 1.25e-2 means 1.25 x 10-2, or 0.0125.

For hexadecimal numbers with an exponent of exp, the base number is multiplied by 2exp:

1. 0xFp2 means 15 x 22, or 60.0.

2. 0xFp-2 means 15 x 2-2, or 3.75.

All of these floating-point literals have a decimal value of 12.1875:

1. let decimalDouble = 12.1875
2. let exponentDouble = 1.21875e1
3. let hexadecimalDouble = 0xC.3p0

Numeric literals can contain extra formatting to make them easier to read. Both integers and floats can be padded with extra zeroes and can contain underscores to help with readability. Neither type of formatting affects the underlying value of the literal:

1. let paddedDouble = 000123.456
2. let oneMillion = 1_000_000
3. let justOverOneMillion = 1_000_000.000_000_1

## Numeric Type Conversion

Use the Int type for all general-purpose integer constants and variables in your code, even if they are known to be non-negative. Using the default integer type in everyday situations means that integer constants and variables are immediately interoperable in your code and will match the inferred type for integer literal values.

Use other integer types only when they are specifically needed for the task at hand, because of explicitly-sized data from an external source, or for performance, memory usage, or other necessary optimization. Using explicitly-sized types in these situations helps to catch any accidental value overflows and implicitly documents the nature of the data being used.

### Integer Conversion

The range of numbers that can be stored in an integer constant or variable is different for each numeric type. An Int8 constant or variable can store numbers between -128 and 127, whereas a UInt8 constant or variable can store numbers between 0 and 255. A number that will not fit into a constant or variable of a sized integer type is reported as an error when your code is compiled:

1. let cannotBeNegative: UInt8 = -1
2. // UInt8 cannot store negative numbers, and so this will report an error
3. let tooBig: Int8 = Int8.max + 1
4. // Int8 cannot store a number larger than its maximum value,
5. // and so this will also report an error

Because each numeric type can store a different range of values, you must opt in to numeric type conversion on a case-by-case basis. This opt-in approach prevents hidden conversion errors and helps make type conversion intentions explicit in your code.

To convert one specific number type to another, you initialize a new number of the desired type with the existing value. In the example below, the constant twoThousand is of type UInt16, whereas the constant one is of type UInt8. They cannot be added together directly, because they are not of the same type. Instead, this example calls UInt16(one) to create a new UInt16 initialized with the value of one, and uses this value in place of the original:

1. let twoThousand: UInt16 = 2_000
2. let one: UInt8 = 1
3. let twoThousandAndOne = twoThousand + UInt16(one)

Because both sides of the addition are now of type UInt16, the addition is allowed. The output constant (twoThousandAndOne) is inferred to be of type UInt16, because it is the sum of two UInt16 values.

SomeType(ofInitialValue) is the default way to call the initializer of a Swift type and pass in an initial value. Behind the scenes, UInt16 has an initializer that accepts a UInt8 value, and so this initializer is used to make a new UInt16 from an existing UInt8. You can’t pass in any type here, however—it has to be a type for which UInt16 provides an initializer. Extending existing types to provide initializers that accept new types (including your own type definitions) is covered in Extensions.

### Integer and Floating-Point Conversion

Conversions between integer and floating-point numeric types must be made explicit:

1. let three = 3
2. let pointOneFourOneFiveNine = 0.14159
3. let pi = Double(three) + pointOneFourOneFiveNine
4. // pi equals 3.14159, and is inferred to be of type Double

Here, the value of the constant three is used to create a new value of type Double, so that both sides of the addition are of the same type. Without this conversion in place, the addition would not be allowed.

Floating-point to integer conversion must also be made explicit. An integer type can be initialized with a Double or Float value:

1. let integerPi = Int(pi)
2. // integerPi equals 3, and is inferred to be of type Int

Floating-point values are always truncated when used to initialize a new integer value in this way. This means that 4.75 becomes 4, and -3.9 becomes -3.

## Type Aliases

Type aliases define an alternative name for an existing type. You define type aliases with the typealias keyword.

Type aliases are useful when you want to refer to an existing type by a name that is contextually more appropriate, such as when working with data of a specific size from an external source:

1. typealias AudioSample = UInt16

Once you define a type alias, you can use the alias anywhere you might use the original name:

1. var maxAmplitudeFound = AudioSample.min
2. // maxAmplitudeFound is now 0

Here, AudioSample is defined as an alias for UInt16. Because it is an alias, the call to AudioSample.min actually calls UInt16.min, which provides an initial value of 0 for the maxAmplitudeFound variable.

## Booleans

Swift has a basic Boolean type, called Bool. Boolean values are referred to as logical, because they can only ever be true or false. Swift provides two Boolean constant values, true and false:

1. let orangesAreOrange = true
2. let turnipsAreDelicious = false

The types of orangesAreOrange and turnipsAreDelicious have been inferred as Bool from the fact that they were initialized with Boolean literal values. As with Int and Double above, you don’t need to declare constants or variables as Bool if you set them to true or false as soon as you create them. Type inference helps make Swift code more concise and readable when it initializes constants or variables with other values whose type is already known.

Boolean values are particularly useful when you work with conditional statements such as the if statement:

1. if turnipsAreDelicious {
2.  print("Mmm, tasty turnips!")
3. } else {
4.  print("Eww, turnips are horrible.")
5. }
6. // prints "Eww, turnips are horrible."

Conditional statements such as the if statement are covered in more detail in Control Flow.

Swift’s type safety prevents non-Boolean values from being substituted for Bool. The following example reports a compile-time error:

1. let i = 1
2. if i {
3.  // this example will not compile, and will report an error
4. }

However, the alternative example below is valid:

1. let i = 1
2. if i == 1 {
3.  // this example will compile successfully
4. }

The result of the i == 1 comparison is of type Bool, and so this second example passes the type-check. Comparisons like i == 1 are discussed in Basic Operators.

As with other examples of type safety in Swift, this approach avoids accidental errors and ensures that the intention of a particular section of code is always clear.

## Tuples

Tuples group multiple values into a single compound value. The values within a tuple can be of any type and do not have to be of the same type as each other.

In this example, (404, "Not Found") is a tuple that describes an HTTP status code. An HTTP status code is a special value returned by a web server whenever you request a web page. A status code of 404 Not Found is returned if you request a webpage that doesn’t exist.

1. let http404Error = (404, "Not Found")
2. // http404Error is of type (Int, String), and equals (404, "Not Found")

The (404, "Not Found") tuple groups together an Int and a String to give the HTTP status code two separate values: a number and a human-readable description. It can be described as “a tuple of type (Int, String)”.

You can create tuples from any permutation of types, and they can contain as many different types as you like. There’s nothing stopping you from having a tuple of type (Int, Int, Int), or (String, Bool), or indeed any other permutation you require.

You can decompose a tuple’s contents into separate constants or variables, which you then access as usual:

1. let (statusCode, statusMessage) = http404Error
2. print("The status code is \(statusCode)")
3. // prints "The status code is 404"
4. print("The status message is \(statusMessage)")
5. // prints "The status message is Not Found"

If you only need some of the tuple’s values, ignore parts of the tuple with an underscore (_) when you decompose the tuple:

1. let (justTheStatusCode, _) = http404Error
2. print("The status code is \(justTheStatusCode)")
3. // prints "The status code is 404"

Alternatively, access the individual element values in a tuple using index numbers starting at zero:

1. print("The status code is \(http404Error.0)")
2. // prints "The status code is 404"
3. print("The status message is \(http404Error.1)")
4. // prints "The status message is Not Found"

You can name the individual elements in a tuple when the tuple is defined:

1. let http200Status = (statusCode: 200, description: "OK")

If you name the elements in a tuple, you can use the element names to access the values of those elements:

1. print("The status code is \(http200Status.statusCode)")
2. // prints "The status code is 200"
3. print("The status message is \(http200Status.description)")
4. // prints "The status message is OK"

Tuples are particularly useful as the return values of functions. A function that tries to retrieve a web page might return the (Int, String) tuple type to describe the success or failure of the page retrieval. By returning a tuple with two distinct values, each of a different type, the function provides more useful information about its outcome than if it could only return a single value of a single type. For more information, see Functions with Multiple Return Values.

## Optionals

You use optionals in situations where a value may be absent. An optional says:

1. There is a value, and it equals x

or

1. There isn’t a value at all

Here’s an example of how optionals can be used to cope with the absence of a value. Swift’s String type has an initializer which tries to convert a String value into an Int value. However, not every string can be converted into an integer. The string "123" can be converted into the numeric value 123, but the string "hello, world" does not have an obvious numeric value to convert to.

The example below uses the initializer to try to convert a String into an Int:

1. let possibleNumber = "123"
2. let convertedNumber = Int(possibleNumber)
3. // convertedNumber is inferred to be of type "Int?", or "optional Int"

Because the initializer might fail, it returns an optional Int, rather than an Int. An optional Int is written as Int?, not Int. The question mark indicates that the value it contains is optional, meaning that it might contain some Int value, or it might contain no value at all. (It can’t contain anything else, such as a Bool value or a String value. It’s either an Int, or it’s nothing at all.)

### nil

You set an optional variable to a valueless state by assigning it the special value nil:

1. var serverResponseCode: Int? = 404
2. // serverResponseCode contains an actual Int value of 404
3. serverResponseCode = nil
4. // serverResponseCode now contains no value

If you define an optional variable without providing a default value, the variable is automatically set to nil for you:

1. var surveyAnswer: String?
2. // surveyAnswer is automatically set to nil

### If Statements and Forced Unwrapping

You can use an if statement to find out whether an optional contains a value by comparing the optional against nil. You perform this comparison with the “equal to” operator (==) or the “not equal to” operator (!=).

If an optional has a value, it is considered to be “not equal to” nil:

1. if convertedNumber != nil {
2.  print("convertedNumber contains some integer value.")
3. }
4. // prints "convertedNumber contains some integer value."

Once you’re sure that the optional does contain a value, you can access its underlying value by adding an exclamation mark (!) to the end of the optional’s name. The exclamation mark effectively says, “I know that this optional definitely has a value; please use it.” This is known as forced unwrapping of the optional’s value:

1. if convertedNumber != nil {
2.  print("convertedNumber has an integer value of \(convertedNumber!).")
3. }
4. // prints "convertedNumber has an integer value of 123."

For more on the if statement, see Control Flow.

### Optional Binding

You use optional binding to find out whether an optional contains a value, and if so, to make that value available as a temporary constant or variable. Optional binding can be used with if and while statements to check for a value inside an optional, and to extract that value into a constant or variable, as part of a single action. if and while statements are described in more detail in Control Flow.

Write an optional binding for an if statement as follows:

1. if let constantName = someOptional {
2.     statements
3. }

You can rewrite the possibleNumber example from the Optionals section to use optional binding rather than forced unwrapping:

1. if let actualNumber = Int(possibleNumber) {
2.  print("\'\(possibleNumber)\' has an integer value of \(actualNumber)")
3. } else {
4.  print("\'\(possibleNumber)\' could not be converted to an integer")
5. }
6. // prints "'123' has an integer value of 123"

This code can be read as:

“If the optional Int returned by Int(possibleNumber) contains a value, set a new constant called actualNumber to the value contained in the optional.”

If the conversion is successful, the actualNumber constant becomes available for use within the first branch of the if statement. It has already been initialized with the value contained within the optional, and so there is no need to use the ! suffix to access its value. In this example, actualNumber is simply used to print the result of the conversion.

You can use both constants and variables with optional binding. If you wanted to manipulate the value of actualNumber within the first branch of the if statement, you could write if var actualNumber instead, and the value contained within the optional would be made available as a variable rather than a constant.

Multiple optional bindings can appear in a single if statement as a comma-separated list of assignment expressions.

1. if let constantName = someOptional, anotherConstantName = someOtherOptional {
2.     statements
3. }

### Implicitly Unwrapped Optionals

As described above, optionals indicate that a constant or variable is allowed to have “no value”. Optionals can be checked with an if statement to see if a value exists, and can be conditionally unwrapped with optional binding to access the optional’s value if it does exist.

Sometimes it is clear from a program’s structure that an optional will always have a value, after that value is first set. In these cases, it is useful to remove the need to check and unwrap the optional’s value every time it is accessed, because it can be safely assumed to have a value all of the time.

These kinds of optionals are defined as implicitly unwrapped optionals. You write an implicitly unwrapped optional by placing an exclamation mark (String!) rather than a question mark (String?) after the type that you want to make optional.

Implicitly unwrapped optionals are useful when an optional’s value is confirmed to exist immediately after the optional is first defined and can definitely be assumed to exist at every point thereafter. The primary use of implicitly unwrapped optionals in Swift is during class initialization, as described in Unowned References and Implicitly Unwrapped Optional Properties.

An implicitly unwrapped optional is a normal optional behind the scenes, but can also be used like a nonoptional value, without the need to unwrap the optional value each time it is accessed. The following example shows the difference in behavior between an optional string and an implicitly unwrapped optional string when accessing their wrapped value as an explicit String:

1. let possibleString: String? = "An optional string."
2. let forcedString: String = possibleString! // requires an exclamation mark
3. 
4. let assumedString: String! = "An implicitly unwrapped optional string."
5. let implicitString: String = assumedString // no need for an exclamation mark

You can think of an implicitly unwrapped optional as giving permission for the optional to be unwrapped automatically whenever it is used. Rather than placing an exclamation mark after the optional’s name each time you use it, you place an exclamation mark after the optional’s type when you declare it.

You can still treat an implicitly unwrapped optional like a normal optional, to check if it contains a value:

1. if assumedString != nil {
2.  print(assumedString)
3. }
4. // prints "An implicitly unwrapped optional string."

You can also use an implicitly unwrapped optional with optional binding, to check and unwrap its value in a single statement:

1. if let definiteString = assumedString {
2.  print(definiteString)
3. }
4. // prints "An implicitly unwrapped optional string."

## Error Handling

You use error handling to respond to error conditions your program may encounter during execution.

In contrast to optionals, which can use the presence or absence or a value to communicate success or failure of a function, error handling allows you to determine the underlying cause of failure, and, if necessary, propagate the error to another part of your program.

When a function encounters an error condition, it throws an error. That function’s caller can then catch the error and respond appropriately.

1. func canThrowAnError() throws {
2.  // this function may or may not throw an error
3. }

A function indicates that it can throw an error by including the throws keyword in its declaration. When you call a function that can throw an error, you prepend the try keyword to the expression.

Swift automatically propagates errors out of their current scope until they are handled by a catch clause.

1. do {
2.  try canThrowAnError()
3.  // no error was thrown
4. } catch {
5.  // an error was thrown
6. }

A do statement creates a new containing scope, which allows errors to be propagated to one or more catch clauses.

Here’s an example of how error handling can be used to respond to different error conditions:

1. func makeASandwich() throws {
2.  // ...
3. }
4. 
5. do {
6.  try makeASandwich()
7.  eatASandwich()
8. } catch Error.OutOfCleanDishes {
9.  washDishes()
10. } catch Error.MissingIngredients(let ingredients) {
11.  buyGroceries(ingredients)
12. }

In this example, the makeASandwich() function will throw an error if no clean dishes are available or if any ingredients are missing. Because makeASandwich() throws, the function call is wrapped in a try expression. By wrapping the function call in a do statement, any errors that are thrown will be propagated to the provided catch clauses.

If no error is thrown, the eatASandwich() function is called. If an error is thrown and it matches the Error.OutOfCleanDishes case, then the washDishes() function will be called. If an error is thrown and it matches the Error.MissingIngredients case, then the buyGroceries(_:) function is called with the associated [String] value captured by the catch pattern.

Throwing, catching, and propagating errors is covered in greater detail in Error Handling.

## Assertions

In some cases, it is simply not possible for your code to continue execution if a particular condition is not satisfied. In these situations, you can trigger an assertion in your code to end code execution and to provide an opportunity to debug the cause of the absent or invalid value.

### Debugging with Assertions

An assertion is a runtime check that a logical condition definitely evaluates to true. Literally put, an assertion “asserts” that a condition is true. You use an assertion to make sure that an essential condition is satisfied before executing any further code. If the condition evaluates to true, code execution continues as usual; if the condition evaluates to false, code execution ends, and your app is terminated.

If your code triggers an assertion while running in a debug environment, such as when you build and run an app in Xcode, you can see exactly where the invalid state occurred and query the state of your app at the time that the assertion was triggered. An assertion also lets you provide a suitable debug message as to the nature of the assert.

You write an assertion by calling the global assert(_:_:) function. You pass the assert(_:_:) function an expression that evaluates to true or false and a message that should be displayed if the result of the condition is false:

1. let age = -3
2. assert(age >= 0, "A person's age cannot be less than zero")
3. // this causes the assertion to trigger, because age is not >= 0

In this example, code execution will continue only if age >= 0 evaluates to true, that is, if the value of age is non-negative. If the value of age is negative, as in the code above, then age >= 0 evaluates to false, and the assertion is triggered, terminating the application.

The assertion message can be omitted if desired, as in the following example:

1. assert(age >= 0)

### When to Use Assertions

Use an assertion whenever a condition has the potential to be false, but must definitely be true in order for your code to continue execution. Suitable scenarios for an assertion check include:

1. An integer subscript index is passed to a custom subscript implementation, but the subscript index value could be too low or too high.

2. A value is passed to a function, but an invalid value means that the function cannot fulfill its task.

3. An optional value is currently nil, but a non-nil value is essential for subsequent code to execute successfully.

# Basic Operators

An operator is a special symbol or phrase that you use to check, change, or combine values. For example, the addition operator (+) adds two numbers together (as in let i = 1 + 2). More complex examples include the logical AND operator && (as in if enteredDoorCode && passedRetinaScan) and the increment operator ++i, which is a shortcut to increase the value of i by 1.

Swift supports most standard C operators and improves several capabilities to eliminate common coding errors. The assignment operator (=) does not return a value, to prevent it from being mistakenly used when the equal to operator (==) is intended. Arithmetic operators (+, -, *, /, % and so forth) detect and disallow value overflow, to avoid unexpected results when working with numbers that become larger or smaller than the allowed value range of the type that stores them. You can opt in to value overflow behavior by using Swift’s overflow operators, as described in Overflow Operators.

Unlike C, Swift lets you perform remainder (%) calculations on floating-point numbers. Swift also provides two range operators (a..<b and a...b) not found in C, as a shortcut for expressing a range of values.

This chapter describes the common operators in Swift. Advanced Operators covers Swift’s advanced operators, and describes how to define your own custom operators and implement the standard operators for your own custom types.

## Terminology

Operators are unary, binary, or ternary:

1. Unary operators operate on a single target (such as -a). Unary prefix operators appear immediately before their target (such as !b), and unary postfix operators appear immediately after their target (such as i++).

2. Binary operators operate on two targets (such as 2 + 3) and are infix because they appear in between their two targets.

3. Ternary operators operate on three targets. Like C, Swift has only one ternary operator, the ternary conditional operator (a ? b : c).

The values that operators affect are operands. In the expression 1 + 2, the + symbol is a binary operator and its two operands are the values 1 and 2.

## Assignment Operator

The assignment operator (a = b) initializes or updates the value of a with the value of b:

1. let b = 10
2. var a = 5
3. a = b
4. // a is now equal to 10

If the right side of the assignment is a tuple with multiple values, its elements can be decomposed into multiple constants or variables at once:

1. let (x, y) = (1, 2)
2. // x is equal to 1, and y is equal to 2

Unlike the assignment operator in C and Objective-C, the assignment operator in Swift does not itself return a value. The following statement is not valid:

1. if x = y {
2.  // this is not valid, because x = y does not return a value
3. }

This feature prevents the assignment operator (=) from being used by accident when the equal to operator (==) is actually intended. By making if x = y invalid, Swift helps you to avoid these kinds of errors in your code.

## Arithmetic Operators

Swift supports the four standard arithmetic operators for all number types:

1. Addition (+)

2. Subtraction (-)

3. Multiplication (*)

4. Division (/)

1. 1 + 2 // equals 3
2. 5 - 3 // equals 2
3. 2 * 3 // equals 6
4. 10.0 / 2.5 // equals 4.0

Unlike the arithmetic operators in C and Objective-C, the Swift arithmetic operators do not allow values to overflow by default. You can opt in to value overflow behavior by using Swift’s overflow operators (such as a &+ b). See Overflow Operators.

The addition operator is also supported for String concatenation:

1. "hello, " + "world" // equals "hello, world"

### Remainder Operator

The remainder operator (a % b) works out how many multiples of b will fit inside a and returns the value that is left over (known as the remainder).

Here’s how the remainder operator works. To calculate 9 % 4, you first work out how many 4s will fit inside 9:

You can fit two 4s inside 9, and the remainder is 1 (shown in orange).

In Swift, this would be written as:

1. 9 % 4 // equals 1

To determine the answer for a % b, the % operator calculates the following equation and returns remainder as its output:

a = (b x some multiplier) + remainder

where some multiplier is the largest number of multiples of b that will fit inside a.

Inserting 9 and 4 into this equation yields:

9 = (4 x 2) + 1

The same method is applied when calculating the remainder for a negative value of a:

1. -9 % 4 // equals -1

Inserting -9 and 4 into the equation yields:

-9 = (4 x -2) + -1

giving a remainder value of -1.

The sign of b is ignored for negative values of b. This means that a % b and a % -b always give the same answer.

### Floating-Point Remainder Calculations

Unlike the remainder operator in C and Objective-C, Swift’s remainder operator can also operate on floating-point numbers:

1. 8 % 2.5 // equals 0.5

In this example, 8 divided by 2.5 equals 3, with a remainder of 0.5, so the remainder operator returns a Double value of 0.5.

### Increment and Decrement Operators

Like C, Swift provides an increment operator (++) and a decrement operator (--) as a shortcut to increase or decrease the value of a numeric variable by 1. You can use these operators with variables of any integer or floating-point type.

1. var i = 0
2. ++i // i now equals 1

Each time you call ++i, the value of i is increased by 1. Essentially, ++i is shorthand for saying i = i + 1. Likewise, --i can be used as shorthand for i = i - 1.

The ++ and -- symbols can be used as prefix operators or as postfix operators. ++i and i++ are both valid ways to increase the value of i by 1. Similarly, --i and i-- are both valid ways to decrease the value of i by 1.

Note that these operators modify i and also return a value. If you only want to increment or decrement the value stored in i, you can ignore the returned value. However, if you do use the returned value, it will be different based on whether you used the prefix or postfix version of the operator, according to the following rules:

1. If the operator is written before the variable, it increments the variable before returning its value.

2. If the operator is written after the variable, it increments the variable after returning its value.

For example:

1. var a = 0
2. let b = ++a
3. // a and b are now both equal to 1
4. let c = a++
5. // a is now equal to 2, but c has been set to the pre-increment value of 1

In the example above, let b = ++a increments a before returning its value. This is why both a and b are equal to the new value of 1.

However, let c = a++ increments a after returning its value. This means that c gets the old value of 1, and a is then updated to equal 2.

Unless you need the specific behavior of i++, it is recommended that you use ++i and --i in all cases, because they have the typical expected behavior of modifying i and returning the result.

### Unary Minus Operator

The sign of a numeric value can be toggled using a prefixed -, known as the unary minus operator:

1. let three = 3
2. let minusThree = -three // minusThree equals -3
3. let plusThree = -minusThree // plusThree equals 3, or "minus minus three"

The unary minus operator (-) is prepended directly before the value it operates on, without any white space.

### Unary Plus Operator

The unary plus operator (+) simply returns the value it operates on, without any change:

1. let minusSix = -6
2. let alsoMinusSix = +minusSix // alsoMinusSix equals -6

Although the unary plus operator doesn’t actually do anything, you can use it to provide symmetry in your code for positive numbers when also using the unary minus operator for negative numbers.

## Compound Assignment Operators

Like C, Swift provides compound assignment operators that combine assignment (=) with another operation. One example is the addition assignment operator (+=):

1. var a = 1
2. a += 2
3. // a is now equal to 3

The expression a += 2 is shorthand for a = a + 2. Effectively, the addition and the assignment are combined into one operator that performs both tasks at the same time.

A complete list of compound assignment operators can be found in Expressions.

## Comparison Operators

Swift supports all standard C comparison operators:

1. Equal to (a == b)

2. Not equal to (a != b)

3. Greater than (a > b)

4. Less than (a < b)

5. Greater than or equal to (a >= b)

6. Less than or equal to (a <= b)

Each of the comparison operators returns a Bool value to indicate whether or not the statement is true:

1. 1 == 1 // true, because 1 is equal to 1
2. 2 != 1 // true, because 2 is not equal to 1
3. 2 > 1 // true, because 2 is greater than 1
4. 1 < 2 // true, because 1 is less than 2
5. 1 >= 1 // true, because 1 is greater than or equal to 1
6. 2 <= 1 // false, because 2 is not less than or equal to 1

Comparison operators are often used in conditional statements, such as the if statement:

1. let name = "world"
2. if name == "world" {
3.  print("hello, world")
4. } else {
5.  print("I'm sorry \(name), but I don't recognize you")
6. }
7. // prints "hello, world", because name is indeed equal to "world"

For more on the if statement, see Control Flow.

## Ternary Conditional Operator

The ternary conditional operator is a special operator with three parts, which takes the form question ? answer1 : answer2. It is a shortcut for evaluating one of two expressions based on whether question is true or false. If question is true, it evaluates answer1 and returns its value; otherwise, it evaluates answer2 and returns its value.

The ternary conditional operator is shorthand for the code below:

1. if question {
2.  answer1
3. } else {
4.  answer2
5. }

Here’s an example, which calculates the height for a table row. The row height should be 50 points taller than the content height if the row has a header, and 20 points taller if the row doesn’t have a header:

1. let contentHeight = 40
2. let hasHeader = true
3. let rowHeight = contentHeight + (hasHeader ? 50 : 20)
4. // rowHeight is equal to 90

The preceding example is shorthand for the code below:

1. let contentHeight = 40
2. let hasHeader = true
3. var rowHeight = contentHeight
4. if hasHeader {
5.  rowHeight = rowHeight + 50
6. } else {
7.  rowHeight = rowHeight + 20
8. }
9. // rowHeight is equal to 90

The first example’s use of the ternary conditional operator means that rowHeight can be set to the correct value on a single line of code. This is more concise than the second example, and removes the need for rowHeight to be a variable, because its value does not need to be modified within an if statement.

The ternary conditional operator provides an efficient shorthand for deciding which of two expressions to consider. Use the ternary conditional operator with care, however. Its conciseness can lead to hard-to-read code if overused. Avoid combining multiple instances of the ternary conditional operator into one compound statement.

## Nil Coalescing Operator

The nil coalescing operator (a ?? b) unwraps an optional a if it contains a value, or returns a default value b if a is nil. The expression a is always of an optional type. The expression b must match the type that is stored inside a.

The nil coalescing operator is shorthand for the code below:

1. a != nil ? a! : b

The code above uses the ternary conditional operator and forced unwrapping (a!) to access the value wrapped inside a when a is not nil, and to return b otherwise. The nil coalescing operator provides a more elegant way to encapsulate this conditional checking and unwrapping in a concise and readable form.

The example below uses the nil coalescing operator to choose between a default color name and an optional user-defined color name:

1. let defaultColorName = "red"
2. var userDefinedColorName: String? // defaults to nil
3. 
4. var colorNameToUse = userDefinedColorName ?? defaultColorName
5. // userDefinedColorName is nil, so colorNameToUse is set to the default of "red"

The userDefinedColorName variable is defined as an optional String, with a default value of nil. Because userDefinedColorName is of an optional type, you can use the nil coalescing operator to consider its value. In the example above, the operator is used to determine an initial value for a String variable called colorNameToUse. Because userDefinedColorName is nil, the expression userDefinedColorName ?? defaultColorName returns the value of defaultColorName, or "red".

If you assign a non-nil value to userDefinedColorName and perform the nil coalescing operator check again, the value wrapped inside userDefinedColorName is used instead of the default:

1. userDefinedColorName = "green"
2. colorNameToUse = userDefinedColorName ?? defaultColorName
3. // userDefinedColorName is not nil, so colorNameToUse is set to "green"

## Range Operators

Swift includes two range operators, which are shortcuts for expressing a range of values.

### Closed Range Operator

The closed range operator (a...b) defines a range that runs from a to b, and includes the values a and b. The value of a must not be greater than b.

The closed range operator is useful when iterating over a range in which you want all of the values to be used, such as with a for-in loop:

1. for index in 1...5 {
2.  print("\(index) times 5 is \(index * 5)")
3. }
4. // 1 times 5 is 5
5. // 2 times 5 is 10
6. // 3 times 5 is 15
7. // 4 times 5 is 20
8. // 5 times 5 is 25

For more on for-in loops, see Control Flow.

### Half-Open Range Operator

The half-open range operator (a..<b) defines a range that runs from a to b, but does not include b. It is said to be half-open because it contains its first value, but not its final value. As with the closed range operator, the value of a must not be greater than b. If the value of a is equal to b, then the resulting range will be empty.

Half-open ranges are particularly useful when you work with zero-based lists such as arrays, where it is useful to count up to (but not including) the length of the list:

1. let names = ["Anna", "Alex", "Brian", "Jack"]
2. let count = names.count
3. for i in 0..<count {
4.  print("Person \(i + 1) is called \(names[i])")
5. }
6. // Person 1 is called Anna
7. // Person 2 is called Alex
8. // Person 3 is called Brian
9. // Person 4 is called Jack

Note that the array contains four items, but 0..<count only counts as far as 3 (the index of the last item in the array), because it is a half-open range. For more on arrays, see Arrays.

## Logical Operators

Logical operators modify or combine the Boolean logic values true and false. Swift supports the three standard logical operators found in C-based languages:

1. Logical NOT (!a)

2. Logical AND (a && b)

3. Logical OR (a || b)

### Logical NOT Operator

The logical NOT operator (!a) inverts a Boolean value so that true becomes false, and false becomes true.

The logical NOT operator is a prefix operator, and appears immediately before the value it operates on, without any white space. It can be read as “not a”, as seen in the following example:

1. let allowedEntry = false
2. if !allowedEntry {
3.  print("ACCESS DENIED")
4. }
5. // prints "ACCESS DENIED"

The phrase if !allowedEntry can be read as “if not allowed entry.” The subsequent line is only executed if “not allowed entry” is true; that is, if allowedEntry is false.

As in this example, careful choice of Boolean constant and variable names can help to keep code readable and concise, while avoiding double negatives or confusing logic statements.

### Logical AND Operator

The logical AND operator (a && b) creates logical expressions where both values must be true for the overall expression to also be true.

If either value is false, the overall expression will also be false. In fact, if the first value is false, the second value won’t even be evaluated, because it can’t possibly make the overall expression equate to true. This is known as short-circuit evaluation.

This example considers two Bool values and only allows access if both values are true:

1. let enteredDoorCode = true
2. let passedRetinaScan = false
3. if enteredDoorCode && passedRetinaScan {
4.  print("Welcome!")
5. } else {
6.  print("ACCESS DENIED")
7. }
8. // prints "ACCESS DENIED"

### Logical OR Operator

The logical OR operator (a || b) is an infix operator made from two adjacent pipe characters. You use it to create logical expressions in which only one of the two values has to be true for the overall expression to be true.

Like the Logical AND operator above, the Logical OR operator uses short-circuit evaluation to consider its expressions. If the left side of a Logical OR expression is true, the right side is not evaluated, because it cannot change the outcome of the overall expression.

In the example below, the first Bool value (hasDoorKey) is false, but the second value (knowsOverridePassword) is true. Because one value is true, the overall expression also evaluates to true, and access is allowed:

1. let hasDoorKey = false
2. let knowsOverridePassword = true
3. if hasDoorKey || knowsOverridePassword {
4.  print("Welcome!")
5. } else {
6.  print("ACCESS DENIED")
7. }
8. // prints "Welcome!"

### Combining Logical Operators

You can combine multiple logical operators to create longer compound expressions:

1. if enteredDoorCode && passedRetinaScan || hasDoorKey || knowsOverridePassword {
2.  print("Welcome!")
3. } else {
4.  print("ACCESS DENIED")
5. }
6. // prints "Welcome!"

This example uses multiple && and || operators to create a longer compound expression. However, the && and || operators still operate on only two values, so this is actually three smaller expressions chained together. The example can be read as:

If we’ve entered the correct door code and passed the retina scan, or if we have a valid door key, or if we know the emergency override password, then allow access.

Based on the values of enteredDoorCode, passedRetinaScan, and hasDoorKey, the first two subexpressions are false. However, the emergency override password is known, so the overall compound expression still evaluates to true.

### Explicit Parentheses

It is sometimes useful to include parentheses when they are not strictly needed, to make the intention of a complex expression easier to read. In the door access example above, it is useful to add parentheses around the first part of the compound expression to make its intent explicit:

1. if (enteredDoorCode && passedRetinaScan) || hasDoorKey || knowsOverridePassword {
2.  print("Welcome!")
3. } else {
4.  print("ACCESS DENIED")
5. }
6. // prints "Welcome!"

The parentheses make it clear that the first two values are considered as part of a separate possible state in the overall logic. The output of the compound expression doesn’t change, but the overall intention is clearer to the reader. Readability is always preferred over brevity; use parentheses where they help to make your intentions clear.

# Strings and Characters

A string is an ordered collection of characters, such as "hello, world" or "albatross". Swift strings are represented by the String type, which in turn represents a collection of values of Character type.

Swift’s String and Character types provide a fast, Unicode-compliant way to work with text in your code. The syntax for string creation and manipulation is lightweight and readable, with a string literal syntax that is similar to C. String concatenation is as simple as adding together two strings with the + operator, and string mutability is managed by choosing between a constant or a variable, just like any other value in Swift.

Despite this simplicity of syntax, Swift’s String type is a fast, modern string implementation. Every string is composed of encoding-independent Unicode characters, and provides support for accessing those characters in various Unicode representations.

You can also use strings to insert constants, variables, literals, and expressions into longer strings, in a process known as string interpolation. This makes it easy to create custom string values for display, storage, and printing.

## String Literals

You can include predefined String values within your code as string literals. A string literal is a fixed sequence of textual characters surrounded by a pair of double quotes ("").

Use a string literal as an initial value for a constant or variable:

1. let someString = "Some string literal value"

Note that Swift infers a type of String for the someString constant, because it is initialized with a string literal value.

## Initializing an Empty String

To create an empty String value as the starting point for building a longer string, either assign an empty string literal to a variable, or initialize a new String instance with initializer syntax:

1. var emptyString = "" // empty string literal
2. var anotherEmptyString = String() // initializer syntax
3. // these two strings are both empty, and are equivalent to each other

Find out whether a String value is empty by checking its Boolean isEmpty property:

1. if emptyString.isEmpty {
2.  print("Nothing to see here")
3. }
4. // prints "Nothing to see here"

## String Mutability

You indicate whether a particular String can be modified (or mutated) by assigning it to a variable (in which case it can be modified), or to a constant (in which case it cannot be modified):

1. var variableString = "Horse"
2. variableString += " and carriage"
3. // variableString is now "Horse and carriage"
4. 
5. let constantString = "Highlander"
6. constantString += " and another Highlander"
7. // this reports a compile-time error - a constant string cannot be modified

## Strings Are Value Types

Swift’s String type is a value type. If you create a new String value, that String value is copied when it is passed to a function or method, or when it is assigned to a constant or variable. In each case, a new copy of the existing String value is created, and the new copy is passed or assigned, not the original version. Value types are described in Structures and Enumerations Are Value Types.

Swift’s copy-by-default String behavior ensures that when a function or method passes you a String value, it is clear that you own that exact String value, regardless of where it came from. You can be confident that the string you are passed will not be modified unless you modify it yourself.

Behind the scenes, Swift’s compiler optimizes string usage so that actual copying takes place only when absolutely necessary. This means you always get great performance when working with strings as value types.

## Working with Characters

You can access the individual Character values for a String by iterating over its characters property with a for-in loop:

1. for character in "Dog!🐶".characters {
2.  print(character)
3. }
4. // D
5. // o
6. // g
7. // !
8. // 🐶

The for-in loop is described in For Loops.

Alternatively, you can create a stand-alone Character constant or variable from a single-character string literal by providing a Character type annotation:

1. let exclamationMark: Character = "!"

String values can be constructed by passing an array of Character values as an argument to its initializer:

1. let catCharacters: [Character] = ["C", "a", "t", "!", "🐱"]
2. let catString = String(catCharacters)
3. print(catString)
4. // prints "Cat!🐱"

## Concatenating Strings and Characters

String values can be added together (or concatenated) with the addition operator (+) to create a new String value:

1. let string1 = "hello"
2. let string2 = " there"
3. var welcome = string1 + string2
4. // welcome now equals "hello there"

You can also append a String value to an existing String variable with the addition assignment operator (+=):

1. var instruction = "look over"
2. instruction += string2
3. // instruction now equals "look over there"

You can append a Character value to a String variable with the String type’s append() method:

1. let exclamationMark: Character = "!"
2. welcome.append(exclamationMark)
3. // welcome now equals "hello there!"

## String Interpolation

String interpolation is a way to construct a new String value from a mix of constants, variables, literals, and expressions by including their values inside a string literal. Each item that you insert into the string literal is wrapped in a pair of parentheses, prefixed by a backslash:

1. let multiplier = 3
2. let message = "\(multiplier) times 2.5 is \(Double(multiplier) * 2.5)"
3. // message is "3 times 2.5 is 7.5"

In the example above, the value of multiplier is inserted into a string literal as \(multiplier). This placeholder is replaced with the actual value of multiplier when the string interpolation is evaluated to create an actual string.

The value of multiplier is also part of a larger expression later in the string. This expression calculates the value of Double(multiplier) * 2.5 and inserts the result (7.5) into the string. In this case, the expression is written as \(Double(multiplier) * 2.5) when it is included inside the string literal.

## Unicode

Unicode is an international standard for encoding, representing, and processing text in different writing systems. It enables you to represent almost any character from any language in a standardized form, and to read and write those characters to and from an external source such as a text file or web page. Swift’s String and Character types are fully Unicode-compliant, as described in this section.

### Unicode Scalars

Behind the scenes, Swift’s native String type is built from Unicode scalar values. A Unicode scalar is a unique 21-bit number for a character or modifier, such as U+0061 for LATIN SMALL LETTER A ("a"), or U+1F425 for FRONT-FACING BABY CHICK ("🐥").

Note that not all 21-bit Unicode scalars are assigned to a character—some scalars are reserved for future assignment. Scalars that have been assigned to a character typically also have a name, such as LATIN SMALL LETTER A and FRONT-FACING BABY CHICK in the examples above.

### Special Characters in String Literals

String literals can include the following special characters:

1. The escaped special characters \0 (null character), \\ (backslash), \t (horizontal tab), \n (line feed), \r (carriage return), \" (double quote) and \' (single quote)

2. An arbitrary Unicode scalar, written as \u{n}, where n is a 1–8 digit hexadecimal number with a value equal to a valid Unicode code point

The code below shows four examples of these special characters. The wiseWords constant contains two escaped double quote characters. The dollarSign, blackHeart, and sparklingHeart constants demonstrate the Unicode scalar format:

1. let wiseWords = "\"Imagination is more important than knowledge\" - Einstein"
2. // "Imagination is more important than knowledge" - Einstein
3. let dollarSign = "\u{24}" // , Unicode scalar U+0024 4. let blackHeart = "\u{2665}" // ♥, Unicode scalar U+2665 5. let sparklingHeart = "\u{1F496}" // 💖, Unicode scalar U+1F496 ### Extended Grapheme Clusters Every instance of Swift’s Character type represents a single extended grapheme cluster. An extended grapheme cluster is a sequence of one or more Unicode scalars that (when combined) produce a single human-readable character. Here’s an example. The letter é can be represented as the single Unicode scalar é (LATIN SMALL LETTER E WITH ACUTE, or U+00E9). However, the same letter can also be represented as a pair of scalars—a standard letter e (LATIN SMALL LETTER E, or U+0065), followed by the COMBINING ACUTE ACCENT scalar (U+0301). The COMBINING ACUTE ACCENT scalar is graphically applied to the scalar that precedes it, turning an e into an é when it is rendered by a Unicode-aware text-rendering system. In both cases, the letter é is represented as a single Swift Character value that represents an extended grapheme cluster. In the first case, the cluster contains a single scalar; in the second case, it is a cluster of two scalars: 1. let eAcute: Character = "\u{E9}" // é 2. let combinedEAcute: Character = "\u{65}\u{301}" // e followed by ́ 3. // eAcute is é, combinedEAcute is é Extended grapheme clusters are a flexible way to represent many complex script characters as a single Character value. For example, Hangul syllables from the Korean alphabet can be represented as either a precomposed or decomposed sequence. Both of these representations qualify as a single Character value in Swift: 1. let precomposed: Character = "\u{D55C}" // 한 2. let decomposed: Character = "\u{1112}\u{1161}\u{11AB}" // ᄒ, ᅡ, ᆫ 3. // precomposed is 한, decomposed is 한 Extended grapheme clusters enable scalars for enclosing marks (such as COMBINING ENCLOSING CIRCLE, or U+20DD) to enclose other Unicode scalars as part of a single Character value: 1. let enclosedEAcute: Character = "\u{E9}\u{20DD}" 2. // enclosedEAcute is é⃝ Unicode scalars for regional indicator symbols can be combined in pairs to make a single Character value, such as this combination of REGIONAL INDICATOR SYMBOL LETTER U (U+1F1FA) and REGIONAL INDICATOR SYMBOL LETTER S (U+1F1F8): 1. let regionalIndicatorForUS: Character = "\u{1F1FA}\u{1F1F8}" 2. // regionalIndicatorForUS is 🇺🇸 ## Counting Characters To retrieve a count of the Character values in a string, use the count property of the string’s characters property: 1. let unusualMenagerie = "Koala 🐨, Snail 🐌, Penguin 🐧, Dromedary 🐪" 2. print("unusualMenagerie has \(unusualMenagerie.characters.count) characters") 3. // prints "unusualMenagerie has 40 characters" Note that Swift’s use of extended grapheme clusters for Character values means that string concatenation and modification may not always affect a string’s character count. For example, if you initialize a new string with the four-character word cafe, and then append a COMBINING ACUTE ACCENT (U+0301) to the end of the string, the resulting string will still have a character count of 4, with a fourth character of é, not e: 1. var word = "cafe" 2. print("the number of characters in \(word) is \(word.characters.count)") 3. // prints "the number of characters in cafe is 4" 4.   5. word += "\u{301}" // COMBINING ACUTE ACCENT, U+0301 6.   7. print("the number of characters in \(word) is \(word.characters.count)") 8. // prints "the number of characters in café is 4" ## Accessing and Modifying a String You access and modify a string through its methods and properties, or by using subscript syntax. ### String Indices Each String value has an associated index type, String.Index, which corresponds to the positions of each Character it contains. As mentioned above, different characters can require different amounts of memory to store, so in order to determine which Character is at a particular position, you must iterate over each Unicode scalar from the start or end of that String. For this reason, Swift strings cannot be indexed by integer values. Use the startIndex property to access the position of the first Character of a String. The endIndex property is the position after the last character in a String. As a result, the endIndex property isn’t a valid argument to a string’s subscript. If a String is empty, startIndex and endIndex are equal. A String.Index value can access its immediately preceding index by calling the predecessor() method, and its immediately succeeding index by calling the successor() method. Any index in a String can be accessed from any other index by chaining these methods together, or by using the global advance(start:n:) function. Attempting to access an index outside of a string’s range will trigger a runtime error. You can use subscript syntax to access the Character at a particular String index. 1. let greeting = "Guten Tag!" 2. greeting[greeting.startIndex] 3. // G 4. greeting[greeting.endIndex.predecessor()] 5. // ! 6. greeting[greeting.startIndex.successor()] 7. // u 8. let index = advance(greeting.startIndex, 7) 9. greeting[index] 10. // a Attempting to access a Character at an index outside of a string’s range will trigger a runtime error. 1. greeting[greeting.endIndex] // error 2. greeting.endIndex.successor() // error Use the indices property of the characters property to create a Range of all of the indexes used to access individual characters in a string. 1. for index in greeting.characters.indices { 2.  print("\(greeting[index]) ", appendNewline: false) 3. } 4. // prints "G u t e n T a g !" ### Inserting and Removing To insert a character into a string at a specified index, use the insert(_:atIndex:) method. 1. var welcome = "hello" 2. welcome.insert("!", atIndex: welcome.endIndex) 3. // welcome now equals "hello!" To insert another string at a specified index, use the splice(_:atIndex:) method. 1. welcome.splice(" there".characters, atIndex: welcome.endIndex.predecessor()) 2. // welcome now equals "hello there!" To remove a character from a string at a specified index, use the removeAtIndex(_:) method. 1. welcome.removeAtIndex(welcome.endIndex.predecessor()) 2. // welcome now equals "hello there" To remove a substring at a specified range, use the removeRange(_:) method: 1. let range = advance(welcome.endIndex, -6)..<welcome.endIndex 2. welcome.removeRange(range) 3. // welcome now equals "hello" ## Comparing Strings Swift provides three ways to compare textual values: string and character equality, prefix equality, and suffix equality. ### String and Character Equality String and character equality is checked with the “equal to” operator (==) and the “not equal to” operator (!=), as described in Comparison Operators: 1. let quotation = "We're a lot alike, you and I." 2. let sameQuotation = "We're a lot alike, you and I." 3. if quotation == sameQuotation { 4.  print("These two strings are considered equal") 5. } 6. // prints "These two strings are considered equal" Two String values (or two Character values) are considered equal if their extended grapheme clusters are canonically equivalent. Extended grapheme clusters are canonically equivalent if they have the same linguistic meaning and appearance, even if they are composed from different Unicode scalars behind the scenes. For example, LATIN SMALL LETTER E WITH ACUTE (U+00E9) is canonically equivalent to LATIN SMALL LETTER E (U+0065) followed by COMBINING ACUTE ACCENT (U+0301). Both of these extended grapheme clusters are valid ways to represent the character é, and so they are considered to be canonically equivalent: 1. // "Voulez-vous un café?" using LATIN SMALL LETTER E WITH ACUTE 2. let eAcuteQuestion = "Voulez-vous un caf\u{E9}?" 3.   4. // "Voulez-vous un café?" using LATIN SMALL LETTER E and COMBINING ACUTE ACCENT 5. let combinedEAcuteQuestion = "Voulez-vous un caf\u{65}\u{301}?" 6.   7. if eAcuteQuestion == combinedEAcuteQuestion { 8.  print("These two strings are considered equal") 9. } 10. // prints "These two strings are considered equal" Conversely, LATIN CAPITAL LETTER A (U+0041, or "A"), as used in English, is not equivalent to CYRILLIC CAPITAL LETTER A (U+0410, or "А"), as used in Russian. The characters are visually similar, but do not have the same linguistic meaning: 1. let latinCapitalLetterA: Character = "\u{41}" 2.   3. let cyrillicCapitalLetterA: Character = "\u{0410}" 4.   5. if latinCapitalLetterA != cyrillicCapitalLetterA { 6.  print("These two characters are not equivalent") 7. } 8. // prints "These two characters are not equivalent" ### Prefix and Suffix Equality To check whether a string has a particular string prefix or suffix, call the string’s hasPrefix(_:) and hasSuffix(_:) methods, both of which take a single argument of type String and return a Boolean value. The examples below consider an array of strings representing the scene locations from the first two acts of Shakespeare’s Romeo and Juliet: 1. let romeoAndJuliet = [ 2.  "Act 1 Scene 1: Verona, A public place", 3.  "Act 1 Scene 2: Capulet's mansion", 4.  "Act 1 Scene 3: A room in Capulet's mansion", 5.  "Act 1 Scene 4: A street outside Capulet's mansion", 6.  "Act 1 Scene 5: The Great Hall in Capulet's mansion", 7.  "Act 2 Scene 1: Outside Capulet's mansion", 8.  "Act 2 Scene 2: Capulet's orchard", 9.  "Act 2 Scene 3: Outside Friar Lawrence's cell", 10.  "Act 2 Scene 4: A street in Verona", 11.  "Act 2 Scene 5: Capulet's mansion", 12.  "Act 2 Scene 6: Friar Lawrence's cell" 13. ] You can use the hasPrefix(_:) method with the romeoAndJuliet array to count the number of scenes in Act 1 of the play: 1. var act1SceneCount = 0 2. for scene in romeoAndJuliet { 3.  if scene.hasPrefix("Act 1 ") { 4.  ++act1SceneCount 5.  } 6. } 7. print("There are \(act1SceneCount) scenes in Act 1") 8. // prints "There are 5 scenes in Act 1" Similarly, use the hasSuffix(_:) method to count the number of scenes that take place in or around Capulet’s mansion and Friar Lawrence’s cell: 1. var mansionCount = 0 2. var cellCount = 0 3. for scene in romeoAndJuliet { 4.  if scene.hasSuffix("Capulet's mansion") { 5.  ++mansionCount 6.  } else if scene.hasSuffix("Friar Lawrence's cell") { 7.  ++cellCount 8.  } 9. } 10. print("\(mansionCount) mansion scenes; \(cellCount) cell scenes") 11. // prints "6 mansion scenes; 2 cell scenes" ## Unicode Representations of Strings When a Unicode string is written to a text file or some other storage, the Unicode scalars in that string are encoded in one of several Unicode-defined encoding forms. Each form encodes the string in small chunks known as code units. These include the UTF-8 encoding form (which encodes a string as 8-bit code units), the UTF-16 encoding form (which encodes a string as 16-bit code units), and the UTF-32 encoding form (which encodes a string as 32-bit code units). Swift provides several different ways to access Unicode representations of strings. You can iterate over the string with a for-in statement, to access its individual Character values as Unicode extended grapheme clusters. This process is described in Working with Characters. Alternatively, access a String value in one of three other Unicode-compliant representations: 1. A collection of UTF-8 code units (accessed with the string’s utf8 property) 2. A collection of UTF-16 code units (accessed with the string’s utf16 property) 3. A collection of 21-bit Unicode scalar values, equivalent to the string’s UTF-32 encoding form (accessed with the string’s unicodeScalars property) Each example below shows a different representation of the following string, which is made up of the characters D, o, g, ‼ (DOUBLE EXCLAMATION MARK, or Unicode scalar U+203C), and the 🐶 character (DOG FACE, or Unicode scalar U+1F436): 1. let dogString = "Dog‼🐶" ### UTF-8 Representation You can access a UTF-8 representation of a String by iterating over its utf8 property. This property is of type String.UTF8View, which is a collection of unsigned 8-bit (UInt8) values, one for each byte in the string’s UTF-8 representation: 1. for codeUnit in dogString.utf8 { 2.  print("\(codeUnit) ", appendNewline: false) 3. } 4. print("") 5. // 68 111 103 226 128 188 240 159 144 182 In the example above, the first three decimal codeUnit values (68, 111, 103) represent the characters D, o, and g, whose UTF-8 representation is the same as their ASCII representation. The next three decimal codeUnit values (226, 128, 188) are a three-byte UTF-8 representation of the DOUBLE EXCLAMATION MARK character. The last four codeUnit values (240, 159, 144, 182) are a four-byte UTF-8 representation of the DOG FACE character. ### UTF-16 Representation You can access a UTF-16 representation of a String by iterating over its utf16 property. This property is of type String.UTF16View, which is a collection of unsigned 16-bit (UInt16) values, one for each 16-bit code unit in the string’s UTF-16 representation: 1. for codeUnit in dogString.utf16 { 2.  print("\(codeUnit) ", appendNewline: false) 3. } 4. print("") 5. // 68 111 103 8252 55357 56374 Again, the first three codeUnit values (68, 111, 103) represent the characters D, o, and g, whose UTF-16 code units have the same values as in the string’s UTF-8 representation (because these Unicode scalars represent ASCII characters). The fourth codeUnit value (8252) is a decimal equivalent of the hexadecimal value 203C, which represents the Unicode scalar U+203C for the DOUBLE EXCLAMATION MARK character. This character can be represented as a single code unit in UTF-16. The fifth and sixth codeUnit values (55357 and 56374) are a UTF-16 surrogate pair representation of the DOG FACE character. These values are a high-surrogate value of U+D83D (decimal value 55357) and a low-surrogate value of U+DC36 (decimal value 56374). ### Unicode Scalar Representation You can access a Unicode scalar representation of a String value by iterating over its unicodeScalars property. This property is of type UnicodeScalarView, which is a collection of values of type UnicodeScalar. Each UnicodeScalar has a value property that returns the scalar’s 21-bit value, represented within a UInt32 value: 1. for scalar in dogString.unicodeScalars { 2.  print("\(scalar.value) ", appendNewline: false) 3. } 4. print("") 5. // 68 111 103 8252 128054 The value properties for the first three UnicodeScalar values (68, 111, 103) once again represent the characters D, o, and g. The fourth codeUnit value (8252) is again a decimal equivalent of the hexadecimal value 203C, which represents the Unicode scalar U+203C for the DOUBLE EXCLAMATION MARK character. The value property of the fifth and final UnicodeScalar, 128054, is a decimal equivalent of the hexadecimal value 1F436, which represents the Unicode scalar U+1F436 for the DOG FACE character. As an alternative to querying their value properties, each UnicodeScalar value can also be used to construct a new String value, such as with string interpolation: 1. for scalar in dogString.unicodeScalars { 2.  print("\(scalar) ") 3. } 4. // D 5. // o 6. // g 7. // ‼ 8. // 🐶 # Collection Types Swift provides three primary collection types, known as arrays, sets, and dictionaries, for storing collections of values. Arrays are ordered collections of values. Sets are unordered collections of unique values. Dictionaries are unordered collections of key-value associations. Arrays, sets, and dictionaries in Swift are always clear about the types of values and keys that they can store. This means that you cannot insert a value of the wrong type into a collection by mistake. It also means you can be confident about the type of values you will retrieve from a collection. ## Mutability of Collections If you create an array, a set, or a dictionary, and assign it to a variable, the collection that is created will be mutable. This means that you can change (or mutate) the collection after it is created by adding, removing, or changing items in the collection. If you assign an array, a set, or a dictionary to a constant, that collection is immutable, and its size and contents cannot be changed. ## Arrays An array stores values of the same type in an ordered list. The same value can appear in an array multiple times at different positions. ### Array Type Shorthand Syntax The type of a Swift array is written in full as Array<Element>, where Element is the type of values the array is allowed to store. You can also write the type of an array in shorthand form as [Element]. Although the two forms are functionally identical, the shorthand form is preferred and is used throughout this guide when referring to the type of an array. ### Creating an Empty Array You can create an empty array of a certain type using initializer syntax: 1. var someInts = [Int]() 2. print("someInts is of type [Int] with \(someInts.count) items.") 3. // prints "someInts is of type [Int] with 0 items." Note that the type of the someInts variable is inferred to be [Int] from the type of the initializer. Alternatively, if the context already provides type information, such as a function argument or an already typed variable or constant, you can create an empty array with an empty array literal, which is written as [] (an empty pair of square brackets): 1. someInts.append(3) 2. // someInts now contains 1 value of type Int 3. someInts = [] 4. // someInts is now an empty array, but is still of type [Int] ### Creating an Array with a Default Value Swift’s Array type also provides an initializer for creating an array of a certain size with all of its values set to the same default value. You pass this initializer the number of items to be added to the new array (called count) and a default value of the appropriate type (called repeatedValue): 1. var threeDoubles = [Double](count: 3, repeatedValue: 0.0) 2. // threeDoubles is of type [Double], and equals [0.0, 0.0, 0.0] ### Creating an Array by Adding Two Arrays Together You can create a new array by adding together two existing arrays with compatible types with the addition operator (+). The new array’s type is inferred from the type of the two arrays you add together: 1. var anotherThreeDoubles = [Double](count: 3, repeatedValue: 2.5) 2. // anotherThreeDoubles is inferred as [Double], and equals [2.5, 2.5, 2.5] 3.   4. var sixDoubles = threeDoubles + anotherThreeDoubles 5. // sixDoubles is inferred as [Double], and equals [0.0, 0.0, 0.0, 2.5, 2.5, 2.5] ### Creating an Array with an Array Literal You can also initialize an array with an array literal, which is a shorthand way to write one or more values as an array collection. An array literal is written as a list of values, separated by commas, surrounded by a pair of square brackets: 1. [value 1, value 2, value 3] The example below creates an array called shoppingList to store String values: 1. var shoppingList: [String] = ["Eggs", "Milk"] 2. // shoppingList has been initialized with two initial items The shoppingList variable is declared as “an array of string values”, written as [String]. Because this particular array has specified a value type of String, it is allowed to store String values only. Here, the shoppingList array is initialized with two String values ("Eggs" and "Milk"), written within an array literal. In this case, the array literal contains two String values and nothing else. This matches the type of the shoppingList variable’s declaration (an array that can only contain String values), and so the assignment of the array literal is permitted as a way to initialize shoppingList with two initial items. Thanks to Swift’s type inference, you don’t have to write the type of the array if you’re initializing it with an array literal containing values of the same type. The initialization of shoppingList could have been written in a shorter form instead: 1. var shoppingList = ["Eggs", "Milk"] Because all values in the array literal are of the same type, Swift can infer that [String] is the correct type to use for the shoppingList variable. ### Accessing and Modifying an Array You access and modify an array through its methods and properties, or by using subscript syntax. To find out the number of items in an array, check its read-only count property: 1. print("The shopping list contains \(shoppingList.count) items.") 2. // prints "The shopping list contains 2 items." Use the Boolean isEmpty property as a shortcut for checking whether the count property is equal to 0: 1. if shoppingList.isEmpty { 2.  print("The shopping list is empty.") 3. } else { 4.  print("The shopping list is not empty.") 5. } 6. // prints "The shopping list is not empty." You can add a new item to the end of an array by calling the array’s append(_:) method: 1. shoppingList.append("Flour") 2. // shoppingList now contains 3 items, and someone is making pancakes Alternatively, append an array of one or more compatible items with the addition assignment operator (+=): 1. shoppingList += ["Baking Powder"] 2. // shoppingList now contains 4 items 3. shoppingList += ["Chocolate Spread", "Cheese", "Butter"] 4. // shoppingList now contains 7 items Retrieve a value from the array by using subscript syntax, passing the index of the value you want to retrieve within square brackets immediately after the name of the array: 1. var firstItem = shoppingList[0] 2. // firstItem is equal to "Eggs" You can use subscript syntax to change an existing value at a given index: 1. shoppingList[0] = "Six eggs" 2. // the first item in the list is now equal to "Six eggs" rather than "Eggs" You can also use subscript syntax to change a range of values at once, even if the replacement set of values has a different length than the range you are replacing. The following example replaces "Chocolate Spread", "Cheese", and "Butter" with "Bananas" and "Apples": 1. shoppingList[4...6] = ["Bananas", "Apples"] 2. // shoppingList now contains 6 items To insert an item into the array at a specified index, call the array’s insert(_:atIndex:) method: 1. shoppingList.insert("Maple Syrup", atIndex: 0) 2. // shoppingList now contains 7 items 3. // "Maple Syrup" is now the first item in the list This call to the insert(_:atIndex:) method inserts a new item with a value of "Maple Syrup" at the very beginning of the shopping list, indicated by an index of 0. Similarly, you remove an item from the array with the removeAtIndex(_:) method. This method removes the item at the specified index and returns the removed item (although you can ignore the returned value if you do not need it): 1. let mapleSyrup = shoppingList.removeAtIndex(0) 2. // the item that was at index 0 has just been removed 3. // shoppingList now contains 6 items, and no Maple Syrup 4. // the mapleSyrup constant is now equal to the removed "Maple Syrup" string Any gaps in an array are closed when an item is removed, and so the value at index 0 is once again equal to "Six eggs": 1. firstItem = shoppingList[0] 2. // firstItem is now equal to "Six eggs" If you want to remove the final item from an array, use the removeLast() method rather than the removeAtIndex(_:) method to avoid the need to query the array’s count property. Like the removeAtIndex(_:) method, removeLast() returns the removed item: 1. let apples = shoppingList.removeLast() 2. // the last item in the array has just been removed 3. // shoppingList now contains 5 items, and no apples 4. // the apples constant is now equal to the removed "Apples" string ### Iterating Over an Array You can iterate over the entire set of values in an array with the for-in loop: 1. for item in shoppingList { 2.  print(item) 3. } 4. // Six eggs 5. // Milk 6. // Flour 7. // Baking Powder 8. // Bananas If you need the integer index of each item as well as its value, use the enumerate() method to iterate over the array instead. The enumerate() method returns a tuple for each item in the array composed of the index and the value for that item. You can decompose the tuple into temporary constants or variables as part of the iteration: 1. for (index, value) in shoppingList.enumerate() { 2.  print("Item \(index + 1): \(value)") 3. } 4. // Item 1: Six eggs 5. // Item 2: Milk 6. // Item 3: Flour 7. // Item 4: Baking Powder 8. // Item 5: Bananas For more about the for-in loop, see For Loops. ## Sets A set stores distinct values of the same type in a collection with no defined ordering. You can use sets as an alternative to arrays when the order of items is not important, or when you need to ensure that an item only appears once. ### Hash Values for Set Types A type must be hashable in order to be stored in a set—that is, the type must provide a way to compute a hash value for itself. A hash value is an Int value that is the same for all objects that compare equally, such that if a == b, it follows that a.hashValue == b.hashValue. All of Swift’s basic types (such as String, Int, Double, and Bool) are hashable by default, and can be used as set value types or dictionary key types. Enumeration member values without associated values (as described in Enumerations) are also hashable by default. ### Set Type Syntax The type of a Swift set is written as Set<Element>, where Element is the type that the set is allowed to store. Unlike arrays, sets do not have an equivalent shorthand form. ### Creating and Initializing an Empty Set You can create an empty set of a certain type using initializer syntax: 1. var letters = Set<Character>() 2. print("letters is of type Set<Character> with \(letters.count) items.") 3. // prints "letters is of type Set<Character> with 0 items." Alternatively, if the context already provides type information, such as a function argument or an already typed variable or constant, you can create an empty set with an empty array literal: 1. letters.insert("a") 2. // letters now contains 1 value of type Character 3. letters = [] 4. // letters is now an empty set, but is still of type Set<Character> ### Creating a Set with an Array Literal You can also initialize a set with an array literal, as a shorthand way to write one or more values as a set collection. The example below creates a set called favoriteGenres to store String values: 1. var favoriteGenres: Set<String> = ["Rock", "Classical", "Hip hop"] 2. // favoriteGenres has been initialized with three initial items The favoriteGenres variable is declared as “a set of String values”, written as Set<String>. Because this particular set has specified a value type of String, it is only allowed to store String values. Here, the favoriteGenres set is initialized with three String values ("Rock", "Classical", and "Hip hop"), written within an array literal. A set type cannot be inferred from an array literal alone, so the type Set must be explicitly declared. However, because of Swift’s type inference, you don’t have to write the type of the set if you’re initializing it with an array literal containing values of the same type. The initialization of favoriteGenres could have been written in a shorter form instead: 1. var favoriteGenres: Set = ["Rock", "Classical", "Hip hop"] Because all values in the array literal are of the same type, Swift can infer that Set<String> is the correct type to use for the favoriteGenres variable. ### Accessing and Modifying a Set You access and modify a set through its methods and properties. To find out the number of items in a set, check its read-only count property: 1. print("I have \(favoriteGenres.count) favorite music genres.") 2. // prints "I have 3 favorite music genres." Use the Boolean isEmpty property as a shortcut for checking whether the count property is equal to 0: 1. if favoriteGenres.isEmpty { 2.  print("As far as music goes, I'm not picky.") 3. } else { 4.  print("I have particular music preferences.") 5. } 6. // prints "I have particular music preferences." You can add a new item into a set by calling the set’s insert(_:) method: 1. favoriteGenres.insert("Jazz") 2. // favoriteGenres now contains 4 items You can remove an item from a set by calling the set’s remove(_:) method, which removes the item if it’s a member of the set, and returns the removed value, or returns nil if the set did not contain it. Alternatively, all items in a set can be removed with its removeAll() method. 1. if let removedGenre = favoriteGenres.remove("Rock") { 2.  print("\(removedGenre)? I'm over it.") 3. } else { 4.  print("I never much cared for that.") 5. } 6. // prints "Rock? I'm over it." To check whether a set contains a particular item, use the contains(_:) method. 1. if favoriteGenres.contains("Funk") { 2.  print("I get up on the good foot.") 3. } else { 4.  print("It's too funky in here.") 5. } 6. // prints "It's too funky in here." ### Iterating Over a Set You can iterate over the values in a set with a for-in loop. 1. for genre in favoriteGenres { 2.  print("\(genre)") 3. } 4. // Classical 5. // Jazz 6. // Hip hop For more about the for-in loop, see For Loops. Swift’s Set type does not have a defined ordering. To iterate over the values of a set in a specific order, use the sort() method, which returns an ordered collection of the provided sequence. 1. for genre in favoriteGenres.sort() { 2.  print("\(genre)") 3. } 4. // Classical 5. // Hip hop 6. // Jazz ## Performing Set Operations You can efficiently perform fundamental set operations, such as combining two sets together, determining which values two sets have in common, or determining whether two sets contain all, some, or none of the same values. ### Fundamental Set Operations The illustration below depicts two sets–a and b– with the results of various set operations represented by the shaded regions. 1. Use the intersect(_:) method to create a new set with only the values common to both sets. 2. Use the exclusiveOr(_:) method to create a new set with values in either set, but not both. 3. Use the union(_:) method to create a new set with all of the values in both sets. 4. Use the subtract(_:) method to create a new set with values not in the specified set. 1. let oddDigits: Set = [1, 3, 5, 7, 9] 2. let evenDigits: Set = [0, 2, 4, 6, 8] 3. let singleDigitPrimeNumbers: Set = [2, 3, 5, 7] 4.   5. oddDigits.union(evenDigits).sort() 6. // [0, 1, 2, 3, 4, 5, 6, 7, 8, 9] 7. oddDigits.intersect(evenDigits).sort() 8. // [] 9. oddDigits.subtract(singleDigitPrimeNumbers).sort() 10. // [1, 9] 11. oddDigits.exclusiveOr(singleDigitPrimeNumbers).sort() 12. // [1, 2, 9] ### Set Membership and Equality The illustration below depicts three sets–a, b and c– with overlapping regions representing elements shared among sets. Set a is a superset of set b, because a contains all elements in b. Conversely, set b is a subset of set a, because all elements in b are also contained by a. Set b and set c are disjoint with one another, because they share no elements in common. 1. Use the “is equal” operator (==) to determine whether two sets contain all of the same values. 2. Use the isSubsetOf(_:) method to determine whether all of the values of a set are contained in the specified set. 3. Use the isSupersetOf(_:) method to determine whether a set contains all of the values in a specified set. 4. Use the isStrictSubsetOf(_:) or isStrictSupersetOf(_:) methods to determine whether a set is a subset or superset, but not equal to, a specified set. 5. Use the isDisjointWith(_:) method to determine whether two sets have any values in common. 1. let houseAnimals: Set = ["🐶", "🐱"] 2. let farmAnimals: Set = ["🐮", "🐔", "🐑", "🐶", "🐱"] 3. let cityAnimals: Set = ["🐦", "🐭"] 4.   5. houseAnimals.isSubsetOf(farmAnimals) 6. // true 7. farmAnimals.isSupersetOf(houseAnimals) 8. // true 9. farmAnimals.isDisjointWith(cityAnimals) 10. // true ## Dictionaries A dictionary stores associations between keys of the same type and values of the same type in a collection with no defined ordering. Each value is associated with a unique key, which acts as an identifier for that value within the dictionary. Unlike items in an array, items in a dictionary do not have a specified order. You use a dictionary when you need to look up values based on their identifier, in much the same way that a real-world dictionary is used to look up the definition for a particular word. ### Dictionary Type Shorthand Syntax The type of a Swift dictionary is written in full as Dictionary<Key, Value>, where Key is the type of value that can be used as a dictionary key, and Value is the type of value that the dictionary stores for those keys. You can also write the type of a dictionary in shorthand form as [Key: Value]. Although the two forms are functionally identical, the shorthand form is preferred and is used throughout this guide when referring to the type of a dictionary. ### Creating an Empty Dictionary As with arrays, you can create an empty Dictionary of a certain type by using initializer syntax: 1. var namesOfIntegers = [Int: String]() 2. // namesOfIntegers is an empty [Int: String] dictionary This example creates an empty dictionary of type [Int: String] to store human-readable names of integer values. Its keys are of type Int, and its values are of type String. If the context already provides type information, you can create an empty dictionary with an empty dictionary literal, which is written as [:] (a colon inside a pair of square brackets): 1. namesOfIntegers[16] = "sixteen" 2. // namesOfIntegers now contains 1 key-value pair 3. namesOfIntegers = [:] 4. // namesOfIntegers is once again an empty dictionary of type [Int: String] ### Creating a Dictionary with a Dictionary Literal You can also initialize a dictionary with a dictionary literal, which has a similar syntax to the array literal seen earlier. A dictionary literal is a shorthand way to write one or more key-value pairs as a Dictionary collection. A key-value pair is a combination of a key and a value. In a dictionary literal, the key and value in each key-value pair are separated by a colon. The key-value pairs are written as a list, separated by commas, surrounded by a pair of square brackets: 1. [key 1: value 1, key 2: value 2, key 3: value 3] The example below creates a dictionary to store the names of international airports. In this dictionary, the keys are three-letter International Air Transport Association codes, and the values are airport names: 1. var airports: [String: String] = ["YYZ": "Toronto Pearson", "DUB": "Dublin"] The airports dictionary is declared as having a type of [String: String], which means “a Dictionary whose keys are of type String, and whose values are also of type String”. The airports dictionary is initialized with a dictionary literal containing two key-value pairs. The first pair has a key of "YYZ" and a value of "Toronto Pearson". The second pair has a key of "DUB" and a value of "Dublin". This dictionary literal contains two String: String pairs. This key-value type matches the type of the airports variable declaration (a dictionary with only String keys, and only String values), and so the assignment of the dictionary literal is permitted as a way to initialize the airports dictionary with two initial items. As with arrays, you don’t have to write the type of the dictionary if you’re initializing it with a dictionary literal whose keys and values have consistent types. The initialization of airports could have been written in a shorter form instead: 1. var airports = ["YYZ": "Toronto Pearson", "DUB": "Dublin"] Because all keys in the literal are of the same type as each other, and likewise all values are of the same type as each other, Swift can infer that [String: String] is the correct type to use for the airports dictionary. ### Accessing and Modifying a Dictionary You access and modify a dictionary through its methods and properties, or by using subscript syntax. As with an array, you find out the number of items in a Dictionary by checking its read-only count property: 1. print("The airports dictionary contains \(airports.count) items.") 2. // prints "The airports dictionary contains 2 items." Use the Boolean isEmpty property as a shortcut for checking whether the count property is equal to 0: 1. if airports.isEmpty { 2.  print("The airports dictionary is empty.") 3. } else { 4.  print("The airports dictionary is not empty.") 5. } 6. // prints "The airports dictionary is not empty." You can add a new item to a dictionary with subscript syntax. Use a new key of the appropriate type as the subscript index, and assign a new value of the appropriate type: 1. airports["LHR"] = "London" 2. // the airports dictionary now contains 3 items You can also use subscript syntax to change the value associated with a particular key: 1. airports["LHR"] = "London Heathrow" 2. // the value for "LHR" has been changed to "London Heathrow" As an alternative to subscripting, use a dictionary’s updateValue(_:forKey:) method to set or update the value for a particular key. Like the subscript examples above, the updateValue(_:forKey:) method sets a value for a key if none exists, or updates the value if that key already exists. Unlike a subscript, however, the updateValue(_:forKey:) method returns the old value after performing an update. This enables you to check whether or not an update took place. The updateValue(_:forKey:) method returns an optional value of the dictionary’s value type. For a dictionary that stores String values, for example, the method returns a value of type String?, or “optional String”. This optional value contains the old value for that key if one existed before the update, or nil if no value existed: 1. if let oldValue = airports.updateValue("Dublin Airport", forKey: "DUB") { 2.  print("The old value for DUB was \(oldValue).") 3. } 4. // prints "The old value for DUB was Dublin." You can also use subscript syntax to retrieve a value from the dictionary for a particular key. Because it is possible to request a key for which no value exists, a dictionary’s subscript returns an optional value of the dictionary’s value type. If the dictionary contains a value for the requested key, the subscript returns an optional value containing the existing value for that key. Otherwise, the subscript returns nil: 1. if let airportName = airports["DUB"] { 2.  print("The name of the airport is \(airportName).") 3. } else { 4.  print("That airport is not in the airports dictionary.") 5. } 6. // prints "The name of the airport is Dublin Airport." You can use subscript syntax to remove a key-value pair from a dictionary by assigning a value of nil for that key: 1. airports["APL"] = "Apple International" 2. // "Apple International" is not the real airport for APL, so delete it 3. airports["APL"] = nil 4. // APL has now been removed from the dictionary Alternatively, remove a key-value pair from a dictionary with the removeValueForKey(_:) method. This method removes the key-value pair if it exists and returns the removed value, or returns nil if no value existed: 1. if let removedValue = airports.removeValueForKey("DUB") { 2.  print("The removed airport's name is \(removedValue).") 3. } else { 4.  print("The airports dictionary does not contain a value for DUB.") 5. } 6. // prints "The removed airport's name is Dublin Airport." ### Iterating Over a Dictionary You can iterate over the key-value pairs in a dictionary with a for-in loop. Each item in the dictionary is returned as a (key, value) tuple, and you can decompose the tuple’s members into temporary constants or variables as part of the iteration: 1. for (airportCode, airportName) in airports { 2.  print("\(airportCode): \(airportName)") 3. } 4. // YYZ: Toronto Pearson 5. // LHR: London Heathrow For more about the for-in loop, see For Loops. You can also retrieve an iterable collection of a dictionary’s keys or values by accessing its keys and values properties: 1. for airportCode in airports.keys { 2.  print("Airport code: \(airportCode)") 3. } 4. // Airport code: YYZ 5. // Airport code: LHR 6.   7. for airportName in airports.values { 8.  print("Airport name: \(airportName)") 9. } 10. // Airport name: Toronto Pearson 11. // Airport name: London Heathrow If you need to use a dictionary’s keys or values with an API that takes an Array instance, initialize a new array with the keys or values property: 1. let airportCodes = [String](airports.keys) 2. // airportCodes is ["YYZ", "LHR"] 3.   4. let airportNames = [String](airports.values) 5. // airportNames is ["Toronto Pearson", "London Heathrow"] Swift’s Dictionary type does not have a defined ordering. To iterate over the keys or values of a dictionary in a specific order, use the sort() method on its keys or values property. # Control Flow Swift provides all the familiar control flow statements from C-like languages. These include for and while loops to perform a task multiple times; if, guard, and switch statements to execute different branches of code based on certain conditions; and statements such as break and continue to transfer the flow of execution to another point in your code. In addition to the traditional for loop found in C, Swift adds a for-in loop that makes it easy to iterate over arrays, dictionaries, ranges, strings, and other sequences. Swift’s switch statement is also considerably more powerful than its counterpart in C. The cases of a switch statement do not “fall through” to the next case in Swift, avoiding common C errors caused by missing break statements. Cases can match many different patterns, including interval matches, tuples, and casts to a specific type. Matched values in a switch case can be bound to temporary constants or variables for use within the case’s body, and complex matching conditions can be expressed with a where clause for each case. ## For Loops Swift provides two kinds of loop that perform a set of statements a certain number of times: 1. The for-in loop performs a set of statements for each item in a sequence. 2. The for loop performs a set of statements until a specific condition is met, typically by incrementing a counter each time the loop ends. ### For-In You use the for-in loop to iterate over a sequence, such as ranges of numbers, items in an array, or characters in a string. This example prints the first few entries in the five-times-table: 1. for index in 1...5 { 2.  print("\(index) times 5 is \(index * 5)") 3. } 4. // 1 times 5 is 5 5. // 2 times 5 is 10 6. // 3 times 5 is 15 7. // 4 times 5 is 20 8. // 5 times 5 is 25 The sequence being iterated is a range of numbers from 1 to 5, inclusive, as indicated by the use of the closed range operator (...). The value of index is set to the first number in the range (1), and the statements inside the loop are executed. In this case, the loop contains only one statement, which prints an entry from the five-times-table for the current value of index. After the statement is executed, the value of index is updated to contain the second value in the range (2), and the print(_:) function is called again. This process continues until the end of the range is reached. In the example above, index is a constant whose value is automatically set at the start of each iteration of the loop. As such, it does not have to be declared before it is used. It is implicitly declared simply by its inclusion in the loop declaration, without the need for a let declaration keyword. If you don’t need each value from a sequence, you can ignore the values by using an underscore in place of a variable name: 1. let base = 3 2. let power = 10 3. var answer = 1 4. for _ in 1...power { 5.  answer *= base 6. } 7. print("\(base) to the power of \(power) is \(answer)") 8. // prints "3 to the power of 10 is 59049" This example calculates the value of one number to the power of another (in this case, 3 to the power of 10). It multiplies a starting value of 1 (that is, 3 to the power of 0) by 3, ten times, using a closed range that starts with 1 and ends with 10. This calculation doesn’t need to know the individual counter values each time through the loop—it simply needs to execute the loop the correct number of times. The underscore character _ (used in place of a loop variable) causes the individual values to be ignored and does not provide access to the current value during each iteration of the loop. Use a for-in loop with an array to iterate over its items: 1. let names = ["Anna", "Alex", "Brian", "Jack"] 2. for name in names { 3.  print("Hello, \(name)!") 4. } 5. // Hello, Anna! 6. // Hello, Alex! 7. // Hello, Brian! 8. // Hello, Jack! You can also iterate over a dictionary to access its key-value pairs. Each item in the dictionary is returned as a (key, value) tuple when the dictionary is iterated, and you can decompose the (key, value) tuple’s members as explicitly named constants for use within the body of the for-in loop. Here, the dictionary’s keys are decomposed into a constant called animalName, and the dictionary’s values are decomposed into a constant called legCount: 1. let numberOfLegs = ["spider": 8, "ant": 6, "cat": 4] 2. for (animalName, legCount) in numberOfLegs { 3.  print("\(animalName)s have \(legCount) legs") 4. } 5. // ants have 6 legs 6. // cats have 4 legs 7. // spiders have 8 legs Items in a Dictionary may not necessarily be iterated in the same order as they were inserted. The contents of a Dictionary are inherently unordered, and iterating over them does not guarantee the order in which they will be retrieved. For more on arrays and dictionaries, see Collection Types.) ### For In addition to for-in loops, Swift supports traditional C-style for loops with a condition and an incrementer: 1. for var index = 0; index < 3; ++index { 2.  print("index is \(index)") 3. } 4. // index is 0 5. // index is 1 6. // index is 2 Here’s the general form of this loop format: 1. for initialization; condition; increment { 2.  statements 3. } Semicolons separate the three parts of the loop’s definition, as in C. However, unlike C, Swift doesn’t need parentheses around the entire “initialization; condition; increment” block. The loop is executed as follows: 1. When the loop is first entered, the initialization expression is evaluated once, to set up any constants or variables that are needed for the loop. 2. The condition expression is evaluated. If it evaluates to false, the loop ends, and code execution continues after the for loop’s closing brace (}). If the expression evaluates to true, code execution continues by executing the statements inside the braces. 3. After all statements are executed, the increment expression is evaluated. It might increase or decrease the value of a counter, or set one of the initialized variables to a new value based on the outcome of the statements. After the increment expression has been evaluated, execution returns to step 2, and the condition expression is evaluated again. Constants and variables declared within the initialization expression (such as var index = 0) are only valid within the scope of the for loop itself. To retrieve the final value of index after the loop ends, you must declare index before the loop’s scope begins: 1. var index: Int 2. for index = 0; index < 3; ++index { 3.  print("index is \(index)") 4. } 5. // index is 0 6. // index is 1 7. // index is 2 8. print("The loop statements were executed \(index) times") 9. // prints "The loop statements were executed 3 times" Note that the final value of index after this loop is completed is 3, not 2. The last time the increment statement ++index is called, it sets index to 3, which causes index < 3 to equate to false, ending the loop. ## While Loops A while loop performs a set of statements until a condition becomes false. These kinds of loops are best used when the number of iterations is not known before the first iteration begins. Swift provides two kinds of while loop: 1. while evaluates its condition at the start of each pass through the loop. 2. repeat-while evaluates its condition at the end of each pass through the loop. ### While A while loop starts by evaluating a single condition. If the condition is true, a set of statements is repeated until the condition becomes false. Here’s the general form of a while loop: 1. while condition { 2.  statements 3. } This example plays a simple game of Snakes and Ladders (also known as Chutes and Ladders): The rules of the game are as follows: 1. The board has 25 squares, and the aim is to land on or beyond square 25. 2. Each turn, you roll a six-sided dice and move by that number of squares, following the horizontal path indicated by the dotted arrow above. 3. If your turn ends at the bottom of a ladder, you move up that ladder. 4. If your turn ends at the head of a snake, you move down that snake. The game board is represented by an array of Int values. Its size is based on a constant called finalSquare, which is used to initialize the array and also to check for a win condition later in the example. The board is initialized with 26 zero Int values, not 25 (one each at indexes 0 through 25 inclusive): 1. let finalSquare = 25 2. var board = [Int](count: finalSquare + 1, repeatedValue: 0) Some squares are then set to have more specific values for the snakes and ladders. Squares with a ladder base have a positive number to move you up the board, whereas squares with a snake head have a negative number to move you back down the board: 1. board[03] = +08; board[06] = +11; board[09] = +09; board[10] = +02 2. board[14] = -10; board[19] = -11; board[22] = -02; board[24] = -08 Square 3 contains the bottom of a ladder that moves you up to square 11. To represent this, board[03] is equal to +08, which is equivalent to an integer value of 8 (the difference between 3 and 11). The unary plus operator (+i) balances with the unary minus operator (-i), and numbers lower than 10 are padded with zeros so that all board definitions align. (Neither stylistic tweak is strictly necessary, but they lead to neater code.) The player’s starting square is “square zero”, which is just off the bottom left corner of the board. The first dice roll always moves the player on to the board: 1. var square = 0 2. var diceRoll = 0 3. while square < finalSquare { 4.  // roll the dice 5.  if ++diceRoll == 7 { diceRoll = 1 } 6.  // move by the rolled amount 7.  square += diceRoll 8.  if square < board.count { 9.  // if we're still on the board, move up or down for a snake or a ladder 10.  square += board[square] 11.  } 12. } 13. print("Game over!") This example uses a very simple approach to dice rolling. Instead of a random number generator, it starts with a diceRoll value of 0. Each time through the while loop, diceRoll is incremented with the prefix increment operator (++i), and is then checked to see if it has become too large. The return value of ++diceRoll is equal to the value of diceRoll after it is incremented. Whenever this return value equals 7, the dice roll has become too large, and is reset to a value of 1. This gives a sequence of diceRoll values that is always 1, 2, 3, 4, 5, 6, 1, 2 and so on. After rolling the dice, the player moves forward by diceRoll squares. It’s possible that the dice roll may have moved the player beyond square 25, in which case the game is over. To cope with this scenario, the code checks that square is less than the board array’s count property before adding the value stored in board[square] onto the current square value to move the player up or down any ladders or snakes. Had this check not been performed, board[square] might try to access a value outside the bounds of the board array, which would trigger an error. If square is now equal to 26, the code would try to check the value of board[26], which is larger than the size of the array. The current while loop execution then ends, and the loop’s condition is checked to see if the loop should be executed again. If the player has moved on or beyond square number 25, the loop’s condition evaluates to false, and the game ends. A while loop is appropriate in this case because the length of the game is not clear at the start of the while loop. Instead, the loop is executed until a particular condition is satisfied. ### Repeat-While The other variation of the while loop, known as the repeat-while loop, performs a single pass through the loop block first, before considering the loop’s condition. It then continues to repeat the loop until the condition is false. Here’s the general form of a repeat-while loop: 1. repeat { 2.  statements 3. } while condition Here’s the Snakes and Ladders example again, written as a repeat-while loop rather than a while loop. The values of finalSquare, board, square, and diceRoll are initialized in exactly the same way as with a while loop: 1. let finalSquare = 25 2. var board = [Int](count: finalSquare + 1, repeatedValue: 0) 3. board[03] = +08; board[06] = +11; board[09] = +09; board[10] = +02 4. board[14] = -10; board[19] = -11; board[22] = -02; board[24] = -08 5. var square = 0 6. var diceRoll = 0 In this version of the game, the first action in the loop is to check for a ladder or a snake. No ladder on the board takes the player straight to square 25, and so it is not possible to win the game by moving up a ladder. Therefore, it is safe to check for a snake or a ladder as the first action in the loop. At the start of the game, the player is on “square zero”. board[0] always equals 0, and has no effect: 1. repeat { 2.  // move up or down for a snake or ladder 3.  square += board[square] 4.  // roll the dice 5.  if ++diceRoll == 7 { diceRoll = 1 } 6.  // move by the rolled amount 7.  square += diceRoll 8. } while square < finalSquare 9. print("Game over!") After the code checks for snakes and ladders, the dice is rolled, and the player is moved forward by diceRoll squares. The current loop execution then ends. The loop’s condition (while square < finalSquare) is the same as before, but this time it is not evaluated until the end of the first run through the loop. The structure of the repeat-while loop is better suited to this game than the while loop in the previous example. In the repeat-while loop above, square += board[square] is always executed immediately after the loop’s while condition confirms that square is still on the board. This behavior removes the need for the array bounds check seen in the earlier version of the game. ## Conditional Statements It is often useful to execute different pieces of code based on certain conditions. You might want to run an extra piece of code when an error occurs, or to display a message when a value becomes too high or too low. To do this, you make parts of your code conditional. Swift provides two ways to add conditional branches to your code, known as the if statement and the switch statement. Typically, you use the if statement to evaluate simple conditions with only a few possible outcomes. The switch statement is better suited to more complex conditions with multiple possible permutations, and is useful in situations where pattern-matching can help select an appropriate code branch to execute. ### If In its simplest form, the if statement has a single if condition. It executes a set of statements only if that condition is true: 1. var temperatureInFahrenheit = 30 2. if temperatureInFahrenheit <= 32 { 3.  print("It's very cold. Consider wearing a scarf.") 4. } 5. // prints "It's very cold. Consider wearing a scarf." The preceding example checks whether the temperature is less than or equal to 32 degrees Fahrenheit (the freezing point of water). If it is, a message is printed. Otherwise, no message is printed, and code execution continues after the if statement’s closing brace. The if statement can provide an alternative set of statements, known as an else clause, for when the if condition is false. These statements are indicated by the else keyword: 1. temperatureInFahrenheit = 40 2. if temperatureInFahrenheit <= 32 { 3.  print("It's very cold. Consider wearing a scarf.") 4. } else { 5.  print("It's not that cold. Wear a t-shirt.") 6. } 7. // prints "It's not that cold. Wear a t-shirt." One of these two branches is always executed. Because the temperature has increased to 40 degrees Fahrenheit, it is no longer cold enough to advise wearing a scarf, and so the else branch is triggered instead. You can chain multiple if statements together, to consider additional clauses: 1. temperatureInFahrenheit = 90 2. if temperatureInFahrenheit <= 32 { 3.  print("It's very cold. Consider wearing a scarf.") 4. } else if temperatureInFahrenheit >= 86 { 5.  print("It's really warm. Don't forget to wear sunscreen.") 6. } else { 7.  print("It's not that cold. Wear a t-shirt.") 8. } 9. // prints "It's really warm. Don't forget to wear sunscreen." Here, an additional if statement is added to respond to particularly warm temperatures. The final else clause remains, and prints a response for any temperatures that are neither too warm nor too cold. The final else clause is optional, however, and can be excluded if the set of conditions does not need to be complete: 1. temperatureInFahrenheit = 72 2. if temperatureInFahrenheit <= 32 { 3.  print("It's very cold. Consider wearing a scarf.") 4. } else if temperatureInFahrenheit >= 86 { 5.  print("It's really warm. Don't forget to wear sunscreen.") 6. } In this example, the temperature is neither too cold nor too warm to trigger the if or else if conditions, and so no message is printed. ### Switch A switch statement considers a value and compares it against several possible matching patterns. It then executes an appropriate block of code, based on the first pattern that matches successfully. A switch statement provides an alternative to the if statement for responding to multiple potential states. In its simplest form, a switch statement compares a value against one or more values of the same type: 1. switch some value to consider { 2. case value 1: 3.  respond to value 1 4. case value 2, 5. value 3: 6.  respond to value 2 or 3 7. default: 8.  otherwise, do something else 9. } Every switch statement consists of multiple possible cases, each of which begins with the case keyword. In addition to comparing against specific values, Swift provides several ways for each case to specify more complex matching patterns. These options are described later in this section. The body of each switch case is a separate branch of code execution, in a similar manner to the branches of an if statement. The switch statement determines which branch should be selected. This is known as switching on the value that is being considered. Every switch statement must be exhaustive. That is, every possible value of the type being considered must be matched by one of the switch cases. If it is not appropriate to provide a switch case for every possible value, you can define a default catch-all case to cover any values that are not addressed explicitly. This catch-all case is indicated by the keyword default, and must always appear last. This example uses a switch statement to consider a single lowercase character called someCharacter: 1. let someCharacter: Character = "e" 2. switch someCharacter { 3. case "a", "e", "i", "o", "u": 4.  print("\(someCharacter) is a vowel") 5. case "b", "c", "d", "f", "g", "h", "j", "k", "l", "m", 6. "n", "p", "q", "r", "s", "t", "v", "w", "x", "y", "z": 7.  print("\(someCharacter) is a consonant") 8. default: 9.  print("\(someCharacter) is not a vowel or a consonant") 10. } 11. // prints "e is a vowel" The switch statement’s first case matches all five lowercase vowels in the English language. Similarly, its second case matches all lowercase English consonants. It is not practical to write all other possible characters as part of a switch case, and so this switch statement provides a default case to match all other characters that are not vowels or consonants. This provision ensures that the switch statement is exhaustive. ### No Implicit Fallthrough In contrast with switch statements in C and Objective-C, switch statements in Swift do not fall through the bottom of each case and into the next one by default. Instead, the entire switch statement finishes its execution as soon as the first matching switch case is completed, without requiring an explicit break statement. This makes the switch statement safer and easier to use than in C, and avoids executing more than one switch case by mistake. The body of each case must contain at least one executable statement. It is not valid to write the following code, because the first case is empty: 1. let anotherCharacter: Character = "a" 2. switch anotherCharacter { 3. case "a": 4. case "A": 5.  print("The letter A") 6. default: 7.  print("Not the letter A") 8. } 9. // this will report a compile-time error Unlike a switch statement in C, this switch statement does not match both "a" and "A". Rather, it reports a compile-time error that case "a": does not contain any executable statements. This approach avoids accidental fallthrough from one case to another, and makes for safer code that is clearer in its intent. Multiple matches for a single switch case can be separated by commas, and can be written over multiple lines if the list is long: 1. switch some value to consider { 2. case value 1, 3. value 2: 4.  statements 5. } ### Interval Matching Values in switch cases can be checked for their inclusion in an interval. This example uses number intervals to provide a natural-language count for numbers of any size: 1. let approximateCount = 62 2. let countedThings = "moons orbiting Saturn" 3. var naturalCount: String 4. switch approximateCount { 5. case 0: 6.  naturalCount = "no" 7. case 1..<5: 8.  naturalCount = "a few" 9. case 5..<12: 10.  naturalCount = "several" 11. case 12..<100: 12.  naturalCount = "dozens of" 13. case 100..<1000: 14.  naturalCount = "hundreds of" 15. default: 16.  naturalCount = "many" 17. } 18. print("There are \(naturalCount) \(countedThings).") 19. // prints "There are dozens of moons orbiting Saturn." In the above example, approximateCount is evaluated in a switch statement. Each case compares that value to a number or interval. Because the value of approximateCount falls between 12 and 100, naturalCount is assigned the value "dozens of", and execution is transferred out of the switch statement. ### Tuples You can use tuples to test multiple values in the same switch statement. Each element of the tuple can be tested against a different value or interval of values. Alternatively, use the underscore (_) identifier to match any possible value. The example below takes an (x, y) point, expressed as a simple tuple of type (Int, Int), and categorizes it on the graph that follows the example: 1. let somePoint = (1, 1) 2. switch somePoint { 3. case (0, 0): 4.  print("(0, 0) is at the origin") 5. case (_, 0): 6.  print("(\(somePoint.0), 0) is on the x-axis") 7. case (0, _): 8.  print("(0, \(somePoint.1)) is on the y-axis") 9. case (-2...2, -2...2): 10.  print("(\(somePoint.0), \(somePoint.1)) is inside the box") 11. default: 12.  print("(\(somePoint.0), \(somePoint.1)) is outside of the box") 13. } 14. // prints "(1, 1) is inside the box" The switch statement determines if the point is at the origin (0, 0); on the red x-axis; on the orange y-axis; inside the blue 4-by-4 box centered on the origin; or outside of the box. Unlike C, Swift allows multiple switch cases to consider the same value or values. In fact, the point (0, 0) could match all four of the cases in this example. However, if multiple matches are possible, the first matching case is always used. The point (0, 0) would match case (0, 0) first, and so all other matching cases would be ignored. ### Value Bindings A switch case can bind the value or values it matches to temporary constants or variables, for use in the body of the case. This is known as value binding, because the values are “bound” to temporary constants or variables within the case’s body. The example below takes an (x, y) point, expressed as a tuple of type (Int, Int) and categorizes it on the graph that follows: 1. let anotherPoint = (2, 0) 2. switch anotherPoint { 3. case (let x, 0): 4.  print("on the x-axis with an x value of \(x)") 5. case (0, let y): 6.  print("on the y-axis with a y value of \(y)") 7. case let (x, y): 8.  print("somewhere else at (\(x), \(y))") 9. } 10. // prints "on the x-axis with an x value of 2" The switch statement determines if the point is on the red x-axis, on the orange y-axis, or elsewhere, on neither axis. The three switch cases declare placeholder constants x and y, which temporarily take on one or both tuple values from anotherPoint. The first case, case (let x, 0), matches any point with a y value of 0 and assigns the point’s x value to the temporary constant x. Similarly, the second case, case (0, let y), matches any point with an x value of 0 and assigns the point’s y value to the temporary constant y. Once the temporary constants are declared, they can be used within the case’s code block. Here, they are used as shorthand for printing the values with the print(_:) function. Note that this switch statement does not have a default case. The final case, case let (x, y), declares a tuple of two placeholder constants that can match any value. As a result, it matches all possible remaining values, and a default case is not needed to make the switch statement exhaustive. In the example above, x and y are declared as constants with the let keyword, because there is no need to modify their values within the body of the case. However, they could have been declared as variables instead, with the var keyword. If this had been done, a temporary variable would have been created and initialized with the appropriate value. Any changes to that variable would only have an effect within the body of the case. ### Where A switch case can use a where clause to check for additional conditions. The example below categorizes an (x, y) point on the following graph: 1. let yetAnotherPoint = (1, -1) 2. switch yetAnotherPoint { 3. case let (x, y) where x == y: 4.  print("(\(x), \(y)) is on the line x == y") 5. case let (x, y) where x == -y: 6.  print("(\(x), \(y)) is on the line x == -y") 7. case let (x, y): 8.  print("(\(x), \(y)) is just some arbitrary point") 9. } 10. // prints "(1, -1) is on the line x == -y" The switch statement determines if the point is on the green diagonal line where x == y, on the purple diagonal line where x == -y, or neither. The three switch cases declare placeholder constants x and y, which temporarily take on the two tuple values from yetAnotherPoint. These constants are used as part of a where clause, to create a dynamic filter. The switch case matches the current value of point only if the where clause’s condition evaluates to true for that value. As in the previous example, the final case matches all possible remaining values, and so a default case is not needed to make the switch statement exhaustive. ## Control Transfer Statements Control transfer statements change the order in which your code is executed, by transferring control from one piece of code to another. Swift has five control transfer statements: 1. continue 2. break 3. fallthrough 4. return 5. throw The continue, break, and fallthrough statements are described below. The return statement is described in Functions, and the throw statement is described in Throwing Errors. ### Continue The continue statement tells a loop to stop what it is doing and start again at the beginning of the next iteration through the loop. It says “I am done with the current loop iteration” without leaving the loop altogether. The following example removes all vowels and spaces from a lowercase string to create a cryptic puzzle phrase: 1. let puzzleInput = "great minds think alike" 2. var puzzleOutput = "" 3. for character in puzzleInput.characters { 4.  switch character { 5.  case "a", "e", "i", "o", "u", " ": 6.  continue 7.  default: 8.  puzzleOutput.append(character) 9.  } 10. } 11. print(puzzleOutput) 12. // prints "grtmndsthnklk" The code above calls the continue keyword whenever it matches a vowel or a space, causing the current iteration of the loop to end immediately and to jump straight to the start of the next iteration. This behavior enables the switch block to match (and ignore) only the vowel and space characters, rather than requiring the block to match every character that should get printed. ### Break The break statement ends execution of an entire control flow statement immediately. The break statement can be used inside a switch statement or loop statement when you want to terminate the execution of the switch or loop statement earlier than would otherwise be the case. ### Break in a Loop Statement When used inside a loop statement, break ends the loop’s execution immediately, and transfers control to the first line of code after the loop’s closing brace (}). No further code from the current iteration of the loop is executed, and no further iterations of the loop are started. ### Break in a Switch Statement When used inside a switch statement, break causes the switch statement to end its execution immediately, and to transfer control to the first line of code after the switch statement’s closing brace (}). This behavior can be used to match and ignore one or more cases in a switch statement. Because Swift’s switch statement is exhaustive and does not allow empty cases, it is sometimes necessary to deliberately match and ignore a case in order to make your intentions explicit. You do this by writing the break statement as the entire body of the case you want to ignore. When that case is matched by the switch statement, the break statement inside the case ends the switch statement’s execution immediately. The following example switches on a Character value and determines whether it represents a number symbol in one of four languages. Multiple values are covered in a single switch case for brevity: 1. let numberSymbol: Character = "三" // Simplified Chinese for the number 3 2. var possibleIntegerValue: Int? 3. switch numberSymbol { 4. case "1", "١", "一", "๑": 5.  possibleIntegerValue = 1 6. case "2", "٢", "二", "๒": 7.  possibleIntegerValue = 2 8. case "3", "٣", "三", "๓": 9.  possibleIntegerValue = 3 10. case "4", "٤", "四", "๔": 11.  possibleIntegerValue = 4 12. default: 13.  break 14. } 15. if let integerValue = possibleIntegerValue { 16.  print("The integer value of \(numberSymbol) is \(integerValue).") 17. } else { 18.  print("An integer value could not be found for \(numberSymbol).") 19. } 20. // prints "The integer value of 三 is 3." This example checks numberSymbol to determine whether it is a Latin, Arabic, Chinese, or Thai symbol for the numbers 1 to 4. If a match is found, one of the switch statement’s cases sets an optional Int? variable called possibleIntegerValue to an appropriate integer value. After the switch statement completes its execution, the example uses optional binding to determine whether a value was found. The possibleIntegerValue variable has an implicit initial value of nil by virtue of being an optional type, and so the optional binding will succeed only if possibleIntegerValue was set to an actual value by one of the switch statement’s first four cases. It is not practical to list every possible Character value in the example above, so a default case provides a catchall for any characters that are not matched. This default case does not need to perform any action, and so it is written with a single break statement as its body. As soon as the default case is matched, the break statement ends the switch statement’s execution, and code execution continues from the if let statement. ### Fallthrough Switch statements in Swift do not fall through the bottom of each case and into the next one. Instead, the entire switch statement completes its execution as soon as the first matching case is completed. By contrast, C requires you to insert an explicit break statement at the end of every switch case to prevent fallthrough. Avoiding default fallthrough means that Swift switch statements are much more concise and predictable than their counterparts in C, and thus they avoid executing multiple switch cases by mistake. If you really need C-style fallthrough behavior, you can opt in to this behavior on a case-by-case basis with the fallthrough keyword. The example below uses fallthrough to create a textual description of a number: 1. let integerToDescribe = 5 2. var description = "The number \(integerToDescribe) is" 3. switch integerToDescribe { 4. case 2, 3, 5, 7, 11, 13, 17, 19: 5.  description += " a prime number, and also" 6.  fallthrough 7. default: 8.  description += " an integer." 9. } 10. print(description) 11. // prints "The number 5 is a prime number, and also an integer." This example declares a new String variable called description and assigns it an initial value. The function then considers the value of integerToDescribe using a switch statement. If the value of integerToDescribe is one of the prime numbers in the list, the function appends text to the end of description, to note that the number is prime. It then uses the fallthrough keyword to “fall into” the default case as well. The default case adds some extra text to the end of the description, and the switch statement is complete. If the value of integerToDescribe is not in the list of known prime numbers, it is not matched by the first switch case at all. There are no other specific cases, and so integerToDescribe is matched by the catchall default case. After the switch statement has finished executing, the number’s description is printed using the print(_:) function. In this example, the number 5 is correctly identified as a prime number. ### Labeled Statements You can nest loops and conditional statements inside other loops and conditional statements in Swift to create complex control flow structures. However, loops and conditional statements can both use the break statement to end their execution prematurely. Therefore, it is sometimes useful to be explicit about which loop or conditional statement you want a break statement to terminate. Similarly, if you have multiple nested loops, it can be useful to be explicit about which loop the continue statement should affect. To achieve these aims, you can mark a loop statement or conditional statement with a statement label. With a conditional statement, you can use a statement label with the break statement to end the execution of the labeled statement. With a loop statement, you can use a statement label with the break or continue statement to end or continue the execution of the labeled statement. A labeled statement is indicated by placing a label on the same line as the statement’s introducer keyword, followed by a colon. Here’s an example of this syntax for a while loop, although the principle is the same for all loops and switch statements: 1. label name: while condition { 2.  statements 3. } The following example uses the break and continue statements with a labeled while loop for an adapted version of the Snakes and Ladders game that you saw earlier in this chapter. This time around, the game has an extra rule: 1. To win, you must land exactly on square 25. If a particular dice roll would take you beyond square 25, you must roll again until you roll the exact number needed to land on square 25. The game board is the same as before: The values of finalSquare, board, square, and diceRoll are initialized in the same way as before: 1. let finalSquare = 25 2. var board = [Int](count: finalSquare + 1, repeatedValue: 0) 3. board[03] = +08; board[06] = +11; board[09] = +09; board[10] = +02 4. board[14] = -10; board[19] = -11; board[22] = -02; board[24] = -08 5. var square = 0 6. var diceRoll = 0 This version of the game uses a while loop and a switch statement to implement the game’s logic. The while loop has a statement label called gameLoop, to indicate that it is the main game loop for the Snakes and Ladders game. The while loop’s condition is while square != finalSquare, to reflect that you must land exactly on square 25: 1. gameLoop: while square != finalSquare { 2.  if ++diceRoll == 7 { diceRoll = 1 } 3.  switch square + diceRoll { 4.  case finalSquare: 5.  // diceRoll will move us to the final square, so the game is over 6.  break gameLoop 7.  case let newSquare where newSquare > finalSquare: 8.  // diceRoll will move us beyond the final square, so roll again 9.  continue gameLoop 10.  default: 11.  // this is a valid move, so find out its effect 12.  square += diceRoll 13.  square += board[square] 14.  } 15. } 16. print("Game over!") The dice is rolled at the start of each loop. Rather than moving the player immediately, a switch statement is used to consider the result of the move, and to work out if the move is allowed: 1. If the dice roll will move the player onto the final square, the game is over. The break gameLoop statement transfers control to the first line of code outside of the while loop, which ends the game. 2. If the dice roll will move the player beyond the final square, the move is invalid, and the player needs to roll again. The continue gameLoop statement ends the current while loop iteration and begins the next iteration of the loop. 3. In all other cases, the dice roll is a valid move. The player moves forward by diceRoll squares, and the game logic checks for any snakes and ladders. The loop then ends, and control returns to the while condition to decide whether another turn is required. ## Early Exit A guard statement, like an if statement, executes statements depending on the Boolean value of an expression. You use a guard statement to require that a condition must be true in order for the code after the guard statement to be executed. Unlike an if statement, a guard statement always has an else clause—the code inside the else clause is executed if the condition is not true. 1. func greet(person: [String: String]) { 2.  guard let name = person["name"] else { 3.  return 4.  } 5.   6.  print("Hello \(name)!") 7.   8.  guard let location = person["location"] else { 9.  print("I hope the weather is nice near you.") 10.  return 11.  } 12.   13.  print("I hope the weather is nice in \(location).") 14. } 15.   16. greet(["name": "John"]) 17. // prints "Hello John!" 18. // prints "I hope the weather is nice near you." 19. greet(["name": "Jane", "location": "Cupertino"]) 20. // prints "Hello Jane!" 21. // prints "I hope the weather is nice in Cupertino." If the guard statement’s condition is met, code execution continues after the guard statement’s closing brace. Any variables or constants that were assigned values using an optional binding as part of the condition are available for the rest of the code block that the guard statement appears in. If that condition is not met, the code inside the else branch is executed. That branch must transfer control to exit the code block that that guard statement appears in. It can do this with a control transfer statement such as return, break, continue, or throw, or it can call a function or method that doesn’t return, such as fatalError(). Using a guard statement for requirements improves the readability of your code, compared to doing the same check with an if statement. It lets you write the code that’s typically executed without wrapping it in an else block, and it lets you keep the code that handles a violated requirement next to the requirement. ## Checking API Availability Swift has built-in support for checking API availability, which ensures that you don’t accidentally use APIs that are unavailable on a given deployment target. The compiler uses availability information in the SDK to verify that all of the APIs used in your code are available on the deployment target specified by your project. Swift reports an error at compile time if you try to use an API that isn’t available. You use an availability condition in an if or guard statement to conditionally execute a block of code, depending on whether the APIs you want to use are available at run time. The compiler uses the information from the availability condition when it verifies that the APIs in that block of code are available. 1. if #available(iOS 9, OSX 10.10, *) { 2.  // Use iOS 9 APIs on iOS, and use OS X v10.10 APIs on OS X 3. } else { 4.  // Fall back to earlier iOS and OS X APIs 5. } The availability condition above specifies that on iOS, the body of the if executes only on iOS 9 and later; on OS X, only on OS X v10.10 and later. The last argument, *, is required and specifies that on any other platform, the body of the if executes on the minimum deployment target specified by your target. In its general form, the availability condition takes a list of platform names and versions. You use iOS, OSX, and watchOS for the platform names. In addition to specifying major version numbers like iOS 8, you can specify minor versions numbers like iOS 8.3 and OS X v10.10.3. 1. if #available(platform name version, ..., *) { 2.  statements to execute if the APIs are available 3. } else { 4.  fallback statements to execute if the APIs are unavailable 5. } # Functions Functions are self-contained chunks of code that perform a specific task. You give a function a name that identifies what it does, and this name is used to “call” the function to perform its task when needed. Swift’s unified function syntax is flexible enough to express anything from a simple C-style function with no parameter names to a complex Objective-C-style method with local and external parameter names for each parameter. Parameters can provide default values to simplify function calls and can be passed as in-out parameters, which modify a passed variable once the function has completed its execution. Every function in Swift has a type, consisting of the function’s parameter types and return type. You can use this type like any other type in Swift, which makes it easy to pass functions as parameters to other functions, and to return functions from functions. Functions can also be written within other functions to encapsulate useful functionality within a nested function scope. ## Defining and Calling Functions When you define a function, you can optionally define one or more named, typed values that the function takes as input (known as parameters), and/or a type of value that the function will pass back as output when it is done (known as its return type). Every function has a function name, which describes the task that the function performs. To use a function, you “call” that function with its name and pass it input values (known as arguments) that match the types of the function’s parameters. A function’s arguments must always be provided in the same order as the function’s parameter list. The function in the example below is called sayHello(_:), because that’s what it does—it takes a person’s name as input and returns a greeting for that person. To accomplish this, you define one input parameter—a String value called personName—and a return type of String, which will contain a greeting for that person: 1. func sayHello(personName: String) -> String { 2.  let greeting = "Hello, " + personName + "!" 3.  return greeting 4. } All of this information is rolled up into the function’s definition, which is prefixed with the func keyword. You indicate the function’s return type with the return arrow -> (a hyphen followed by a right angle bracket), which is followed by the name of the type to return. The definition describes what the function does, what it expects to receive, and what it returns when it is done. The definition makes it easy for the function to be called unambiguously from elsewhere in your code: 1. print(sayHello("Anna")) 2. // prints "Hello, Anna!" 3. print(sayHello("Brian")) 4. // prints "Hello, Brian!" You call the sayHello(_:) function by passing it a String argument value in parentheses, such as sayHello("Anna"). Because the function returns a String value, sayHello(_:) can be wrapped in a call to the print(_:) function to print that string and see its return value, as shown above. The body of the sayHello(_:) function starts by defining a new String constant called greeting and setting it to a simple greeting message for personName. This greeting is then passed back out of the function using the return keyword. As soon as return greeting is called, the function finishes its execution and returns the current value of greeting. You can call the sayHello(_:) function multiple times with different input values. The example above shows what happens if it is called with an input value of "Anna", and an input value of "Brian". The function returns a tailored greeting in each case. To simplify the body of this function, combine the message creation and the return statement into one line: 1. func sayHelloAgain(personName: String) -> String { 2.  return "Hello again, " + personName + "!" 3. } 4. print(sayHelloAgain("Anna")) 5. // prints "Hello again, Anna!" ## Function Parameters and Return Values Function parameters and return values are extremely flexible in Swift. You can define anything from a simple utility function with a single unnamed parameter to a complex function with expressive parameter names and different parameter options. ### Multiple Input Parameters Functions can have multiple input parameters, which are written within the function’s parentheses, separated by commas. This function takes a start and an end index for a half-open range, and works out how many elements the range contains: 1. func halfOpenRangeLength(start start: Int, end: Int) -> Int { 2.  return end - start 3. } 4. print(halfOpenRangeLength(start: 1, end: 10)) 5. // prints "9" ### Functions Without Parameters Functions are not required to define input parameters. Here’s a function with no input parameters, which always returns the same String message whenever it is called: 1. func sayHelloWorld() -> String { 2.  return "hello, world" 3. } 4. print(sayHelloWorld()) 5. // prints "hello, world" The function definition still needs parentheses after the function’s name, even though it does not take any parameters. The function name is also followed by an empty pair of parentheses when the function is called. ### Functions With Multiple Parameters Functions can have multiple input parameters, which are written within the function’s parentheses, separated by commas. This function takes a person’s name and whether they have already been greeted as input, and returns an appropriate greeting for that person: 1. func sayHello(personName: String, alreadyGreeted: Bool) -> String { 2.  if alreadyGreeted { 3.  return sayHelloAgain(personName) 4.  } else { 5.  return sayHello(personName) 6.  } 7. } 8. print(sayHello("Tim", alreadyGreeted: true)) 9. // prints "Hello again, Tim!" You call the sayHello(_:alreadyGreeted:) function by passing it both a String argument value and a Bool argument value labeled alreadyGreeted in parentheses, separated by commas. When calling a function with more than one parameter, any argument after the first is labeled according to its corresponding parameter name. Function parameter naming is described in more detail in Function Parameter Names. ### Functions Without Return Values Functions are not required to define a return type. Here’s a version of the sayHello(_:) function, called sayGoodbye(_:), which prints its own String value rather than returning it: 1. func sayGoodbye(personName: String) { 2.  print("Goodbye, \(personName)!") 3. } 4. sayGoodbye("Dave") 5. // prints "Goodbye, Dave!" Because it does not need to return a value, the function’s definition does not include the return arrow (->) or a return type. The return value of a function can be ignored when it is called: 1. func printAndCount(stringToPrint: String) -> Int { 2.  print(stringToPrint) 3.  return stringToPrint.characters.count 4. } 5. func printWithoutCounting(stringToPrint: String) { 6.  printAndCount(stringToPrint) 7. } 8. printAndCount("hello, world") 9. // prints "hello, world" and returns a value of 12 10. printWithoutCounting("hello, world") 11. // prints "hello, world" but does not return a value The first function, printAndCount(_:), prints a string, and then returns its character count as an Int. The second function, printWithoutCounting, calls the first function, but ignores its return value. When the second function is called, the message is still printed by the first function, but the returned value is not used. ### Functions with Multiple Return Values You can use a tuple type as the return type for a function to return multiple values as part of one compound return value. The example below defines a function called minMax(_:), which finds the smallest and largest numbers in an array of Int values: 1. func minMax(array: [Int]) -> (min: Int, max: Int) { 2.  var currentMin = array[0] 3.  var currentMax = array[0] 4.  for value in array[1..<array.count] { 5.  if value < currentMin { 6.  currentMin = value 7.  } else if value > currentMax { 8.  currentMax = value 9.  } 10.  } 11.  return (currentMin, currentMax) 12. } The minMax(_:) function returns a tuple containing two Int values. These values are labeled min and max so that they can be accessed by name when querying the function’s return value. The body of the minMax(_:) function starts by setting two working variables called currentMin and currentMax to the value of the first integer in the array. The function then iterates over the remaining values in the array and checks each value to see if it is smaller or larger than the values of currentMin and currentMax respectively. Finally, the overall minimum and maximum values are returned as a tuple of two Int values. Because the tuple’s member values are named as part of the function’s return type, they can be accessed with dot syntax to retrieve the minimum and maximum found values: 1. let bounds = minMax([8, -6, 2, 109, 3, 71]) 2. print("min is \(bounds.min) and max is \(bounds.max)") 3. // prints "min is -6 and max is 109" Note that the tuple’s members do not need to be named at the point that the tuple is returned from the function, because their names are already specified as part of the function’s return type. ### Optional Tuple Return Types If the tuple type to be returned from a function has the potential to have “no value” for the entire tuple, you can use an optional tuple return type to reflect the fact that the entire tuple can be nil. You write an optional tuple return type by placing a question mark after the tuple type’s closing parenthesis, such as (Int, Int)? or (String, Int, Bool)?. The minMax(_:) function above returns a tuple containing two Int values. However, the function does not perform any safety checks on the array it is passed. If the array argument contains an empty array, the minMax(_:) function, as defined above, will trigger a runtime error when attempting to access array[0]. To handle this “empty array” scenario safely, write the minMax(_:) function with an optional tuple return type and return a value of nil when the array is empty: 1. func minMax(array: [Int]) -> (min: Int, max: Int)? { 2.  if array.isEmpty { return nil } 3.  var currentMin = array[0] 4.  var currentMax = array[0] 5.  for value in array[1..<array.count] { 6.  if value < currentMin { 7.  currentMin = value 8.  } else if value > currentMax { 9.  currentMax = value 10.  } 11.  } 12.  return (currentMin, currentMax) 13. } You can use optional binding to check whether this version of the minMax(_:) function returns an actual tuple value or nil: 1. if let bounds = minMax([8, -6, 2, 109, 3, 71]) { 2.  print("min is \(bounds.min) and max is \(bounds.max)") 3. } 4. // prints "min is -6 and max is 109" ## Function Parameter Names Function parameters have both an external parameter name and a local parameter name. An external parameter name is used to label arguments passed to a function call. A local parameter name is used in the implementation of the function. 1. func someFunction(firstParameterName: Int, secondParameterName: Int) { 2.  // function body goes here 3.  // firstParameterName and secondParameterName refer to 4.  // the argument values for the first and second parameters 5. } 6. someFunction(1, secondParameterName: 2) By default, the first parameter omits its external name, and the second and subsequent parameters use their local name as their external name. All parameters must have unique local names, but may share external parameter in common. ### Specifying External Parameter Names You write an external parameter name before the local parameter name it supports, separated by a space: 1. func someFunction(externalParameterName localParameterName: Int) { 2.  // function body goes here, and can use localParameterName 3.  // to refer to the argument value for that parameter 4. } Here’s a version of the sayHello(_:) function that takes the names of two people and returns a greeting for both of them: 1. func sayHello(to person: String, and anotherPerson: String) -> String { 2.  return "Hello \(person) and \(anotherPerson)!" 3. } 4. print(sayHello(to: "Bill", and: "Ted")) 5. // prints "Hello Bill and Ted!" By specifying external parameter names for both parameters, both the first and second arguments to the sayHello(to:and:) function must be labeled when you call it. The use of external parameter names can allow a function to be called in an expressive, sentence-like manner, while still providing a function body that is readable and clear in intent. ### Omitting External Parameter Names If you do not want to use an external name for the second or subsequent parameters of a function, write an underscore (_) instead of an explicit external name for that parameter. 1. func someFunction(firstParameterName: Int, _ secondParameterName: Int) { 2.  // function body goes here 3.  // firstParameterName and secondParameterName refer to 4.  // the argument values for the first and second parameters 5. } 6. someFunction(1, 2) ### Default Parameter Values You can define a default value for any parameter in a function by assigning a value to the parameter after that parameter’s type. If a default value is defined, you can omit that parameter when calling the function. 1. func someFunction(parameterWithDefault: Int = 12) { 2.  // function body goes here 3.  // if no arguments are passed to the function call, 4.  // value of parameterWithDefault is 42 5. } 6. someFunction(6) // parameterWithDefault is 6 7. someFunction() // parameterWithDefault is 12 ### Variadic Parameters A variadic parameter accepts zero or more values of a specified type. You use a variadic parameter to specify that the parameter can be passed a varying number of input values when the function is called. Write variadic parameters by inserting three period characters (...) after the parameter’s type name. The values passed to a variadic parameter are made available within the function’s body as an array of the appropriate type. For example, a variadic parameter with a name of numbers and a type of Double... is made available within the function’s body as a constant array called numbers of type [Double]. The example below calculates the arithmetic mean (also known as the average) for a list of numbers of any length: 1. func arithmeticMean(numbers: Double...) -> Double { 2.  var total: Double = 0 3.  for number in numbers { 4.  total += number 5.  } 6.  return total / Double(numbers.count) 7. } 8. arithmeticMean(1, 2, 3, 4, 5) 9. // returns 3.0, which is the arithmetic mean of these five numbers 10. arithmeticMean(3, 8.25, 18.75) 11. // returns 10.0, which is the arithmetic mean of these three numbers ### Constant and Variable Parameters Function parameters are constants by default. Trying to change the value of a function parameter from within the body of that function results in a compile-time error. This means that you can’t change the value of a parameter by mistake. However, sometimes it is useful for a function to have a variable copy of a parameter’s value to work with. You can avoid defining a new variable yourself within the function by specifying one or more parameters as variable parameters instead. Variable parameters are available as variables rather than as constants, and give a new modifiable copy of the parameter’s value for your function to work with. Define variable parameters by prefixing the parameter name with the keyword var: 1. func alignRight(var string: String, totalLength: Int, pad: Character) -> String { 2.  let amountToPad = totalLength - string.characters.count 3.  if amountToPad < 1 { 4.  return string 5.  } 6.  let padString = String(pad) 7.  for _ in 1...amountToPad { 8.  string = padString + string 9.  } 10.  return string 11. } 12. let originalString = "hello" 13. let paddedString = alignRight(originalString, totalLength: 10, pad: "-") 14. // paddedString is equal to "-----hello" 15. // originalString is still equal to "hello" This example defines a new function called alignRight(_:totalLength:pad:), which aligns an input string to the right edge of a longer output string. Any space on the left is filled with a specified padding character. In this example, the string "hello" is converted to the string "-----hello". The alignRight(_:totalLength:pad:) function defines the input parameter string to be a variable parameter. This means that string is now available as a local variable, initialized with the passed-in string value, and can be manipulated within the body of the function. The function starts by working out how many characters need to be added to the left of string in order to right-align it within the overall string. This value is stored in a local constant called amountToPad. If no padding is needed (that is, if amountToPad is less than 1), the function simply returns the input value of string without any padding. Otherwise, the function creates a new temporary String constant called padString, initialized with the pad character, and adds amountToPad copies of padString to the left of the existing string. (A String value cannot be added on to a Character value, and so the padString constant is used to ensure that both sides of the + operator are String values.) ### In-Out Parameters Variable parameters, as described above, can only be changed within the function itself. If you want a function to modify a parameter’s value, and you want those changes to persist after the function call has ended, define that parameter as an in-out parameter instead. You write an in-out parameter by placing the inout keyword at the start of its parameter definition. An in-out parameter has a value that is passed in to the function, is modified by the function, and is passed back out of the function to replace the original value. You can only pass a variable as the argument for an in-out parameter. You cannot pass a constant or a literal value as the argument, because constants and literals cannot be modified. You place an ampersand (&) directly before a variable’s name when you pass it as an argument to an inout parameter, to indicate that it can be modified by the function. Here’s an example of a function called swapTwoInts(_:_:), which has two in-out integer parameters called a and b: 1. func swapTwoInts(inout a: Int, inout _ b: Int) { 2.  let temporaryA = a 3.  a = b 4.  b = temporaryA 5. } The swapTwoInts(_:_:) function simply swaps the value of b into a, and the value of a into b. The function performs this swap by storing the value of a in a temporary constant called temporaryA, assigning the value of b to a, and then assigning temporaryA to b. You can call the swapTwoInts(_:_:) function with two variables of type Int to swap their values. Note that the names of someInt and anotherInt are prefixed with an ampersand when they are passed to the swapTwoInts(_:_:) function: 1. var someInt = 3 2. var anotherInt = 107 3. swapTwoInts(&someInt, &anotherInt) 4. print("someInt is now \(someInt), and anotherInt is now \(anotherInt)") 5. // prints "someInt is now 107, and anotherInt is now 3" The example above shows that the original values of someInt and anotherInt are modified by the swapTwoInts(_:_:) function, even though they were originally defined outside of the function. ## Function Types Every function has a specific function type, made up of the parameter types and the return type of the function. For example: 1. func addTwoInts(a: Int, _ b: Int) -> Int { 2.  return a + b 3. } 4. func multiplyTwoInts(a: Int, _ b: Int) -> Int { 5.  return a * b 6. } This example defines two simple mathematical functions called addTwoInts and multiplyTwoInts. These functions each take two Int values, and return an Int value, which is the result of performing an appropriate mathematical operation. The type of both of these functions is (Int, Int) -> Int. This can be read as: “A function type that has two parameters, both of type Int, and that returns a value of type Int.” Here’s another example, for a function with no parameters or return value: 1. func printHelloWorld() { 2.  print("hello, world") 3. } The type of this function is () -> Void, or “a function that has no parameters, and returns Void.” ### Using Function Types You use function types just like any other types in Swift. For example, you can define a constant or variable to be of a function type and assign an appropriate function to that variable: 1. var mathFunction: (Int, Int) -> Int = addTwoInts This can be read as: “Define a variable called mathFunction, which has a type of ‘a function that takes two Int values, and returns an Int value.’ Set this new variable to refer to the function called addTwoInts.” The addTwoInts(_:_:) function has the same type as the mathFunction variable, and so this assignment is allowed by Swift’s type-checker. You can now call the assigned function with the name mathFunction: 1. print("Result: \(mathFunction(2, 3))") 2. // prints "Result: 5" A different function with the same matching type can be assigned to the same variable, in the same way as for non-function types: 1. mathFunction = multiplyTwoInts 2. print("Result: \(mathFunction(2, 3))") 3. // prints "Result: 6" As with any other type, you can leave it to Swift to infer the function type when you assign a function to a constant or variable: 1. let anotherMathFunction = addTwoInts 2. // anotherMathFunction is inferred to be of type (Int, Int) -> Int ### Function Types as Parameter Types You can use a function type such as (Int, Int) -> Int as a parameter type for another function. This enables you to leave some aspects of a function’s implementation for the function’s caller to provide when the function is called. Here’s an example to print the results of the math functions from above: 1. func printMathResult(mathFunction: (Int, Int) -> Int, _ a: Int, _ b: Int) { 2.  print("Result: \(mathFunction(a, b))") 3. } 4. printMathResult(addTwoInts, 3, 5) 5. // prints "Result: 8" This example defines a function called printMathResult(_:_:_:), which has three parameters. The first parameter is called mathFunction, and is of type (Int, Int) -> Int. You can pass any function of that type as the argument for this first parameter. The second and third parameters are called a and b, and are both of type Int. These are used as the two input values for the provided math function. When printMathResult(_:_:_:) is called, it is passed the addTwoInts(_:_:) function, and the integer values 3 and 5. It calls the provided function with the values 3 and 5, and prints the result of 8. The role of printMathResult(_:_:_:) is to print the result of a call to a math function of an appropriate type. It doesn’t matter what that function’s implementation actually does—it matters only that the function is of the correct type. This enables printMathResult(_:_:_:) to hand off some of its functionality to the caller of the function in a type-safe way. ### Function Types as Return Types You can use a function type as the return type of another function. You do this by writing a complete function type immediately after the return arrow (->) of the returning function. The next example defines two simple functions called stepForward(_:) and stepBackward(_:). The stepForward(_:) function returns a value one more than its input value, and the stepBackward(_:) function returns a value one less than its input value. Both functions have a type of (Int) -> Int: 1. func stepForward(input: Int) -> Int { 2.  return input + 1 3. } 4. func stepBackward(input: Int) -> Int { 5.  return input - 1 6. } Here’s a function called chooseStepFunction(_:), whose return type is “a function of type (Int) -> Int”. The chooseStepFunction(_:)function returns the stepForward(_:) function or the stepBackward(_:) function based on a Boolean parameter called backwards: 1. func chooseStepFunction(backwards: Bool) -> (Int) -> Int { 2.  return backwards ? stepBackward : stepForward 3. } You can now use chooseStepFunction(_:) to obtain a function that will step in one direction or the other: 1. var currentValue = 3 2. let moveNearerToZero = chooseStepFunction(currentValue > 0) 3. // moveNearerToZero now refers to the stepBackward() function The preceding example determines whether a positive or negative step is needed to move a variable called currentValue progressively closer to zero. currentValue has an initial value of 3, which means that currentValue > 0 returns true, causing chooseStepFunction(_:) to return the stepBackward(_:) function. A reference to the returned function is stored in a constant called moveNearerToZero. Now that moveNearerToZero refers to the correct function, it can be used to count to zero: 1. print("Counting to zero:") 2. // Counting to zero: 3. while currentValue != 0 { 4.  print("\(currentValue)... ") 5.  currentValue = moveNearerToZero(currentValue) 6. } 7. print("zero!") 8. // 3... 9. // 2... 10. // 1... 11. // zero! ## Nested Functions All of the functions you have encountered so far in this chapter have been examples of global functions, which are defined at a global scope. You can also define functions inside the bodies of other functions, known as nested functions. Nested functions are hidden from the outside world by default, but can still be called and used by their enclosing function. An enclosing function can also return one of its nested functions to allow the nested function to be used in another scope. You can rewrite the chooseStepFunction(_:) example above to use and return nested functions: 1. func chooseStepFunction(backwards: Bool) -> (Int) -> Int { 2.  func stepForward(input: Int) -> Int { return input + 1 } 3.  func stepBackward(input: Int) -> Int { return input - 1 } 4.  return backwards ? stepBackward : stepForward 5. } 6. var currentValue = -4 7. let moveNearerToZero = chooseStepFunction(currentValue > 0) 8. // moveNearerToZero now refers to the nested stepForward() function 9. while currentValue != 0 { 10.  print("\(currentValue)... ") 11.  currentValue = moveNearerToZero(currentValue) 12. } 13. print("zero!") 14. // -4... 15. // -3... 16. // -2... 17. // -1... 18. // zero! # Closures Closures are self-contained blocks of functionality that can be passed around and used in your code. Closures in Swift are similar to blocks in C and Objective-C and to lambdas in other programming languages. Closures can capture and store references to any constants and variables from the context in which they are defined. This is known as closing over those constants and variables, hence the name “closures”. Swift handles all of the memory management of capturing for you. Global and nested functions, as introduced in Functions, are actually special cases of closures. Closures take one of three forms: 1. Global functions are closures that have a name and do not capture any values. 2. Nested functions are closures that have a name and can capture values from their enclosing function. 3. Closure expressions are unnamed closures written in a lightweight syntax that can capture values from their surrounding context. Swift’s closure expressions have a clean, clear style, with optimizations that encourage brief, clutter-free syntax in common scenarios. These optimizations include: 1. Inferring parameter and return value types from context 2. Implicit returns from single-expression closures 3. Shorthand argument names 4. Trailing closure syntax ## Closure Expressions Nested functions, as introduced in Nested Functions, are a convenient means of naming and defining self-contained blocks of code as part of a larger function. However, it is sometimes useful to write shorter versions of function-like constructs without a full declaration and name. This is particularly true when you work with functions that take other functions as one or more of their arguments. Closure expressions are a way to write inline closures in a brief, focused syntax. Closure expressions provide several syntax optimizations for writing closures in a shortened form without loss of clarity or intent. The closure expression examples below illustrate these optimizations by refining a single example of the sort(_:) method over several iterations, each of which expresses the same functionality in a more succinct way. ### The Sort Function Swift’s standard library provides a function called sort, which sorts an array of values of a known type, based on the output of a sorting closure that you provide. Once it completes the sorting process, the sort(_:) method returns a new array of the same type and size as the old one, with its elements in the correct sorted order. The original array is not modified by the sort(_:) method. The closure expression examples below use the sort(_:) method to sort an array of String values in reverse alphabetical order. Here’s the initial array to be sorted: 1. let names = ["Chris", "Alex", "Ewa", "Barry", "Daniella"] The sort(_:) method takes two arguments: 1. An array of values of a known type. 2. A closure that takes two arguments of the same type as the array’s contents, and returns a Bool value to say whether the first value should appear before or after the second value once the values are sorted. The sorting closure needs to return true if the first value should appear before the second value, and false otherwise. This example is sorting an array of String values, and so the sorting closure needs to be a function of type (String, String) -> Bool. One way to provide the sorting closure is to write a normal function of the correct type, and to pass it in as the sort(_:) method’s parameter: 1. func backwards(s1: String, s2: String) -> Bool { 2.  return s1 > s2 3. } 4. var reversed = names.sort(backwards) 5. // reversed is equal to ["Ewa", "Daniella", "Chris", "Barry", "Alex"] If the first string (s1) is greater than the second string (s2), the backwards(_:_:) function will return true, indicating that s1 should appear before s2 in the sorted array. For characters in strings, “greater than” means “appears later in the alphabet than”. This means that the letter "B" is “greater than” the letter "A", and the string "Tom" is greater than the string "Tim". This gives a reverse alphabetical sort, with "Barry" being placed before "Alex", and so on. However, this is a rather long-winded way to write what is essentially a single-expression function (a > b). In this example, it would be preferable to write the sorting closure inline, using closure expression syntax. ### Closure Expression Syntax Closure expression syntax has the following general form: 1. { (parameters) -> return type in 2.  statements 3. } Closure expression syntax can use constant parameters, variable parameters, and inout parameters. Default values cannot be provided. Variadic parameters can be used if you name the variadic parameter and place it last in the parameter list. Tuples can also be used as parameter types and return types. The example below shows a closure expression version of the backwards(_:_:) function from earlier: 1. reversed = names.sort({ (s1: String, s2: String) -> Bool in 2.  return s1 > s2 3. }) Note that the declaration of parameters and return type for this inline closure is identical to the declaration from the backwards(_:_:) function. In both cases, it is written as (s1: String, s2: String) -> Bool. However, for the inline closure expression, the parameters and return type are written inside the curly braces, not outside of them. The start of the closure’s body is introduced by the in keyword. This keyword indicates that the definition of the closure’s parameters and return type has finished, and the body of the closure is about to begin. Because the body of the closure is so short, it can even be written on a single line: 1. reversed = names.sort( { (s1: String, s2: String) -> Bool in return s1 > s2 } ) This illustrates that the overall call to the sort(_:) method has remained the same. A pair of parentheses still wrap the entire set of arguments for the function. However, one of those arguments is now an inline closure. ### Inferring Type From Context Because the sorting closure is passed as an argument to a function, Swift can infer the types of its parameters and the type of the value it returns from the type of the sort(_:) method’s second parameter. This parameter is expecting a function of type (String, String) -> Bool. This means that the (String, String) and Bool types do not need to be written as part of the closure expression’s definition. Because all of the types can be inferred, the return arrow (->) and the parentheses around the names of the parameters can also be omitted: 1. reversed = names.sort( { s1, s2 in return s1 > s2 } ) It is always possible to infer the parameter types and return type when passing a closure to a function as an inline closure expression. As a result, you never need to write an inline closure in its fullest form when the closure is used as a function argument. Nonetheless, you can still make the types explicit if you wish, and doing so is encouraged if it avoids ambiguity for readers of your code. In the case of the sort(_:) method, the purpose of the closure is clear from the fact that sorting is taking place, and it is safe for a reader to assume that the closure is likely to be working with String values, because it is assisting with the sorting of an array of strings. ### Implicit Returns from Single-Expression Closures Single-expression closures can implicitly return the result of their single expression by omitting the return keyword from their declaration, as in this version of the previous example: 1. reversed = names.sort( { s1, s2 in s1 > s2 } ) Here, the function type of the sort(_:) method’s second argument makes it clear that a Bool value must be returned by the closure. Because the closure’s body contains a single expression (s1 > s2) that returns a Bool value, there is no ambiguity, and the return keyword can be omitted. ### Shorthand Argument Names Swift automatically provides shorthand argument names to inline closures, which can be used to refer to the values of the closure’s arguments by the names 0, $1, $2, and so on.

If you use these shorthand argument names within your closure expression, you can omit the closure’s argument list from its definition, and the number and type of the shorthand argument names will be inferred from the expected function type. The in keyword can also be omitted, because the closure expression is made up entirely of its body:

1. reversed = names.sort( { $0 >$1 } )

Here, $0 and $1 refer to the closure’s first and second String arguments.

### Operator Functions

There’s actually an even shorter way to write the closure expression above. Swift’s String type defines its string-specific implementation of the greater-than operator (>) as a function that has two parameters of type String, and returns a value of type Bool. This exactly matches the function type needed for the sort(_:) method’s second parameter. Therefore, you can simply pass in the greater-than operator, and Swift will infer that you want to use its string-specific implementation:

1. reversed = names.sort(>)

For more about operator functions, see Operator Functions.

## Trailing Closures

If you need to pass a closure expression to a function as the function’s final argument and the closure expression is long, it can be useful to write it as a trailing closure instead. A trailing closure is a closure expression that is written outside of (and after) the parentheses of the function call it supports:

1. func someFunctionThatTakesAClosure(closure: () -> Void) {
2.  // function body goes here
3. }
4. 
5. // here's how you call this function without using a trailing closure:
6. 
7. someFunctionThatTakesAClosure({
8.  // closure's body goes here
9. })
10. 
11. // here's how you call this function with a trailing closure instead:
12. 
13. someFunctionThatTakesAClosure() {
14.  // trailing closure's body goes here
15. }

The string-sorting closure from the Closure Expression Syntax section above can be written outside of the sort(_:) method’s parentheses as a trailing closure:

1. reversed = names.sort() { $0 >$1 }

Trailing closures are most useful when the closure is sufficiently long that it is not possible to write it inline on a single line. As an example, Swift’s Array type has a map(_:) method which takes a closure expression as its single argument. The closure is called once for each item in the array, and returns an alternative mapped value (possibly of some other type) for that item. The nature of the mapping and the type of the returned value is left up to the closure to specify.

After applying the provided closure to each array element, the map(_:) method returns a new array containing all of the new mapped values, in the same order as their corresponding values in the original array.

Here’s how you can use the map(_:) method with a trailing closure to convert an array of Int values into an array of String values. The array [16, 58, 510] is used to create the new array ["OneSix", "FiveEight", "FiveOneZero"]:

1. let digitNames = [
2.  0: "Zero", 1: "One", 2: "Two", 3: "Three", 4: "Four",
3.  5: "Five", 6: "Six", 7: "Seven", 8: "Eight", 9: "Nine"
4. ]
5. let numbers = [16, 58, 510]

The code above creates a dictionary of mappings between the integer digits and English-language versions of their names. It also defines an array of integers, ready to be converted into strings.

You can now use the numbers array to create an array of String values, by passing a closure expression to the array’s map(_:) method as a trailing closure. Note that the call to numbers.map does not need to include any parentheses after map, because the map(_:) method has only one parameter, and that parameter is provided as a trailing closure:

1. let strings = numbers.map {
2.  (var number) -> String in
3.  var output = ""
4.  while number > 0 {
5.  output = digitNames[number % 10]! + output
6.  number /= 10
7.  }
8.  return output
9. }
10. // strings is inferred to be of type [String]
11. // its value is ["OneSix", "FiveEight", "FiveOneZero"]

The map(_:) method calls the closure expression once for each item in the array. You do not need to specify the type of the closure’s input parameter, number, because the type can be inferred from the values in the array to be mapped.

In this example, the closure’s number parameter is defined as a variable parameter, as described in Constant and Variable Parameters, so that the parameter’s value can be modified within the closure body, rather than declaring a new local variable and assigning the passed number value to it. The closure expression also specifies a return type of String, to indicate the type that will be stored in the mapped output array.

The closure expression builds a string called output each time it is called. It calculates the last digit of number by using the remainder operator (number % 10), and uses this digit to look up an appropriate string in the digitNames dictionary. The closure can be used to create a string representation of any integer number greater than zero.

The string retrieved from the digitNames dictionary is added to the front of output, effectively building a string version of the number in reverse. (The expression number % 10 gives a value of 6 for 16, 8 for 58, and 0 for 510.)

The number variable is then divided by 10. Because it is an integer, it is rounded down during the division, so 16 becomes 1, 58 becomes 5, and 510 becomes 51.

The process is repeated until number /= 10 is equal to 0, at which point the output string is returned by the closure, and is added to the output array by the map(_:_:) function.

The use of trailing closure syntax in the example above neatly encapsulates the closure’s functionality immediately after the function that closure supports, without needing to wrap the entire closure within the map(_:_:) function’s outer parentheses.

## Capturing Values

A closure can capture constants and variables from the surrounding context in which it is defined. The closure can then refer to and modify the values of those constants and variables from within its body, even if the original scope that defined the constants and variables no longer exists.

In Swift, the simplest form of a closure that can capture values is a nested function, written within the body of another function. A nested function can capture any of its outer function’s arguments and can also capture any constants and variables defined within the outer function.

Here’s an example of a function called makeIncrementer, which contains a nested function called incrementer. The nested incrementer() function captures two values, runningTotal and amount, from its surrounding context. After capturing these values, incrementer is returned by makeIncrementer as a closure that increments runningTotal by amount each time it is called.

1. func makeIncrementer(forIncrement amount: Int) -> Void -> Int {
2.  var runningTotal = 0
3.  func incrementer() -> Int {
4.  runningTotal += amount
5.  return runningTotal
6.  }
7.  return incrementer
8. }

The return type of makeIncrementer is Void -> Int. This means that it returns a function, rather than a simple value. The function it returns has no parameters, and returns an Int value each time it is called. To learn how functions can return other functions, see Function Types as Return Types.

The makeIncrementer(forIncrement:) function defines an integer variable called runningTotal, to store the current running total of the incrementer that will be returned. This variable is initialized with a value of 0.

The makeIncrementer(forIncrement:) function has a single Int parameter with an external name of forIncrement, and a local name of amount. The argument value passed to this parameter specifies how much runningTotal should be incremented by each time the returned incrementer function is called.

makeIncrementer defines a nested function called incrementer, which performs the actual incrementing. This function simply adds amount to runningTotal, and returns the result.

When considered in isolation, the nested incrementer() function might seem unusual:

1. func incrementer() -> Int {
2.  runningTotal += amount
3.  return runningTotal
4. }

The incrementer() function doesn’t have any parameters, and yet it refers to runningTotal and amount from within its function body. It does this by capturing a reference to runningTotal and amount from the surrounding function and using them within its own function body. Capturing by reference ensures that runningTotal and amount do not disappear when the call to makeIncrementer ends, and also ensures that runningTotal is available the next time the incrementer function is called.

Here’s an example of makeIncrementer in action:

1. let incrementByTen = makeIncrementer(forIncrement: 10)

This example sets a constant called incrementByTen to refer to an incrementer function that adds 10 to its runningTotal variable each time it is called. Calling the function multiple times shows this behavior in action:

1. incrementByTen()
2. // returns a value of 10
3. incrementByTen()
4. // returns a value of 20
5. incrementByTen()
6. // returns a value of 30

If you create a second incrementer, it will have its own stored reference to a new, separate runningTotal variable:

1. let incrementBySeven = makeIncrementer(forIncrement: 7)
2. incrementBySeven()
3. // returns a value of 7

Calling the original incrementer (incrementByTen) again continues to increment its own runningTotal variable, and does not affect the variable captured by incrementBySeven:

1. incrementByTen()
2. // returns a value of 40

## Closures Are Reference Types

In the example above, incrementBySeven and incrementByTen are constants, but the closures these constants refer to are still able to increment the runningTotal variables that they have captured. This is because functions and closures are reference types.

Whenever you assign a function or a closure to a constant or a variable, you are actually setting that constant or variable to be a reference to the function or closure. In the example above, it is the choice of closure that incrementByTen refers to that is constant, and not the contents of the closure itself.

This also means that if you assign a closure to two different constants or variables, both of those constants or variables will refer to the same closure:

1. let alsoIncrementByTen = incrementByTen
2. alsoIncrementByTen()
3. // returns a value of 50

# Enumerations

An enumeration defines a common type for a group of related values and enables you to work with those values in a type-safe way within your code.

If you are familiar with C, you will know that C enumerations assign related names to a set of integer values. Enumerations in Swift are much more flexible, and do not have to provide a value for each member of the enumeration. If a value (known as a “raw” value) is provided for each enumeration member, the value can be a string, a character, or a value of any integer or floating-point type.

Alternatively, enumeration members can specify associated values of any type to be stored along with each different member value, much as unions or variants do in other languages. You can define a common set of related members as part of one enumeration, each of which has a different set of values of appropriate types associated with it.

Enumerations in Swift are first-class types in their own right. They adopt many features traditionally supported only by classes, such as computed properties to provide additional information about the enumeration’s current value, and instance methods to provide functionality related to the values the enumeration represents. Enumerations can also define initializers to provide an initial member value; can be extended to expand their functionality beyond their original implementation; and can conform to protocols to provide standard functionality.

For more on these capabilities, see Properties, Methods, Initialization, Extensions, and Protocols.

## Enumeration Syntax

You introduce enumerations with the enum keyword and place their entire definition within a pair of braces:

1. enum SomeEnumeration {
2.  // enumeration definition goes here
3. }

Here’s an example for the four main points of a compass:

1. enum CompassPoint {
2.  case North
3.  case South
4.  case East
5.  case West
6. }

The values defined in an enumeration (such as North, South, East, and West) are the member values (or members) of that enumeration. The case keyword indicates that a new line of member values is about to be defined.

Multiple member values can appear on a single line, separated by commas:

1. enum Planet {
2.  case Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune
3. }

Each enumeration definition defines a brand new type. Like other types in Swift, their names (such as CompassPoint and Planet) should start with a capital letter. Give enumeration types singular rather than plural names, so that they read as self-evident:

1. var directionToHead = CompassPoint.West

The type of directionToHead is inferred when it is initialized with one of the possible values of CompassPoint. Once directionToHead is declared as a CompassPoint, you can set it to a different CompassPoint value using a shorter dot syntax:

1. directionToHead = .East

The type of directionToHead is already known, and so you can drop the type when setting its value. This makes for highly readable code when working with explicitly-typed enumeration values.

## Matching Enumeration Values with a Switch Statement

You can match individual enumeration values with a switch statement:

1. directionToHead = .South
2. switch directionToHead {
3. case .North:
4.  print("Lots of planets have a north")
5. case .South:
6.  print("Watch out for penguins")
7. case .East:
8.  print("Where the sun rises")
9. case .West:
10.  print("Where the skies are blue")
11. }
12. // prints "Watch out for penguins"

You can read this code as:

“Consider the value of directionToHead. In the case where it equals .North, print "Lots of planets have a north". In the case where it equals .South, print "Watch out for penguins".”

…and so on.

As described in Control Flow, a switch statement must be exhaustive when considering an enumeration’s members. If the case for .West is omitted, this code does not compile, because it does not consider the complete list of CompassPoint members. Requiring exhaustiveness ensures that enumeration members are not accidentally omitted.

When it is not appropriate to provide a case for every enumeration member, you can provide a default case to cover any members that are not addressed explicitly:

1. let somePlanet = Planet.Earth
2. switch somePlanet {
3. case .Earth:
4.  print("Mostly harmless")
5. default:
6.  print("Not a safe place for humans")
7. }
8. // prints "Mostly harmless"

## Associated Values

The examples in the previous section show how the members of an enumeration are a defined (and typed) value in their own right. You can set a constant or variable to Planet.Earth, and check for this value later. However, it is sometimes useful to be able to store associated values of other types alongside these member values. This enables you to store additional custom information along with the member value, and permits this information to vary each time you use that member in your code.

You can define Swift enumerations to store associated values of any given type, and the value types can be different for each member of the enumeration if needed. Enumerations similar to these are known as discriminated unions, tagged unions, or variants in other programming languages.

For example, suppose an inventory tracking system needs to track products by two different types of barcode. Some products are labeled with 1D barcodes in UPC-A format, which uses the numbers 0 to 9. Each barcode has a “number system” digit, followed by five “manufacturer code” digits and five “product code” digits. These are followed by a “check” digit to verify that the code has been scanned correctly:

Other products are labeled with 2D barcodes in QR code format, which can use any ISO 8859-1 character and can encode a string up to 2,953 characters long:

It would be convenient for an inventory tracking system to be able to store UPC-A barcodes as a tuple of four integers, and QR code barcodes as a string of any length.

In Swift, an enumeration to define product barcodes of either type might look like this:

1. enum Barcode {
2.  case UPCA(Int, Int, Int, Int)
3.  case QRCode(String)
4. }

“Define an enumeration type called Barcode, which can take either a value of UPCA with an associated value of type (Int, Int, Int, Int), or a value of QRCode with an associated value of type String.”

This definition does not provide any actual Int or String values—it just defines the type of associated values that Barcode constants and variables can store when they are equal to Barcode.UPCA or Barcode.QRCode.

New barcodes can then be created using either type:

1. var productBarcode = Barcode.UPCA(8, 85909, 51226, 3)

This example creates a new variable called productBarcode and assigns it a value of Barcode.UPCA with an associated tuple value of (8, 85909, 51226, 3).

The same product can be assigned a different type of barcode:

1. productBarcode = .QRCode("ABCDEFGHIJKLMNOP")

At this point, the original Barcode.UPCA and its integer values are replaced by the new Barcode.QRCode and its string value. Constants and variables of type Barcode can store either a .UPCA or a .QRCode (together with their associated values), but they can only store one of them at any given time.

The different barcode types can be checked using a switch statement, as before. This time, however, the associated values can be extracted as part of the switch statement. You extract each associated value as a constant (with the let prefix) or a variable (with the var prefix) for use within the switch case’s body:

1. switch productBarcode {
2. case .UPCA(let numberSystem, let manufacturer, let product, let check):
3.  print("UPC-A: \(numberSystem), \(manufacturer), \(product), \(check).")
4. case .QRCode(let productCode):
5.  print("QR code: \(productCode).")
6. }
7. // prints "QR code: ABCDEFGHIJKLMNOP."

If all of the associated values for an enumeration member are extracted as constants, or if all are extracted as variables, you can place a single var or let annotation before the member name, for brevity:

1. switch productBarcode {
2. case let .UPCA(numberSystem, manufacturer, product, check):
3.  print("UPC-A: \(numberSystem), \(manufacturer), \(product), \(check).")
4. case let .QRCode(productCode):
5.  print("QR code: \(productCode).")
6. }
7. // prints "QR code: ABCDEFGHIJKLMNOP."

## Raw Values

The barcode example in Associated Values shows how members of an enumeration can declare that they store associated values of different types. As an alternative to associated values, enumeration members can come prepopulated with default values (called raw values), which are all of the same type.

Here’s an example that stores raw ASCII values alongside named enumeration members:

1. enum ASCIIControlCharacter: Character {
2.  case Tab = "\t"
3.  case LineFeed = "\n"
4.  case CarriageReturn = "\r"
5. }

Here, the raw values for an enumeration called ASCIIControlCharacter are defined to be of type Character, and are set to some of the more common ASCII control characters. Character values are described in Strings and Characters.

Raw values can be strings, characters, or any of the integer or floating-point number types. Each raw value must be unique within its enumeration declaration.

### Implicitly Assigned Raw Values

When you’re working with enumerations that store integer or string raw values, you don’t have to explicitly assign a raw value for each member. When you don’t, Swift will automatically assign the values for you.

For instance, when integers are used for raw values, the implicit value for each member is one more than the previous member. If the first member doesn’t have a value set, it’s value is 0.

The enumeration below is a refinement of the earlier Planet enumeration, with integer raw values to represent each planet’s order from the sun:

1. enum Planet: Int {
2.  case Mercury = 1, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune
3. }

In the example above, Planet.Mercury has an explicit raw value of 1, Planet.Venus has an implicit raw value of 2, and so on.

When strings are used for raw values, the implicit value for each member is the text of that member’s name.

The enumeration below is a refinement of the earlier CompassPoint enumeration, with string raw values to represent each direction’s name:

1. enum CompassPoint: String {
2.  case North, South, East, West
3. }

In the example above, CompassPoint.South has an implicit raw value of "South", and so on.

You access the raw value of an enumeration member with its rawValue property:

1. let earthsOrder = Planet.Earth.rawValue
2. // earthsOrder is 3
3. 
4. let sunsetDirection = CompassPoint.West.rawValue
5. // sunsetDirection is "West"

### Initializing from a Raw Value

If you define an enumeration with a raw-value type, the enumeration automatically receives an initializer that takes a value of the raw value’s type (as a parameter called rawValue) and returns either an enumeration member or nil. You can use this initializer to try to create a new instance of the enumeration.

This example identifies Uranus from its raw value of 7:

1. let possiblePlanet = Planet(rawValue: 7)
2. // possiblePlanet is of type Planet? and equals Planet.Uranus

Not all possible Int values will find a matching planet, however. Because of this, the raw value initializer always returns an optional enumeration member. In the example above, possiblePlanet is of type Planet?, or “optional Planet.”

If you try to find a planet with a position of 9, the optional Planet value returned by the raw value initializer will be nil:

1. let positionToFind = 9
2. if let somePlanet = Planet(rawValue: positionToFind) {
3.  switch somePlanet {
4.  case .Earth:
5.  print("Mostly harmless")
6.  default:
7.  print("Not a safe place for humans")
8.  }
9. } else {
10.  print("There isn't a planet at position \(positionToFind)")
11. }
12. // prints "There isn't a planet at position 9"

This example uses optional binding to try to access a planet with a raw value of 9. The statement if let somePlanet = Planet(rawValue: 9) creates an optional Planet, and sets somePlanet to the value of that optional Planet if it can be retrieved. In this case, it is not possible to retrieve a planet with a position of 9, and so the else branch is executed instead.

## Recursive Enumerations

Enumerations work well for modeling data when there is a fixed number of possibilities that need to be considered, such as the operations used for doing simple integer arithmetic. These operations let you combine simple arithmetic expressions that are made up of integers such as 5 into more complex ones such as 5 + 4.

One important characteristic of arithmetic expressions is that they can be nested. For example, the expression (5 + 4) * 2 has a number on the right hand side of the multiplication and another expression on the left hand side of the multiplication. Because the data is nested, the enumeration used to store the data also needs to support nesting—this means the enumeration needs to be recursive.

A recursive enumeration is an enumeration that has another instance of the enumeration as the associated value for one or more of the enumeration members. The compiler has to insert a layer of indirection when it works with recursive enumerations. You indicate that an enumeration member is recursive by writing indirect before it.

For example, here is an enumeration that stores simple arithmetic expressions:

1. enum ArithmeticExpression {
2.  case Number(Int)
3.  indirect case Addition(ArithmeticExpression, ArithmeticExpression)
4.  indirect case Multiplication(ArithmeticExpression, ArithmeticExpression)
5. }

You can also write indirect before the beginning of the enumeration, to enable indirection for all of the enumeration’s members that need it:

1. indirect enum ArithmeticExpression {
2.  case Number(Int)
3.  case Addition(ArithmeticExpression, ArithmeticExpression)
4.  case Multiplication(ArithmeticExpression, ArithmeticExpression)
5. }

This enumeration can store three kinds of arithmetic expressions: a plain number, the addition of two expressions, and the multiplication of two expressions. The Addition and Multiplication members have associated values that are also arithmetic expressions—these associated values make it possible to nest expressions.

A recursive function is a straightforward way to work with data that has a recursive structure. For example, here’s a function that evaluates an arithmetic expression:

1. func evaluate(expression: ArithmeticExpression) -> Int {
2.  switch expression {
3.  case .Number(let value):
4.  return value
5.  case .Addition(let left, let right):
6.  return evaluate(left) + evaluate(right)
7.  case .Multiplication(let left, let right):
8.  return evaluate(left) * evaluate(right)
9.  }
10. }
11. 
12. // evaluate (5 + 4) * 2
13. let five = ArithmeticExpression.Number(5)
14. let four = ArithmeticExpression.Number(4)
15. let sum = ArithmeticExpression.Addition(five, four)
16. let product = ArithmeticExpression.Multiplication(sum, ArithmeticExpression.Number(2))
17. print(evaluate(product))
18. // prints "18"

This function evaluates a plain number by simply returning the associated value. It evaluates an addition or multiplication by evaluating the expression on the left hand side, evaluating the expression on the right hand side, and then adding them or multiplying them.

# Classes and Structures

Classes and structures are general-purpose, flexible constructs that become the building blocks of your program’s code. You define properties and methods to add functionality to your classes and structures by using exactly the same syntax as for constants, variables, and functions.

Unlike other programming languages, Swift does not require you to create separate interface and implementation files for custom classes and structures. In Swift, you define a class or a structure in a single file, and the external interface to that class or structure is automatically made available for other code to use.

## Comparing Classes and Structures

Classes and structures in Swift have many things in common. Both can:

1. Define properties to store values

2. Define methods to provide functionality

3. Define subscripts to provide access to their values using subscript syntax

4. Define initializers to set up their initial state

5. Be extended to expand their functionality beyond a default implementation

6. Conform to protocols to provide standard functionality of a certain kind

Classes have additional capabilities that structures do not:

1. Inheritance enables one class to inherit the characteristics of another.

2. Type casting enables you to check and interpret the type of a class instance at runtime.

3. Deinitializers enable an instance of a class to free up any resources it has assigned.

4. Reference counting allows more than one reference to a class instance.

For more information, see Inheritance, Type Casting, Deinitialization, and Automatic Reference Counting.

### Definition Syntax

Classes and structures have a similar definition syntax. You introduce classes with the class keyword and structures with the struct keyword. Both place their entire definition within a pair of braces:

1. class SomeClass {
2.  // class definition goes here
3. }
4. struct SomeStructure {
5.  // structure definition goes here
6. }

Here’s an example of a structure definition and a class definition:

1. struct Resolution {
2.  var width = 0
3.  var height = 0
4. }
5. class VideoMode {
6.  var resolution = Resolution()
7.  var interlaced = false
8.  var frameRate = 0.0
9.  var name: String?
10. }

The example above defines a new structure called Resolution, to describe a pixel-based display resolution. This structure has two stored properties called width and height. Stored properties are constants or variables that are bundled up and stored as part of the class or structure. These two properties are inferred to be of type Int by setting them to an initial integer value of 0.

The example above also defines a new class called VideoMode, to describe a specific video mode for video display. This class has four variable stored properties. The first, resolution, is initialized with a new Resolution structure instance, which infers a property type of Resolution. For the other three properties, new VideoMode instances will be initialized with an interlaced setting of false (meaning “non-interlaced video”), a playback frame rate of 0.0, and an optional String value called name. The name property is automatically given a default value of nil, or “no name value”, because it is of an optional type.

### Class and Structure Instances

The Resolution structure definition and the VideoMode class definition only describe what a Resolution or VideoMode will look like. They themselves do not describe a specific resolution or video mode. To do that, you need to create an instance of the structure or class.

The syntax for creating instances is very similar for both structures and classes:

1. let someResolution = Resolution()
2. let someVideoMode = VideoMode()

Structures and classes both use initializer syntax for new instances. The simplest form of initializer syntax uses the type name of the class or structure followed by empty parentheses, such as Resolution() or VideoMode(). This creates a new instance of the class or structure, with any properties initialized to their default values. Class and structure initialization is described in more detail in Initialization.

### Accessing Properties

You can access the properties of an instance using dot syntax. In dot syntax, you write the property name immediately after the instance name, separated by a period (.), without any spaces:

1. print("The width of someResolution is \(someResolution.width)")
2. // prints "The width of someResolution is 0"

In this example, someResolution.width refers to the width property of someResolution, and returns its default initial value of 0.

You can drill down into sub-properties, such as the width property in the resolution property of a VideoMode:

1. print("The width of someVideoMode is \(someVideoMode.resolution.width)")
2. // prints "The width of someVideoMode is 0"

You can also use dot syntax to assign a new value to a variable property:

1. someVideoMode.resolution.width = 1280
2. print("The width of someVideoMode is now \(someVideoMode.resolution.width)")
3. // prints "The width of someVideoMode is now 1280"

### Memberwise Initializers for Structure Types

All structures have an automatically-generated memberwise initializer, which you can use to initialize the member properties of new structure instances. Initial values for the properties of the new instance can be passed to the memberwise initializer by name:

1. let vga = Resolution(width: 640, height: 480)

Unlike structures, class instances do not receive a default memberwise initializer. Initializers are described in more detail in Initialization.

## Structures and Enumerations Are Value Types

A value type is a type whose value is copied when it is assigned to a variable or constant, or when it is passed to a function.

You’ve actually been using value types extensively throughout the previous chapters. In fact, all of the basic types in Swift—integers, floating-point numbers, Booleans, strings, arrays and dictionaries—are value types, and are implemented as structures behind the scenes.

All structures and enumerations are value types in Swift. This means that any structure and enumeration instances you create—and any value types they have as properties—are always copied when they are passed around in your code.

Consider this example, which uses the Resolution structure from the previous example:

1. let hd = Resolution(width: 1920, height: 1080)
2. var cinema = hd

This example declares a constant called hd and sets it to a Resolution instance initialized with the width and height of full HD video (1920 pixels wide by 1080 pixels high).

It then declares a variable called cinema and sets it to the current value of hd. Because Resolution is a structure, a copy of the existing instance is made, and this new copy is assigned to cinema. Even though hd and cinema now have the same width and height, they are two completely different instances behind the scenes.

Next, the width property of cinema is amended to be the width of the slightly-wider 2K standard used for digital cinema projection (2048 pixels wide and 1080 pixels high):

1. cinema.width = 2048

Checking the width property of cinema shows that it has indeed changed to be 2048:

1. print("cinema is now \(cinema.width) pixels wide")
2. // prints "cinema is now 2048 pixels wide"

However, the width property of the original hd instance still has the old value of 1920:

1. print("hd is still \(hd.width) pixels wide")
2. // prints "hd is still 1920 pixels wide"

When cinema was given the current value of hd, the values stored in hd were copied into the new cinema instance. The end result is two completely separate instances, which just happened to contain the same numeric values. Because they are separate instances, setting the width of cinema to 2048 doesn’t affect the width stored in hd.

The same behavior applies to enumerations:

1. enum CompassPoint {
2.  case North, South, East, West
3. }
4. var currentDirection = CompassPoint.West
5. let rememberedDirection = currentDirection
6. currentDirection = .East
7. if rememberedDirection == .West {
8.  print("The remembered direction is still .West")
9. }
10. // prints "The remembered direction is still .West"

When rememberedDirection is assigned the value of currentDirection, it is actually set to a copy of that value. Changing the value of currentDirection thereafter does not affect the copy of the original value that was stored in rememberedDirection.

## Classes Are Reference Types

Unlike value types, reference types are not copied when they are assigned to a variable or constant, or when they are passed to a function. Rather than a copy, a reference to the same existing instance is used instead.

Here’s an example, using the VideoMode class defined above:

1. let tenEighty = VideoMode()
2. tenEighty.resolution = hd
3. tenEighty.interlaced = true
4. tenEighty.name = "1080i"
5. tenEighty.frameRate = 25.0

This example declares a new constant called tenEighty and sets it to refer to a new instance of the VideoMode class. The video mode is assigned a copy of the HD resolution of 1920 by 1080 from before. It is set to be interlaced, and is given a name of "1080i". Finally, it is set to a frame rate of 25.0 frames per second.

Next, tenEighty is assigned to a new constant, called alsoTenEighty, and the frame rate of alsoTenEighty is modified:

1. let alsoTenEighty = tenEighty
2. alsoTenEighty.frameRate = 30.0

Because classes are reference types, tenEighty and alsoTenEighty actually both refer to the same VideoMode instance. Effectively, they are just two different names for the same single instance.

Checking the frameRate property of tenEighty shows that it correctly reports the new frame rate of 30.0 from the underlying VideoMode instance:

1. print("The frameRate property of tenEighty is now \(tenEighty.frameRate)")
2. // prints "The frameRate property of tenEighty is now 30.0"

Note that tenEighty and alsoTenEighty are declared as constants, rather than variables. However, you can still change tenEighty.frameRate and alsoTenEighty.frameRate because the values of the tenEighty and alsoTenEighty constants themselves do not actually change. tenEighty and alsoTenEighty themselves do not “store” the VideoMode instance—instead, they both refer to a VideoMode instance behind the scenes. It is the frameRate property of the underlying VideoMode that is changed, not the values of the constant references to that VideoMode.

### Identity Operators

Because classes are reference types, it is possible for multiple constants and variables to refer to the same single instance of a class behind the scenes. (The same is not true for structures and enumerations, because they are always copied when they are assigned to a constant or variable, or passed to a function.)

It can sometimes be useful to find out if two constants or variables refer to exactly the same instance of a class. To enable this, Swift provides two identity operators:

1. Identical to (===)

2. Not identical to (!==)

Use these operators to check whether two constants or variables refer to the same single instance:

1. if tenEighty === alsoTenEighty {
2.  print("tenEighty and alsoTenEighty refer to the same VideoMode instance.")
3. }
4. // prints "tenEighty and alsoTenEighty refer to the same VideoMode instance."

Note that “identical to” (represented by three equals signs, or ===) does not mean the same thing as “equal to” (represented by two equals signs, or ==):

1. “Identical to” means that two constants or variables of class type refer to exactly the same class instance.

2. “Equal to” means that two instances are considered “equal” or “equivalent” in value, for some appropriate meaning of “equal”, as defined by the type’s designer.

When you define your own custom classes and structures, it is your responsibility to decide what qualifies as two instances being “equal”. The process of defining your own implementations of the “equal to” and “not equal to” operators is described in Equivalence Operators.

### Pointers

If you have experience with C, C++, or Objective-C, you may know that these languages use pointers to refer to addresses in memory. A Swift constant or variable that refers to an instance of some reference type is similar to a pointer in C, but is not a direct pointer to an address in memory, and does not require you to write an asterisk (*) to indicate that you are creating a reference. Instead, these references are defined like any other constant or variable in Swift.

## Choosing Between Classes and Structures

You can use both classes and structures to define custom data types to use as the building blocks of your program’s code.

However, structure instances are always passed by value, and class instances are always passed by reference. This means that they are suited to different kinds of tasks. As you consider the data constructs and functionality that you need for a project, decide whether each data construct should be defined as a class or as a structure.

As a general guideline, consider creating a structure when one or more of these conditions apply:

1. The structure’s primary purpose is to encapsulate a few relatively simple data values.

2. It is reasonable to expect that the encapsulated values will be copied rather than referenced when you assign or pass around an instance of that structure.

3. Any properties stored by the structure are themselves value types, which would also be expected to be copied rather than referenced.

4. The structure does not need to inherit properties or behavior from another existing type.

Examples of good candidates for structures include:

1. The size of a geometric shape, perhaps encapsulating a width property and a height property, both of type Double.

2. A way to refer to ranges within a series, perhaps encapsulating a start property and a length property, both of type Int.

3. A point in a 3D coordinate system, perhaps encapsulating x, y and z properties, each of type Double.

In all other cases, define a class, and create instances of that class to be managed and passed by reference. In practice, this means that most custom data constructs should be classes, not structures.

## Assignment and Copy Behavior for Strings, Arrays, and Dictionaries

Swift’s String, Array, and Dictionary types are implemented as structures. This means that strings, arrays, and dictionaries are copied when they are assigned to a new constant or variable, or when they are passed to a function or method.

This behavior is different from NSString, NSArray, and NSDictionary in Foundation, which are implemented as classes, not structures. NSString, NSArray, and NSDictionary instances are always assigned and passed around as a reference to an existing instance, rather than as a copy.

# Properties

Properties associate values with a particular class, structure, or enumeration. Stored properties store constant and variable values as part of an instance, whereas computed properties calculate (rather than store) a value. Computed properties are provided by classes, structures, and enumerations. Stored properties are provided only by classes and structures.

Stored and computed properties are usually associated with instances of a particular type. However, properties can also be associated with the type itself. Such properties are known as type properties.

In addition, you can define property observers to monitor changes in a property’s value, which you can respond to with custom actions. Property observers can be added to stored properties you define yourself, and also to properties that a subclass inherits from its superclass.

## Stored Properties

In its simplest form, a stored property is a constant or variable that is stored as part of an instance of a particular class or structure. Stored properties can be either variable stored properties (introduced by the var keyword) or constant stored properties (introduced by the let keyword).

You can provide a default value for a stored property as part of its definition, as described in Default Property Values. You can also set and modify the initial value for a stored property during initialization. This is true even for constant stored properties, as described in Assigning Constant Properties During Initialization.

The example below defines a structure called FixedLengthRange, which describes a range of integers whose range length cannot be changed once it is created:

1. struct FixedLengthRange {
2.  var firstValue: Int
3.  let length: Int
4. }
5. var rangeOfThreeItems = FixedLengthRange(firstValue: 0, length: 3)
6. // the range represents integer values 0, 1, and 2
7. rangeOfThreeItems.firstValue = 6
8. // the range now represents integer values 6, 7, and 8

Instances of FixedLengthRange have a variable stored property called firstValue and a constant stored property called length. In the example above, length is initialized when the new range is created and cannot be changed thereafter, because it is a constant property.

### Stored Properties of Constant Structure Instances

If you create an instance of a structure and assign that instance to a constant, you cannot modify the instance’s properties, even if they were declared as variable properties:

1. let rangeOfFourItems = FixedLengthRange(firstValue: 0, length: 4)
2. // this range represents integer values 0, 1, 2, and 3
3. rangeOfFourItems.firstValue = 6
4. // this will report an error, even though firstValue is a variable property

Because rangeOfFourItems is declared as a constant (with the let keyword), it is not possible to change its firstValue property, even though firstValue is a variable property.

This behavior is due to structures being value types. When an instance of a value type is marked as a constant, so are all of its properties.

The same is not true for classes, which are reference types. If you assign an instance of a reference type to a constant, you can still change that instance’s variable properties.

### Lazy Stored Properties

A lazy stored property is a property whose initial value is not calculated until the first time it is used. You indicate a lazy stored property by writing the lazy modifier before its declaration.

Lazy properties are useful when the initial value for a property is dependent on outside factors whose values are not known until after an instance’s initialization is complete. Lazy properties are also useful when the initial value for a property requires complex or computationally expensive setup that should not be performed unless or until it is needed.

The example below uses a lazy stored property to avoid unnecessary initialization of a complex class. This example defines two classes called DataImporter and DataManager, neither of which is shown in full:

1. class DataImporter {
2.  /*
3.  DataImporter is a class to import data from an external file.
4.  The class is assumed to take a non-trivial amount of time to initialize.
5.  */
6.  var fileName = "data.txt"
7.  // the DataImporter class would provide data importing functionality here
8. }
9. 
10. class DataManager {
11.  lazy var importer = DataImporter()
12.  var data = [String]()
13.  // the DataManager class would provide data management functionality here
14. }
15. 
16. let manager = DataManager()
17. manager.data.append("Some data")
18. manager.data.append("Some more data")
19. // the DataImporter instance for the importer property has not yet been created

The DataManager class has a stored property called data, which is initialized with a new, empty array of String values. Although the rest of its functionality is not shown, the purpose of this DataManager class is to manage and provide access to this array of String data.

Part of the functionality of the DataManager class is the ability to import data from a file. This functionality is provided by the DataImporter class, which is assumed to take a non-trivial amount of time to initialize. This might be because a DataImporter instance needs to open a file and read its contents into memory when the DataImporter instance is initialized.

It is possible for a DataManager instance to manage its data without ever importing data from a file, so there is no need to create a new DataImporter instance when the DataManager itself is created. Instead, it makes more sense to create the DataImporter instance if and when it is first used.

Because it is marked with the lazy modifier, the DataImporter instance for the importer property is only created when the importer property is first accessed, such as when its fileName property is queried:

1. print(manager.importer.fileName)
2. // the DataImporter instance for the importer property has now been created
3. // prints "data.txt"

### Stored Properties and Instance Variables

If you have experience with Objective-C, you may know that it provides two ways to store values and references as part of a class instance. In addition to properties, you can use instance variables as a backing store for the values stored in a property.

Swift unifies these concepts into a single property declaration. A Swift property does not have a corresponding instance variable, and the backing store for a property is not accessed directly. This approach avoids confusion about how the value is accessed in different contexts and simplifies the property’s declaration into a single, definitive statement. All information about the property—including its name, type, and memory management characteristics—is defined in a single location as part of the type’s definition.

## Computed Properties

In addition to stored properties, classes, structures, and enumerations can define computed properties, which do not actually store a value. Instead, they provide a getter and an optional setter to retrieve and set other properties and values indirectly.

1. struct Point {
2.  var x = 0.0, y = 0.0
3. }
4. struct Size {
5.  var width = 0.0, height = 0.0
6. }
7. struct Rect {
8.  var origin = Point()
9.  var size = Size()
10.  var center: Point {
11.  get {
12.  let centerX = origin.x + (size.width / 2)
13.  let centerY = origin.y + (size.height / 2)
14.  return Point(x: centerX, y: centerY)
15.  }
16.  set(newCenter) {
17.  origin.x = newCenter.x - (size.width / 2)
18.  origin.y = newCenter.y - (size.height / 2)
19.  }
20.  }
21. }
22. var square = Rect(origin: Point(x: 0.0, y: 0.0),
23.  size: Size(width: 10.0, height: 10.0))
24. let initialSquareCenter = square.center
25. square.center = Point(x: 15.0, y: 15.0)
26. print("square.origin is now at (\(square.origin.x), \(square.origin.y))")
27. // prints "square.origin is now at (10.0, 10.0)"

This example defines three structures for working with geometric shapes:

1. Point encapsulates an (x, y) coordinate.

2. Size encapsulates a width and a height.

3. Rect defines a rectangle by an origin point and a size.

The Rect structure also provides a computed property called center. The current center position of a Rect can always be determined from its origin and size, and so you don’t need to store the center point as an explicit Point value. Instead, Rect defines a custom getter and setter for a computed variable called center, to enable you to work with the rectangle’s center as if it were a real stored property.

The preceding example creates a new Rect variable called square. The square variable is initialized with an origin point of (0, 0), and a width and height of 10. This square is represented by the blue square in the diagram below.

The square variable’s center property is then accessed through dot syntax (square.center), which causes the getter for center to be called, to retrieve the current property value. Rather than returning an existing value, the getter actually calculates and returns a new Point to represent the center of the square. As can be seen above, the getter correctly returns a center point of (5, 5).

The center property is then set to a new value of (15, 15), which moves the square up and to the right, to the new position shown by the orange square in the diagram below. Setting the center property calls the setter for center, which modifies the x and y values of the stored origin property, and moves the square to its new position.

### Shorthand Setter Declaration

If a computed property’s setter does not define a name for the new value to be set, a default name of newValue is used. Here’s an alternative version of the Rect structure, which takes advantage of this shorthand notation:

1. struct AlternativeRect {
2.  var origin = Point()
3.  var size = Size()
4.  var center: Point {
5.  get {
6.  let centerX = origin.x + (size.width / 2)
7.  let centerY = origin.y + (size.height / 2)
8.  return Point(x: centerX, y: centerY)
9.  }
10.  set {
11.  origin.x = newValue.x - (size.width / 2)
12.  origin.y = newValue.y - (size.height / 2)
13.  }
14.  }
15. }

A computed property with a getter but no setter is known as a read-only computed property. A read-only computed property always returns a value, and can be accessed through dot syntax, but cannot be set to a different value.

You can simplify the declaration of a read-only computed property by removing the get keyword and its braces:

1. struct Cuboid {
2.  var width = 0.0, height = 0.0, depth = 0.0
3.  var volume: Double {
4.  return width * height * depth
5.  }
6. }
7. let fourByFiveByTwo = Cuboid(width: 4.0, height: 5.0, depth: 2.0)
8. print("the volume of fourByFiveByTwo is \(fourByFiveByTwo.volume)")
9. // prints "the volume of fourByFiveByTwo is 40.0"

This example defines a new structure called Cuboid, which represents a 3D rectangular box with width, height, and depth properties. This structure also has a read-only computed property called volume, which calculates and returns the current volume of the cuboid. It doesn’t make sense for volume to be settable, because it would be ambiguous as to which values of width, height, and depth should be used for a particular volume value. Nonetheless, it is useful for a Cuboid to provide a read-only computed property to enable external users to discover its current calculated volume.

## Property Observers

Property observers observe and respond to changes in a property’s value. Property observers are called every time a property’s value is set, even if the new value is the same as the property’s current value.

You can add property observers to any stored properties you define, apart from lazy stored properties. You can also add property observers to any inherited property (whether stored or computed) by overriding the property within a subclass. Property overriding is described in Overriding.

You have the option to define either or both of these observers on a property:

1. willSet is called just before the value is stored.

2. didSet is called immediately after the new value is stored.

If you implement a willSet observer, it is passed the new property value as a constant parameter. You can specify a name for this parameter as part of your willSet implementation. If you choose not to write the parameter name and parentheses within your implementation, the parameter will still be made available with a default parameter name of newValue.

Similarly, if you implement a didSet observer, it will be passed a constant parameter containing the old property value. You can name the parameter if you wish, or use the default parameter name of oldValue.

Here’s an example of willSet and didSet in action. The example below defines a new class called StepCounter, which tracks the total number of steps that a person takes while walking. This class might be used with input data from a pedometer or other step counter to keep track of a person’s exercise during their daily routine.

1. class StepCounter {
2.  var totalSteps: Int = 0 {
3.  willSet(newTotalSteps) {
4.  print("About to set totalSteps to \(newTotalSteps)")
5.  }
6.  didSet {
7.  if totalSteps > oldValue {
8.  print("Added \(totalSteps - oldValue) steps")
9.  }
10.  }
11.  }
12. }
13. let stepCounter = StepCounter()
14. stepCounter.totalSteps = 200
15. // About to set totalSteps to 200
16. // Added 200 steps
17. stepCounter.totalSteps = 360
18. // About to set totalSteps to 360
19. // Added 160 steps
20. stepCounter.totalSteps = 896
21. // About to set totalSteps to 896
22. // Added 536 steps

The StepCounter class declares a totalSteps property of type Int. This is a stored property with willSet and didSet observers.

The willSet and didSet observers for totalSteps are called whenever the property is assigned a new value. This is true even if the new value is the same as the current value.

This example’s willSet observer uses a custom parameter name of newTotalSteps for the upcoming new value. In this example, it simply prints out the value that is about to be set.

The didSet observer is called after the value of totalSteps is updated. It compares the new value of totalSteps against the old value. If the total number of steps has increased, a message is printed to indicate how many new steps have been taken. The didSet observer does not provide a custom parameter name for the old value, and the default name of oldValue is used instead.

## Global and Local Variables

The capabilities described above for computing and observing properties are also available to global variables and local variables. Global variables are variables that are defined outside of any function, method, closure, or type context. Local variables are variables that are defined within a function, method, or closure context.

The global and local variables you have encountered in previous chapters have all been stored variables. Stored variables, like stored properties, provide storage for a value of a certain type and allow that value to be set and retrieved.

However, you can also define computed variables and define observers for stored variables, in either a global or local scope. Computed variables calculate rather than store a value, and are written in the same way as computed properties.

## Type Properties

Instance properties are properties that belong to an instance of a particular type. Every time you create a new instance of that type, it has its own set of property values, separate from any other instance.

You can also define properties that belong to the type itself, not to any one instance of that type. There will only ever be one copy of these properties, no matter how many instances of that type you create. These kinds of properties are called type properties.

Type properties are useful for defining values that are universal to all instances of a particular type, such as a constant property that all instances can use (like a static constant in C), or a variable property that stores a value that is global to all instances of that type (like a static variable in C).

Stored type properties can be variables or constants. Computed type properties are always declared as variable properties, in the same way as computed instance properties.

### Type Property Syntax

In C and Objective-C, you define static constants and variables associated with a type as global static variables. In Swift, however, type properties are written as part of the type’s definition, within the type’s outer curly braces, and each type property is explicitly scoped to the type it supports.

You define type properties with the static keyword. For computed type properties for class types, you can use the class keyword instead to allow subclasses to override the superclass’s implementation. The example below shows the syntax for stored and computed type properties:

1. struct SomeStructure {
2.  static var storedTypeProperty = "Some value."
3.  static var computedTypeProperty: Int {
4.  return 1
5.  }
6. }
7. enum SomeEnumeration {
8.  static var storedTypeProperty = "Some value."
9.  static var computedTypeProperty: Int {
10.  return 6
11.  }
12. }
13. class SomeClass {
14.  static var storedTypeProperty = "Some value."
15.  static var computedTypeProperty: Int {
16.  return 27
17.  }
18.  class var overrideableComputedTypeProperty: Int {
19.  return 107
20.  }
21. }

### Querying and Setting Type Properties

Type properties are queried and set with dot syntax, just like instance properties. However, type properties are queried and set on the type, not on an instance of that type. For example:

1. print(SomeStructure.storedTypeProperty)
2. // prints "Some value."
3. SomeStructure.storedTypeProperty = "Another value."
4. print(SomeStructure.storedTypeProperty)
5. // prints "Another value."
6. print(SomeEnumeration.computedTypeProperty)
7. // prints "6"
8. print(SomeClass.computedTypeProperty)
9. // prints "27"

The examples that follow use two stored type properties as part of a structure that models an audio level meter for a number of audio channels. Each channel has an integer audio level between 0 and 10 inclusive.

The figure below illustrates how two of these audio channels can be combined to model a stereo audio level meter. When a channel’s audio level is 0, none of the lights for that channel are lit. When the audio level is 10, all of the lights for that channel are lit. In this figure, the left channel has a current level of 9, and the right channel has a current level of 7:

The audio channels described above are represented by instances of the AudioChannel structure:

1. struct AudioChannel {
2.  static let thresholdLevel = 10
3.  static var maxInputLevelForAllChannels = 0
4.  var currentLevel: Int = 0 {
5.  didSet {
6.  if currentLevel > AudioChannel.thresholdLevel {
7.  // cap the new audio level to the threshold level
8.  currentLevel = AudioChannel.thresholdLevel
9.  }
10.  if currentLevel > AudioChannel.maxInputLevelForAllChannels {
11.  // store this as the new overall maximum input level
12.  AudioChannel.maxInputLevelForAllChannels = currentLevel
13.  }
14.  }
15.  }
16. }

The AudioChannel structure defines two stored type properties to support its functionality. The first, thresholdLevel, defines the maximum threshold value an audio level can take. This is a constant value of 10 for all AudioChannel instances. If an audio signal comes in with a higher value than 10, it will be capped to this threshold value (as described below).

The second type property is a variable stored property called maxInputLevelForAllChannels. This keeps track of the maximum input value that has been received by any AudioChannel instance. It starts with an initial value of 0.

The AudioChannel structure also defines a stored instance property called currentLevel, which represents the channel’s current audio level on a scale of 0 to 10.

The currentLevel property has a didSet property observer to check the value of currentLevel whenever it is set. This observer performs two checks:

1. If the new value of currentLevel is greater than the allowed thresholdLevel, the property observer caps currentLevel to thresholdLevel.

2. If the new value of currentLevel (after any capping) is higher than any value previously received by any AudioChannel instance, the property observer stores the new currentLevel value in the maxInputLevelForAllChannels type property.

You can use the AudioChannel structure to create two new audio channels called leftChannel and rightChannel, to represent the audio levels of a stereo sound system:

1. var leftChannel = AudioChannel()
2. var rightChannel = AudioChannel()

If you set the currentLevel of the left channel to 7, you can see that the maxInputLevelForAllChannels type property is updated to equal 7:

1. leftChannel.currentLevel = 7
2. print(leftChannel.currentLevel)
3. // prints "7"
4. print(AudioChannel.maxInputLevelForAllChannels)
5. // prints "7"

If you try to set the currentLevel of the right channel to 11, you can see that the right channel’s currentLevel property is capped to the maximum value of 10, and the maxInputLevelForAllChannels type property is updated to equal 10:

1. rightChannel.currentLevel = 11
2. print(rightChannel.currentLevel)
3. // prints "10"
4. print(AudioChannel.maxInputLevelForAllChannels)
5. // prints "10"

# Methods

Methods are functions that are associated with a particular type. Classes, structures, and enumerations can all define instance methods, which encapsulate specific tasks and functionality for working with an instance of a given type. Classes, structures, and enumerations can also define type methods, which are associated with the type itself. Type methods are similar to class methods in Objective-C.

The fact that structures and enumerations can define methods in Swift is a major difference from C and Objective-C. In Objective-C, classes are the only types that can define methods. In Swift, you can choose whether to define a class, structure, or enumeration, and still have the flexibility to define methods on the type you create.

## Instance Methods

Instance methods are functions that belong to instances of a particular class, structure, or enumeration. They support the functionality of those instances, either by providing ways to access and modify instance properties, or by providing functionality related to the instance’s purpose. Instance methods have exactly the same syntax as functions, as described in Functions.

You write an instance method within the opening and closing braces of the type it belongs to. An instance method has implicit access to all other instance methods and properties of that type. An instance method can be called only on a specific instance of the type it belongs to. It cannot be called in isolation without an existing instance.

Here’s an example that defines a simple Counter class, which can be used to count the number of times an action occurs:

1. class Counter {
2.  var count = 0
3.  func increment() {
4.  ++count
5.  }
6.  func incrementBy(amount: Int) {
7.  count += amount
8.  }
9.  func reset() {
10.  count = 0
11.  }
12. }

The Counter class defines three instance methods:

1. increment increments the counter by 1.

2. incrementBy(amount: Int) increments the counter by a specified integer amount.

3. reset resets the counter to zero.

The Counter class also declares a variable property, count, to keep track of the current counter value.

You call instance methods with the same dot syntax as properties:

1. let counter = Counter()
2. // the initial counter value is 0
3. counter.increment()
4. // the counter's value is now 1
5. counter.incrementBy(5)
6. // the counter's value is now 6
7. counter.reset()
8. // the counter's value is now 0

### Local and External Parameter Names for Methods

Function parameters can have both a local name (for use within the function’s body) and an external name (for use when calling the function), as described in Specifying External Parameter Names. The same is true for method parameters, because methods are just functions that are associated with a type.

Methods in Swift are very similar to their counterparts in Objective-C. As in Objective-C, the name of a method in Swift typically refers to the method’s first parameter using a preposition such as with, for, or by, as seen in the incrementBy(_:) method from the preceding Counter class example. The use of a preposition enables the method to be read as a sentence when it is called.

Swift gives the first parameter name in a method a local parameter name by default, and gives the second and subsequent parameter names both local and external parameter names by default. This convention matches the typical naming and calling convention you will be familiar with from writing Objective-C methods, and makes for expressive method calls without the need to qualify your parameter names.

Consider this alternative version of the Counter class, which defines a more complex form of the incrementBy(_:) method:

1. class Counter {
2.  var count: Int = 0
3.  func incrementBy(amount: Int, numberOfTimes: Int) {
4.  count += amount * numberOfTimes
5.  }
6. }

This incrementBy(_:numberOfTimes:) method has two parameters—amount and numberOfTimes. By default, Swift treats amount as a local name only, but treats numberOfTimes as both a local and an external name. You call the method as follows:

1. let counter = Counter()
2. counter.incrementBy(5, numberOfTimes: 3)
3. // counter value is now 15

You don’t need to define an external parameter name for the first argument value, because its purpose is clear from the function name incrementBy(_:numberOfTimes:). The second argument, however, is qualified by an external parameter name to make its purpose clear when the method is called.

The behavior described above means that method definitions in Swift are written with the same grammatical style as Objective-C, and are called in a natural, expressive way.

### Modifying External Parameter Name Behavior for Methods

Sometimes it’s useful to provide an external parameter name for a method’s first parameter, even though this is not the default behavior. To do so, you can add an explicit external name yourself.

Conversely, if you do not want to provide an external name for the second or subsequent parameter of a method, override the default behavior by using an underscore character (_) as an explicit external parameter name for that parameter.

### The self Property

Every instance of a type has an implicit property called self, which is exactly equivalent to the instance itself. You use the self property to refer to the current instance within its own instance methods.

The increment() method in the example above could have been written like this:

1. func increment() {
2.  self.count++
3. }

In practice, you don’t need to write self in your code very often. If you don’t explicitly write self, Swift assumes that you are referring to a property or method of the current instance whenever you use a known property or method name within a method. This assumption is demonstrated by the use of count (rather than self.count) inside the three instance methods for Counter.

The main exception to this rule occurs when a parameter name for an instance method has the same name as a property of that instance. In this situation, the parameter name takes precedence, and it becomes necessary to refer to the property in a more qualified way. You use the self property to distinguish between the parameter name and the property name.

Here, self disambiguates between a method parameter called x and an instance property that is also called x:

1. struct Point {
2.  var x = 0.0, y = 0.0
3.  func isToTheRightOfX(x: Double) -> Bool {
4.  return self.x > x
5.  }
6. }
7. let somePoint = Point(x: 4.0, y: 5.0)
8. if somePoint.isToTheRightOfX(1.0) {
9.  print("This point is to the right of the line where x == 1.0")
10. }
11. // prints "This point is to the right of the line where x == 1.0"

Without the self prefix, Swift would assume that both uses of x referred to the method parameter called x.

### Modifying Value Types from Within Instance Methods

Structures and enumerations are value types. By default, the properties of a value type cannot be modified from within its instance methods.

However, if you need to modify the properties of your structure or enumeration within a particular method, you can opt in to mutating behavior for that method. The method can then mutate (that is, change) its properties from within the method, and any changes that it makes are written back to the original structure when the method ends. The method can also assign a completely new instance to its implicit self property, and this new instance will replace the existing one when the method ends.

You can opt in to this behavior by placing the mutating keyword before the func keyword for that method:

1. struct Point {
2.  var x = 0.0, y = 0.0
3.  mutating func moveByX(deltaX: Double, y deltaY: Double) {
4.  x += deltaX
5.  y += deltaY
6.  }
7. }
8. var somePoint = Point(x: 1.0, y: 1.0)
9. somePoint.moveByX(2.0, y: 3.0)
10. print("The point is now at (\(somePoint.x), \(somePoint.y))")
11. // prints "The point is now at (3.0, 4.0)"

The Point structure above defines a mutating moveByX(_:y:) method, which moves a Point instance by a certain amount. Instead of returning a new point, this method actually modifies the point on which it is called. The mutating keyword is added to its definition to enable it to modify its properties.

Note that you cannot call a mutating method on a constant of structure type, because its properties cannot be changed, even if they are variable properties, as described in Stored Properties of Constant Structure Instances:

1. let fixedPoint = Point(x: 3.0, y: 3.0)
2. fixedPoint.moveByX(2.0, y: 3.0)
3. // this will report an error

### Assigning to self Within a Mutating Method

Mutating methods can assign an entirely new instance to the implicit self property. The Point example shown above could have been written in the following way instead:

1. struct Point {
2.  var x = 0.0, y = 0.0
3.  mutating func moveByX(deltaX: Double, y deltaY: Double) {
4.  self = Point(x: x + deltaX, y: y + deltaY)
5.  }
6. }

This version of the mutating moveByX(_:y:) method creates a brand new structure whose x and y values are set to the target location. The end result of calling this alternative version of the method will be exactly the same as for calling the earlier version.

Mutating methods for enumerations can set the implicit self parameter to be a different member from the same enumeration:

1. enum TriStateSwitch {
2.  case Off, Low, High
3.  mutating func next() {
4.  switch self {
5.  case Off:
6.  self = Low
7.  case Low:
8.  self = High
9.  case High:
10.  self = Off
11.  }
12.  }
13. }
14. var ovenLight = TriStateSwitch.Low
15. ovenLight.next()
16. // ovenLight is now equal to .High
17. ovenLight.next()
18. // ovenLight is now equal to .Off

This example defines an enumeration for a three-state switch. The switch cycles between three different power states (Off, Low and High) every time its next() method is called.

## Type Methods

Instance methods, as described above, are methods that are called on an instance of a particular type. You can also define methods that are called on the type itself. These kinds of methods are called type methods. You indicate type methods by writing the keyword static before the method’s func keyword. Classes may also use the class keyword to allow subclasses to override the superclass’s implementation of that method.

Type methods are called with dot syntax, like instance methods. However, you call type methods on the type, not on an instance of that type. Here’s how you call a type method on a class called SomeClass:

1. class SomeClass {
2.  class func someTypeMethod() {
3.  // type method implementation goes here
4.  }
5. }
6. SomeClass.someTypeMethod()

Within the body of a type method, the implicit self property refers to the type itself, rather than an instance of that type. For structures and enumerations, this means that you can use self to disambiguate between type properties and type method parameters, just as you do for instance properties and instance method parameters.

More generally, any unqualified method and property names that you use within the body of a type method will refer to other type-level methods and properties. A type method can call another type method with the other method’s name, without needing to prefix it with the type name. Similarly, type methods on structures and enumerations can access type properties by using the type property’s name without a type name prefix.

The example below defines a structure called LevelTracker, which tracks a player’s progress through the different levels or stages of a game. It is a single-player game, but can store information for multiple players on a single device.

All of the game’s levels (apart from level one) are locked when the game is first played. Every time a player finishes a level, that level is unlocked for all players on the device. The LevelTracker structure uses type properties and methods to keep track of which levels of the game have been unlocked. It also tracks the current level for an individual player.

1. struct LevelTracker {
2.  static var highestUnlockedLevel = 1
3.  static func unlockLevel(level: Int) {
4.  if level > highestUnlockedLevel { highestUnlockedLevel = level }
5.  }
6.  static func levelIsUnlocked(level: Int) -> Bool {
7.  return level <= highestUnlockedLevel
8.  }
9.  var currentLevel = 1
10.  mutating func advanceToLevel(level: Int) -> Bool {
11.  if LevelTracker.levelIsUnlocked(level) {
12.  currentLevel = level
13.  return true
14.  } else {
15.  return false
16.  }
17.  }
18. }

The LevelTracker structure keeps track of the highest level that any player has unlocked. This value is stored in a type property called highestUnlockedLevel.

LevelTracker also defines two type functions to work with the highestUnlockedLevel property. The first is a type function called unlockLevel, which updates the value of highestUnlockedLevel whenever a new level is unlocked. The second is a convenience type function called levelIsUnlocked, which returns true if a particular level number is already unlocked. (Note that these type methods can access the highestUnlockedLevel type property without your needing to write it as LevelTracker.highestUnlockedLevel.)

In addition to its type property and type methods, LevelTracker tracks an individual player’s progress through the game. It uses an instance property called currentLevel to track the level that a player is currently playing.

To help manage the currentLevel property, LevelTracker defines an instance method called advanceToLevel. Before updating currentLevel, this method checks whether the requested new level is already unlocked. The advanceToLevel(_:) method returns a Boolean value to indicate whether or not it was actually able to set currentLevel.

The LevelTracker structure is used with the Player class, shown below, to track and update the progress of an individual player:

1. class Player {
2.  var tracker = LevelTracker()
3.  let playerName: String
4.  func completedLevel(level: Int) {
5.  LevelTracker.unlockLevel(level + 1)
6.  tracker.advanceToLevel(level + 1)
7.  }
8.  init(name: String) {
9.  playerName = name
10.  }
11. }

The Player class creates a new instance of LevelTracker to track that player’s progress. It also provides a method called completedLevel, which is called whenever a player completes a particular level. This method unlocks the next level for all players and updates the player’s progress to move them to the next level. (The Boolean return value of advanceToLevel is ignored, because the level is known to have been unlocked by the call to LevelTracker.unlockLevel on the previous line.)

You can create an instance of the Player class for a new player, and see what happens when the player completes level one:

1. var player = Player(name: "Argyrios")
2. player.completedLevel(1)
3. print("highest unlocked level is now \(LevelTracker.highestUnlockedLevel)")
4. // prints "highest unlocked level is now 2"

If you create a second player, whom you try to move to a level that is not yet unlocked by any player in the game, the attempt to set the player’s current level fails:

1. player = Player(name: "Beto")
2. if player.tracker.advanceToLevel(6) {
3.  print("player is now on level 6")
4. } else {
5.  print("level 6 has not yet been unlocked")
6. }
7. // prints "level 6 has not yet been unlocked"

# Subscripts

Classes, structures, and enumerations can define subscripts, which are shortcuts for accessing the member elements of a collection, list, or sequence. You use subscripts to set and retrieve values by index without needing separate methods for setting and retrieval. For example, you access elements in an Array instance as someArray[index] and elements in a Dictionary instance as someDictionary[key].

You can define multiple subscripts for a single type, and the appropriate subscript overload to use is selected based on the type of index value you pass to the subscript. Subscripts are not limited to a single dimension, and you can define subscripts with multiple input parameters to suit your custom type’s needs.

## Subscript Syntax

Subscripts enable you to query instances of a type by writing one or more values in square brackets after the instance name. Their syntax is similar to both instance method syntax and computed property syntax. You write subscript definitions with the subscript keyword, and specify one or more input parameters and a return type, in the same way as instance methods. Unlike instance methods, subscripts can be read-write or read-only. This behavior is communicated by a getter and setter in the same way as for computed properties:

1. subscript(index: Int) -> Int {
2.  get {
3.  // return an appropriate subscript value here
4.  }
5.  set(newValue) {
6.  // perform a suitable setting action here
7.  }
8. }

The type of newValue is the same as the return value of the subscript. As with computed properties, you can choose not to specify the setter’s (newValue) parameter. A default parameter called newValue is provided to your setter if you do not provide one yourself.

As with read-only computed properties, you can drop the get keyword for read-only subscripts:

1. subscript(index: Int) -> Int {
2.  // return an appropriate subscript value here
3. }

Here’s an example of a read-only subscript implementation, which defines a TimesTable structure to represent an n-times-table of integers:

1. struct TimesTable {
2.  let multiplier: Int
3.  subscript(index: Int) -> Int {
4.  return multiplier * index
5.  }
6. }
7. let threeTimesTable = TimesTable(multiplier: 3)
8. print("six times three is \(threeTimesTable[6])")
9. // prints "six times three is 18"

In this example, a new instance of TimesTable is created to represent the three-times-table. This is indicated by passing a value of 3 to the structure’s initializer as the value to use for the instance’s multiplier parameter.

You can query the threeTimesTable instance by calling its subscript, as shown in the call to threeTimesTable[6]. This requests the sixth entry in the three-times-table, which returns a value of 18, or 3 times 6.

## Subscript Usage

The exact meaning of “subscript” depends on the context in which it is used. Subscripts are typically used as a shortcut for accessing the member elements in a collection, list, or sequence. You are free to implement subscripts in the most appropriate way for your particular class or structure’s functionality.

For example, Swift’s Dictionary type implements a subscript to set and retrieve the values stored in a Dictionary instance. You can set a value in a dictionary by providing a key of the dictionary’s key type within subscript braces, and assigning a value of the dictionary’s value type to the subscript:

1. var numberOfLegs = ["spider": 8, "ant": 6, "cat": 4]
2. numberOfLegs["bird"] = 2

The example above defines a variable called numberOfLegs and initializes it with a dictionary literal containing three key-value pairs. The type of the numberOfLegs dictionary is inferred to be [String: Int]. After creating the dictionary, this example uses subscript assignment to add a String key of "bird" and an Int value of 2 to the dictionary.

For more information about Dictionary subscripting, see Accessing and Modifying a Dictionary.

## Subscript Options

Subscripts can take any number of input parameters, and these input parameters can be of any type. Subscripts can also return any type. Subscripts can use variable parameters and variadic parameters, but cannot use in-out parameters or provide default parameter values.

A class or structure can provide as many subscript implementations as it needs, and the appropriate subscript to be used will be inferred based on the types of the value or values that are contained within the subscript braces at the point that the subscript is used. This definition of multiple subscripts is known as subscript overloading.

While it is most common for a subscript to take a single parameter, you can also define a subscript with multiple parameters if it is appropriate for your type. The following example defines a Matrix structure, which represents a two-dimensional matrix of Double values. The Matrix structure’s subscript takes two integer parameters:

1. struct Matrix {
2.  let rows: Int, columns: Int
3.  var grid: [Double]
4.  init(rows: Int, columns: Int) {
5.  self.rows = rows
6.  self.columns = columns
7.  grid = Array(count: rows * columns, repeatedValue: 0.0)
8.  }
9.  func indexIsValidForRow(row: Int, column: Int) -> Bool {
10.  return row >= 0 && row < rows && column >= 0 && column < columns
11.  }
12.  subscript(row: Int, column: Int) -> Double {
13.  get {
14.  assert(indexIsValidForRow(row, column: column), "Index out of range")
15.  return grid[(row * columns) + column]
16.  }
17.  set {
18.  assert(indexIsValidForRow(row, column: column), "Index out of range")
19.  grid[(row * columns) + column] = newValue
20.  }
21.  }
22. }

Matrix provides an initializer that takes two parameters called rows and columns, and creates an array that is large enough to store rows * columns values of type Double. Each position in the matrix is given an initial value of 0.0. To achieve this, the array’s size, and an initial cell value of 0.0, are passed to an array initializer that creates and initializes a new array of the correct size. This initializer is described in more detail in Creating an Empty Array.

You can construct a new Matrix instance by passing an appropriate row and column count to its initializer:

1. var matrix = Matrix(rows: 2, columns: 2)

The preceding example creates a new Matrix instance with two rows and two columns. The grid array for this Matrix instance is effectively a flattened version of the matrix, as read from top left to bottom right:

Values in the matrix can be set by passing row and column values into the subscript, separated by a comma:

1. matrix[0, 1] = 1.5
2. matrix[1, 0] = 3.2

These two statements call the subscript’s setter to set a value of 1.5 in the top right position of the matrix (where row is 0 and column is 1), and 3.2 in the bottom left position (where row is 1 and column is 0):

The Matrix subscript’s getter and setter both contain an assertion to check that the subscript’s row and column values are valid. To assist with these assertions, Matrix includes a convenience method called indexIsValidForRow(_:column:), which checks whether the requested row and column are inside the bounds of the matrix:

1. func indexIsValidForRow(row: Int, column: Int) -> Bool {
2.  return row >= 0 && row < rows && column >= 0 && column < columns
3. }

An assertion is triggered if you try to access a subscript that is outside of the matrix bounds:

1. let someValue = matrix[2, 2]
2. // this triggers an assert, because [2, 2] is outside of the matrix bounds

# Inheritance

A class can inherit methods, properties, and other characteristics from another class. When one class inherits from another, the inheriting class is known as a subclass, and the class it inherits from is known as its superclass. Inheritance is a fundamental behavior that differentiates classes from other types in Swift.

Classes in Swift can call and access methods, properties, and subscripts belonging to their superclass and can provide their own overriding versions of those methods, properties, and subscripts to refine or modify their behavior. Swift helps to ensure your overrides are correct by checking that the override definition has a matching superclass definition.

Classes can also add property observers to inherited properties in order to be notified when the value of a property changes. Property observers can be added to any property, regardless of whether it was originally defined as a stored or computed property.

## Defining a Base Class

Any class that does not inherit from another class is known as a base class.

The example below defines a base class called Vehicle. This base class defines a stored property called currentSpeed, with a default value of 0.0 (inferring a property type of Double). The currentSpeed property’s value is used by a read-only computed String property called description to create a description of the vehicle.

The Vehicle base class also defines a method called makeNoise. This method does not actually do anything for a base Vehicle instance, but will be customized by subclasses of Vehicle later on:

1. class Vehicle {
2.  var currentSpeed = 0.0
3.  var description: String {
4.  return "traveling at \(currentSpeed) miles per hour"
5.  }
6.  func makeNoise() {
7.  // do nothing - an arbitrary vehicle doesn't necessarily make a noise
8.  }
9. }

You create a new instance of Vehicle with initializer syntax, which is written as a TypeName followed by empty parentheses:

1. let someVehicle = Vehicle()

Having created a new Vehicle instance, you can access its description property to print a human-readable description of the vehicle’s current speed:

1. print("Vehicle: \(someVehicle.description)")
2. // Vehicle: traveling at 0.0 miles per hour

The Vehicle class defines common characteristics for an arbitrary vehicle, but is not much use in itself. To make it more useful, you need to refine it to describe more specific kinds of vehicle.

## Subclassing

Subclassing is the act of basing a new class on an existing class. The subclass inherits characteristics from the existing class, which you can then refine. You can also add new characteristics to the subclass.

To indicate that a subclass has a superclass, write the subclass name before the superclass name, separated by a colon:

1. class SomeSubclass: SomeSuperclass {
2.  // subclass definition goes here
3. }

The following example defines a subclass called Bicycle, with a superclass of Vehicle:

1. class Bicycle: Vehicle {
2.  var hasBasket = false
3. }

The new Bicycle class automatically gains all of the characteristics of Vehicle, such as its currentSpeed and description properties and its makeNoise() method.

In addition to the characteristics it inherits, the Bicycle class defines a new stored property, hasBasket, with a default value of false (inferring a type of Bool for the property).

By default, any new Bicycle instance you create will not have a basket. You can set the hasBasket property to true for a particular Bicycle instance after that instance is created:

1. let bicycle = Bicycle()
2. bicycle.hasBasket = true

You can also modify the inherited currentSpeed property of a Bicycle instance, and query the instance’s inherited description property:

1. bicycle.currentSpeed = 15.0
2. print("Bicycle: \(bicycle.description)")
3. // Bicycle: traveling at 15.0 miles per hour

Subclasses can themselves be subclassed. The next example creates a subclass of Bicycle for a two-seater bicycle known as a “tandem”:

1. class Tandem: Bicycle {
2.  var currentNumberOfPassengers = 0
3. }

Tandem inherits all of the properties and methods from Bicycle, which in turn inherits all of the properties and methods from Vehicle. The Tandem subclass also adds a new stored property called currentNumberOfPassengers, with a default value of 0.

If you create an instance of Tandem, you can work with any of its new and inherited properties, and query the read-only description property it inherits from Vehicle:

1. let tandem = Tandem()
2. tandem.hasBasket = true
3. tandem.currentNumberOfPassengers = 2
4. tandem.currentSpeed = 22.0
5. print("Tandem: \(tandem.description)")
6. // Tandem: traveling at 22.0 miles per hour

## Overriding

A subclass can provide its own custom implementation of an instance method, type method, instance property, type property, or subscript that it would otherwise inherit from a superclass. This is known as overriding.

To override a characteristic that would otherwise be inherited, you prefix your overriding definition with the override keyword. Doing so clarifies that you intend to provide an override and have not provided a matching definition by mistake. Overriding by accident can cause unexpected behavior, and any overrides without the override keyword are diagnosed as an error when your code is compiled.

The override keyword also prompts the Swift compiler to check that your overriding class’s superclass (or one of its parents) has a declaration that matches the one you provided for the override. This check ensures that your overriding definition is correct.

### Accessing Superclass Methods, Properties, and Subscripts

When you provide a method, property, or subscript override for a subclass, it is sometimes useful to use the existing superclass implementation as part of your override. For example, you can refine the behavior of that existing implementation, or store a modified value in an existing inherited variable.

Where this is appropriate, you access the superclass version of a method, property, or subscript by using the super prefix:

1. An overridden method named someMethod() can call the superclass version of someMethod() by calling super.someMethod() within the overriding method implementation.

2. An overridden property called someProperty can access the superclass version of someProperty as super.someProperty within the overriding getter or setter implementation.

3. An overridden subscript for someIndex can access the superclass version of the same subscript as super[someIndex] from within the overriding subscript implementation.

### Overriding Methods

You can override an inherited instance or type method to provide a tailored or alternative implementation of the method within your subclass.

The following example defines a new subclass of Vehicle called Train, which overrides the makeNoise() method that Train inherits from Vehicle:

1. class Train: Vehicle {
2.  override func makeNoise() {
3.  print("Choo Choo")
4.  }
5. }

If you create a new instance of Train and call its makeNoise() method, you can see that the Train subclass version of the method is called:

1. let train = Train()
2. train.makeNoise()
3. // prints "Choo Choo"

### Overriding Properties

You can override an inherited instance or type property to provide your own custom getter and setter for that property, or to add property observers to enable the overriding property to observe when the underlying property value changes.

### Overriding Property Getters and Setters

You can provide a custom getter (and setter, if appropriate) to override any inherited property, regardless of whether the inherited property is implemented as a stored or computed property at source. The stored or computed nature of an inherited property is not known by a subclass—it only knows that the inherited property has a certain name and type. You must always state both the name and the type of the property you are overriding, to enable the compiler to check that your override matches a superclass property with the same name and type.

You can present an inherited read-only property as a read-write property by providing both a getter and a setter in your subclass property override. You cannot, however, present an inherited read-write property as a read-only property.

The following example defines a new class called Car, which is a subclass of Vehicle. The Car class introduces a new stored property called gear, with a default integer value of 1. The Car class also overrides the description property it inherits from Vehicle, to provide a custom description that includes the current gear:

1. class Car: Vehicle {
2.  var gear = 1
3.  override var description: String {
4.  return super.description + " in gear \(gear)"
5.  }
6. }

The override of the description property starts by calling super.description, which returns the Vehicle class’s description property. The Car class’s version of description then adds some extra text onto the end of this description to provide information about the current gear.

If you create an instance of the Car class and set its gear and currentSpeed properties, you can see that its description property returns the tailored description defined within the Car class:

1. let car = Car()
2. car.currentSpeed = 25.0
3. car.gear = 3
4. print("Car: \(car.description)")
5. // Car: traveling at 25.0 miles per hour in gear 3

### Overriding Property Observers

You can use property overriding to add property observers to an inherited property. This enables you to be notified when the value of an inherited property changes, regardless of how that property was originally implemented. For more information on property observers, see Property Observers.

The following example defines a new class called AutomaticCar, which is a subclass of Car. The AutomaticCar class represents a car with an automatic gearbox, which automatically selects an appropriate gear to use based on the current speed:

1. class AutomaticCar: Car {
2.  override var currentSpeed: Double {
3.  didSet {
4.  gear = Int(currentSpeed / 10.0) + 1
5.  }
6.  }
7. }

Whenever you set the currentSpeed property of an AutomaticCar instance, the property’s didSet observer sets the instance’s gear property to an appropriate choice of gear for the new speed. Specifically, the property observer chooses a gear that is the new currentSpeed value divided by 10, rounded down to the nearest integer, plus 1. A speed of 35.0 produces a gear of 4:

1. let automatic = AutomaticCar()
2. automatic.currentSpeed = 35.0
3. print("AutomaticCar: \(automatic.description)")
4. // AutomaticCar: traveling at 35.0 miles per hour in gear 4

## Preventing Overrides

You can prevent a method, property, or subscript from being overridden by marking it as final. Do this by writing the final modifier before the method, property, or subscript’s introducer keyword (such as final var, final func, final class func, and final subscript).

Any attempt to override a final method, property, or subscript in a subclass is reported as a compile-time error. Methods, properties, or subscripts that you add to a class in an extension can also be marked as final within the extension’s definition.

You can mark an entire class as final by writing the final modifier before the class keyword in its class definition (final class). Any attempt to subclass a final class is reported as a compile-time error.

# Initialization

Initialization is the process of preparing an instance of a class, structure, or enumeration for use. This process involves setting an initial value for each stored property on that instance and performing any other setup or initialization that is required before the new instance is ready for use.

You implement this initialization process by defining initializers, which are like special methods that can be called to create a new instance of a particular type. Unlike Objective-C initializers, Swift initializers do not return a value. Their primary role is to ensure that new instances of a type are correctly initialized before they are used for the first time.

Instances of class types can also implement a deinitializer, which performs any custom cleanup just before an instance of that class is deallocated. For more information about deinitializers, see Deinitialization.

## Setting Initial Values for Stored Properties

Classes and structures must set all of their stored properties to an appropriate initial value by the time an instance of that class or structure is created. Stored properties cannot be left in an indeterminate state.

You can set an initial value for a stored property within an initializer, or by assigning a default property value as part of the property’s definition. These actions are described in the following sections.

### Initializers

Initializers are called to create a new instance of a particular type. In its simplest form, an initializer is like an instance method with no parameters, written using the init keyword:

1. init() {
2.  // perform some initialization here
3. }

The example below defines a new structure called Fahrenheit to store temperatures expressed in the Fahrenheit scale. The Fahrenheit structure has one stored property, temperature, which is of type Double:

1. struct Fahrenheit {
2.  var temperature: Double
3.  init() {
4.  temperature = 32.0
5.  }
6. }
7. var f = Fahrenheit()
8. print("The default temperature is \(f.temperature)° Fahrenheit")
9. // prints "The default temperature is 32.0° Fahrenheit"

The structure defines a single initializer, init, with no parameters, which initializes the stored temperature with a value of 32.0 (the freezing point of water when expressed in the Fahrenheit scale).

### Default Property Values

You can set the initial value of a stored property from within an initializer, as shown above. Alternatively, specify a default property value as part of the property’s declaration. You specify a default property value by assigning an initial value to the property when it is defined.

You can write the Fahrenheit structure from above in a simpler form by providing a default value for its temperature property at the point that the property is declared:

1. struct Fahrenheit {
2.  var temperature = 32.0
3. }

## Customizing Initialization

You can customize the initialization process with input parameters and optional property types, or by assigning constant properties during initialization, as described in the following sections.

### Initialization Parameters

You can provide initialization parameters as part of an initializer’s definition, to define the types and names of values that customize the initialization process. Initialization parameters have the same capabilities and syntax as function and method parameters.

The following example defines a structure called Celsius, which stores temperatures expressed in the Celsius scale. The Celsius structure implements two custom initializers called init(fromFahrenheit:) and init(fromKelvin:), which initialize a new instance of the structure with a value from a different temperature scale:

1. struct Celsius {
2.  var temperatureInCelsius: Double
3.  init(fromFahrenheit fahrenheit: Double) {
4.  temperatureInCelsius = (fahrenheit - 32.0) / 1.8
5.  }
6.  init(fromKelvin kelvin: Double) {
7.  temperatureInCelsius = kelvin - 273.15
8.  }
9. }
10. let boilingPointOfWater = Celsius(fromFahrenheit: 212.0)
11. // boilingPointOfWater.temperatureInCelsius is 100.0
12. let freezingPointOfWater = Celsius(fromKelvin: 273.15)
13. // freezingPointOfWater.temperatureInCelsius is 0.0

The first initializer has a single initialization parameter with an external name of fromFahrenheit and a local name of fahrenheit. The second initializer has a single initialization parameter with an external name of fromKelvin and a local name of kelvin. Both initializers convert their single argument into a value in the Celsius scale and store this value in a property called temperatureInCelsius.

### Local and External Parameter Names

As with function and method parameters, initialization parameters can have both a local name for use within the initializer’s body and an external name for use when calling the initializer.

However, initializers do not have an identifying function name before their parentheses in the way that functions and methods do. Therefore, the names and types of an initializer’s parameters play a particularly important role in identifying which initializer should be called. Because of this, Swift provides an automatic external name for every parameter in an initializer if you don’t provide an external name yourself.

The following example defines a structure called Color, with three constant properties called red, green, and blue. These properties store a value between 0.0 and 1.0 to indicate the amount of red, green, and blue in the color.

Color provides an initializer with three appropriately named parameters of type Double for its red, green, and blue components. Color also provides a second initializer with a single white parameter, which is used to provide the same value for all three color components.

1. struct Color {
2.  let red, green, blue: Double
3.  init(red: Double, green: Double, blue: Double) {
4.  self.red = red
5.  self.green = green
6.  self.blue = blue
7.  }
8.  init(white: Double) {
9.  red = white
10.  green = white
11.  blue = white
12.  }
13. }

Both initializers can be used to create a new Color instance, by providing named values for each initializer parameter:

1. let magenta = Color(red: 1.0, green: 0.0, blue: 1.0)
2. let halfGray = Color(white: 0.5)

Note that it is not possible to call these initializers without using external parameter names. External names must always be used in an initializer if they are defined, and omitting them is a compile-time error:

1. let veryGreen = Color(0.0, 1.0, 0.0)
2. // this reports a compile-time error - external names are required

### Initializer Parameters Without External Names

If you do not want to use an external name for an initializer parameter, write an underscore (_) instead of an explicit external name for that parameter to override the default behavior.

Here’s an expanded version of the Celsius example from earlier, with an additional initializer to create a new Celsius instance from a Double value that is already in the Celsius scale:

1. struct Celsius {
2.  var temperatureInCelsius: Double
3.  init(fromFahrenheit fahrenheit: Double) {
4.  temperatureInCelsius = (fahrenheit - 32.0) / 1.8
5.  }
6.  init(fromKelvin kelvin: Double) {
7.  temperatureInCelsius = kelvin - 273.15
8.  }
9.  init(_ celsius: Double) {
10.  temperatureInCelsius = celsius
11.  }
12. }
13. let bodyTemperature = Celsius(37.0)
14. // bodyTemperature.temperatureInCelsius is 37.0

The initializer call Celsius(37.0) is clear in its intent without the need for an external parameter name. It is therefore appropriate to write this initializer as init(_ celsius: Double) so that it can be called by providing an unnamed Double value.

### Optional Property Types

If your custom type has a stored property that is logically allowed to have “no value”—perhaps because its value cannot be set during initialization, or because it is allowed to have “no value” at some later point—declare the property with an optional type. Properties of optional type are automatically initialized with a value of nil, indicating that the property is deliberately intended to have “no value yet” during initialization.

The following example defines a class called SurveyQuestion, with an optional String property called response:

1. class SurveyQuestion {
2.  var text: String
3.  var response: String?
4.  init(text: String) {
5.  self.text = text
6.  }
7.  func ask() {
8.  print(text)
9.  }
10. }
11. let cheeseQuestion = SurveyQuestion(text: "Do you like cheese?")
12. cheeseQuestion.ask()
13. // prints "Do you like cheese?"
14. cheeseQuestion.response = "Yes, I do like cheese."

The response to a survey question cannot be known until it is asked, and so the response property is declared with a type of String?, or “optional String”. It is automatically assigned a default value of nil, meaning “no string yet”, when a new instance of SurveyQuestion is initialized.

### Assigning Constant Properties During Initialization

You can assign a value to a constant property at any point during initialization, as long as it is set to a definite value by the time initialization finishes. Once a constant property is assigned a value, it can’t be further modified.

You can revise the SurveyQuestion example from above to use a constant property rather than a variable property for the text property of the question, to indicate that the question does not change once an instance of SurveyQuestion is created. Even though the text property is now a constant, it can still be set within the class’s initializer:

1. class SurveyQuestion {
2.  let text: String
3.  var response: String?
4.  init(text: String) {
5.  self.text = text
6.  }
7.  func ask() {
8.  print(text)
9.  }
10. }
11. let beetsQuestion = SurveyQuestion(text: "How about beets?")
12. beetsQuestion.ask()
13. // prints "How about beets?"
14. beetsQuestion.response = "I also like beets. (But not with cheese.)"

## Default Initializers

Swift provides a default initializer for any structure or class that provides default values for all of its properties and does not provide at least one initializer itself. The default initializer simply creates a new instance with all of its properties set to their default values.

This example defines a class called ShoppingListItem, which encapsulates the name, quantity, and purchase state of an item in a shopping list:

1. class ShoppingListItem {
2.  var name: String?
3.  var quantity = 1
4.  var purchased = false
5. }
6. var item = ShoppingListItem()

Because all properties of the ShoppingListItem class have default values, and because it is a base class with no superclass, ShoppingListItem automatically gains a default initializer implementation that creates a new instance with all of its properties set to their default values. (The name property is an optional String property, and so it automatically receives a default value of nil, even though this value is not written in the code.) The example above uses the default initializer for the ShoppingListItem class to create a new instance of the class with initializer syntax, written as ShoppingListItem(), and assigns this new instance to a variable called item.

### Memberwise Initializers for Structure Types

Structure types automatically receive a memberwise initializer if they do not define any of their own custom initializers. Unlike a default initializer, the structure receives a memberwise initializer even if it has stored properties that do not have default values.

The memberwise initializer is a shorthand way to initialize the member properties of new structure instances. Initial values for the properties of the new instance can be passed to the memberwise initializer by name.

The example below defines a structure called Size with two properties called width and height. Both properties are inferred to be of type Double by assigning a default value of 0.0.

The Size structure automatically receives an init(width:height:) memberwise initializer, which you can use to initialize a new Size instance:

1. struct Size {
2.  var width = 0.0, height = 0.0
3. }
4. let twoByTwo = Size(width: 2.0, height: 2.0)

## Initializer Delegation for Value Types

Initializers can call other initializers to perform part of an instance’s initialization. This process, known as initializer delegation, avoids duplicating code across multiple initializers.

The rules for how initializer delegation works, and for what forms of delegation are allowed, are different for value types and class types. Value types (structures and enumerations) do not support inheritance, and so their initializer delegation process is relatively simple, because they can only delegate to another initializer that they provide themselves. Classes, however, can inherit from other classes, as described in Inheritance. This means that classes have additional responsibilities for ensuring that all stored properties they inherit are assigned a suitable value during initialization. These responsibilities are described in Class Inheritance and Initialization below.

For value types, you use self.init to refer to other initializers from the same value type when writing your own custom initializers. You can only call self.init from within an initializer.

Note that if you define a custom initializer for a value type, you will no longer have access to the default initializer (or the memberwise initializer, if it is a structure) for that type. This constraint prevents a situation in which additional essential setup provided in a more complex initializer is circumvented by someone accidentally using one of the automatic initializers instead.

The following example defines a custom Rect structure to represent a geometric rectangle. The example requires two supporting structures called Size and Point, both of which provide default values of 0.0 for all of their properties:

1. struct Size {
2.  var width = 0.0, height = 0.0
3. }
4. struct Point {
5.  var x = 0.0, y = 0.0
6. }

You can initialize the Rect structure below in one of three ways—by using its default zero-initialized origin and size property values, by providing a specific origin point and size, or by providing a specific center point and size. These initialization options are represented by three custom initializers that are part of the Rect structure’s definition:

1. struct Rect {
2.  var origin = Point()
3.  var size = Size()
4.  init() {}
5.  init(origin: Point, size: Size) {
6.  self.origin = origin
7.  self.size = size
8.  }
9.  init(center: Point, size: Size) {
10.  let originX = center.x - (size.width / 2)
11.  let originY = center.y - (size.height / 2)
12.  self.init(origin: Point(x: originX, y: originY), size: size)
13.  }
14. }

The first Rect initializer, init(), is functionally the same as the default initializer that the structure would have received if it did not have its own custom initializers. This initializer has an empty body, represented by an empty pair of curly braces {}, and does not perform any initialization. Calling this initializer returns a Rect instance whose origin and size properties are both initialized with the default values of Point(x: 0.0, y: 0.0) and Size(width: 0.0, height: 0.0) from their property definitions:

1. let basicRect = Rect()
2. // basicRect's origin is (0.0, 0.0) and its size is (0.0, 0.0)

The second Rect initializer, init(origin:size:), is functionally the same as the memberwise initializer that the structure would have received if it did not have its own custom initializers. This initializer simply assigns the origin and size argument values to the appropriate stored properties:

1. let originRect = Rect(origin: Point(x: 2.0, y: 2.0),
2.  size: Size(width: 5.0, height: 5.0))
3. // originRect's origin is (2.0, 2.0) and its size is (5.0, 5.0)

The third Rect initializer, init(center:size:), is slightly more complex. It starts by calculating an appropriate origin point based on a center point and a size value. It then calls (or delegates) to the init(origin:size:) initializer, which stores the new origin and size values in the appropriate properties:

1. let centerRect = Rect(center: Point(x: 4.0, y: 4.0),
2.  size: Size(width: 3.0, height: 3.0))
3. // centerRect's origin is (2.5, 2.5) and its size is (3.0, 3.0)

The init(center:size:) initializer could have assigned the new values of origin and size to the appropriate properties itself. However, it is more convenient (and clearer in intent) for the init(center:size:) initializer to take advantage of an existing initializer that already provides exactly that functionality.

## Class Inheritance and Initialization

All of a class’s stored properties—including any properties the class inherits from its superclass—must be assigned an initial value during initialization.

Swift defines two kinds of initializers for class types to help ensure all stored properties receive an initial value. These are known as designated initializers and convenience initializers.

### Designated Initializers and Convenience Initializers

Designated initializers are the primary initializers for a class. A designated initializer fully initializes all properties introduced by that class and calls an appropriate superclass initializer to continue the initialization process up the superclass chain.

Classes tend to have very few designated initializers, and it is quite common for a class to have only one. Designated initializers are “funnel” points through which initialization takes place, and through which the initialization process continues up the superclass chain.

Every class must have at least one designated initializer. In some cases, this requirement is satisfied by inheriting one or more designated initializers from a superclass, as described in Automatic Initializer Inheritance below.

Convenience initializers are secondary, supporting initializers for a class. You can define a convenience initializer to call a designated initializer from the same class as the convenience initializer with some of the designated initializer’s parameters set to default values. You can also define a convenience initializer to create an instance of that class for a specific use case or input value type.

You do not have to provide convenience initializers if your class does not require them. Create convenience initializers whenever a shortcut to a common initialization pattern will save time or make initialization of the class clearer in intent.

### Syntax for Designated and Convenience Initializers

Designated initializers for classes are written in the same way as simple initializers for value types:

1. init(parameters) {
2.     statements
3. }

Convenience initializers are written in the same style, but with the convenience modifier placed before the init keyword, separated by a space:

1. convenience init(parameters) {
2.     statements
3. }

### Initializer Delegation for Class Types

To simplify the relationships between designated and convenience initializers, Swift applies the following three rules for delegation calls between initializers:

Rule 1

A designated initializer must call a designated initializer from its immediate superclass.

Rule 2

A convenience initializer must call another initializer from the same class.

Rule 3

A convenience initializer must ultimately call a designated initializer.

A simple way to remember this is:

1. Designated initializers must always delegate up.

2. Convenience initializers must always delegate across.

These rules are illustrated in the figure below:

Here, the superclass has a single designated initializer and two convenience initializers. One convenience initializer calls another convenience initializer, which in turn calls the single designated initializer. This satisfies rules 2 and 3 from above. The superclass does not itself have a further superclass, and so rule 1 does not apply.

The subclass in this figure has two designated initializers and one convenience initializer. The convenience initializer must call one of the two designated initializers, because it can only call another initializer from the same class. This satisfies rules 2 and 3 from above. Both designated initializers must call the single designated initializer from the superclass, to satisfy rule 1 from above.

The figure below shows a more complex class hierarchy for four classes. It illustrates how the designated initializers in this hierarchy act as “funnel” points for class initialization, simplifying the interrelationships among classes in the chain:

### Two-Phase Initialization

Class initialization in Swift is a two-phase process. In the first phase, each stored property is assigned an initial value by the class that introduced it. Once the initial state for every stored property has been determined, the second phase begins, and each class is given the opportunity to customize its stored properties further before the new instance is considered ready for use.

The use of a two-phase initialization process makes initialization safe, while still giving complete flexibility to each class in a class hierarchy. Two-phase initialization prevents property values from being accessed before they are initialized, and prevents property values from being set to a different value by another initializer unexpectedly.

Swift’s compiler performs four helpful safety-checks to make sure that two-phase initialization is completed without error:

Safety check 1

A designated initializer must ensure that all of the properties introduced by its class are initialized before it delegates up to a superclass initializer.

As mentioned above, the memory for an object is only considered fully initialized once the initial state of all of its stored properties is known. In order for this rule to be satisfied, a designated initializer must make sure that all its own properties are initialized before it hands off up the chain.

Safety check 2

A designated initializer must delegate up to a superclass initializer before assigning a value to an inherited property. If it doesn’t, the new value the designated initializer assigns will be overwritten by the superclass as part of its own initialization.

Safety check 3

A convenience initializer must delegate to another initializer before assigning a value to any property (including properties defined by the same class). If it doesn’t, the new value the convenience initializer assigns will be overwritten by its own class’s designated initializer.

Safety check 4

An initializer cannot call any instance methods, read the values of any instance properties, or refer to self as a value until after the first phase of initialization is complete.

The class instance is not fully valid until the first phase ends. Properties can only be accessed, and methods can only be called, once the class instance is known to be valid at the end of the first phase.

Here’s how two-phase initialization plays out, based on the four safety checks above:

Phase 1

1. A designated or convenience initializer is called on a class.

2. Memory for a new instance of that class is allocated. The memory is not yet initialized.

3. A designated initializer for that class confirms that all stored properties introduced by that class have a value. The memory for these stored properties is now initialized.

4. The designated initializer hands off to a superclass initializer to perform the same task for its own stored properties.

5. This continues up the class inheritance chain until the top of the chain is reached.

6. Once the top of the chain is reached, and the final class in the chain has ensured that all of its stored properties have a value, the instance’s memory is considered to be fully initialized, and phase 1 is complete.

Phase 2

1. Working back down from the top of the chain, each designated initializer in the chain has the option to customize the instance further. Initializers are now able to access self and can modify its properties, call its instance methods, and so on.

2. Finally, any convenience initializers in the chain have the option to customize the instance and to work with self.

Here’s how phase 1 looks for an initialization call for a hypothetical subclass and superclass:

In this example, initialization begins with a call to a convenience initializer on the subclass. This convenience initializer cannot yet modify any properties. It delegates across to a designated initializer from the same class.

The designated initializer makes sure that all of the subclass’s properties have a value, as per safety check 1. It then calls a designated initializer on its superclass to continue the initialization up the chain.

The superclass’s designated initializer makes sure that all of the superclass properties have a value. There are no further superclasses to initialize, and so no further delegation is needed.

As soon as all properties of the superclass have an initial value, its memory is considered fully initialized, and Phase 1 is complete.

Here’s how phase 2 looks for the same initialization call:

The superclass’s designated initializer now has an opportunity to customize the instance further (although it does not have to).

Once the superclass’s designated initializer is finished, the subclass’s designated initializer can perform additional customization (although again, it does not have to).

Finally, once the subclass’s designated initializer is finished, the convenience initializer that was originally called can perform additional customization.

### Initializer Inheritance and Overriding

Unlike subclasses in Objective-C, Swift subclasses do not inherit their superclass initializers by default. Swift’s approach prevents a situation in which a simple initializer from a superclass is inherited by a more specialized subclass and is used to create a new instance of the subclass that is not fully or correctly initialized.

If you want a custom subclass to present one or more of the same initializers as its superclass, you can provide a custom implementation of those initializers within the subclass.

When you write a subclass initializer that matches a superclass designated initializer, you are effectively providing an override of that designated initializer. Therefore, you must write the override modifier before the subclass’s initializer definition. This is true even if you are overriding an automatically provided default initializer, as described in Default Initializers.

As with an overridden property, method or subscript, the presence of the override modifier prompts Swift to check that the superclass has a matching designated initializer to be overridden, and validates that the parameters for your overriding initializer have been specified as intended.

Conversely, if you write a subclass initializer that matches a superclass convenience initializer, that superclass convenience initializer can never be called directly by your subclass, as per the rules described above in Initializer Delegation for Class Types. Therefore, your subclass is not (strictly speaking) providing an override of the superclass initializer. As a result, you do not write the override modifier when providing a matching implementation of a superclass convenience initializer.

The example below defines a base class called Vehicle. This base class declares a stored property called numberOfWheels, with a default Int value of 0. The numberOfWheels property is used by a computed property called description to create a String description of the vehicle’s characteristics:

1. class Vehicle {
2.  var numberOfWheels = 0
3.  var description: String {
4.  return "\(numberOfWheels) wheel(s)"
5.  }
6. }

The Vehicle class provides a default value for its only stored property, and does not provide any custom initializers itself. As a result, it automatically receives a default initializer, as described in Default Initializers. The default initializer (when available) is always a designated initializer for a class, and can be used to create a new Vehicle instance with a numberOfWheels of 0:

1. let vehicle = Vehicle()
2. print("Vehicle: \(vehicle.description)")
3. // Vehicle: 0 wheel(s)

The next example defines a subclass of Vehicle called Bicycle:

1. class Bicycle: Vehicle {
2.  override init() {
3.  super.init()
4.  numberOfWheels = 2
5.  }
6. }

The Bicycle subclass defines a custom designated initializer, init(). This designated initializer matches a designated initializer from the superclass of Bicycle, and so the Bicycle version of this initializer is marked with the override modifier.

The init() initializer for Bicycle starts by calling super.init(), which calls the default initializer for the Bicycle class’s superclass, Vehicle. This ensures that the numberOfWheels inherited property is initialized by Vehicle before Bicycle has the opportunity to modify the property. After calling super.init(), the original value of numberOfWheels is replaced with a new value of 2.

If you create an instance of Bicycle, you can call its inherited description computed property to see how its numberOfWheels property has been updated:

1. let bicycle = Bicycle()
2. print("Bicycle: \(bicycle.description)")
3. // Bicycle: 2 wheel(s)

### Automatic Initializer Inheritance

As mentioned above, subclasses do not inherit their superclass initializers by default. However, superclass initializers are automatically inherited if certain conditions are met. In practice, this means that you do not need to write initializer overrides in many common scenarios, and can inherit your superclass initializers with minimal effort whenever it is safe to do so.

Assuming that you provide default values for any new properties you introduce in a subclass, the following two rules apply:

Rule 1

If your subclass doesn’t define any designated initializers, it automatically inherits all of its superclass designated initializers.

Rule 2

If your subclass provides an implementation of all of its superclass designated initializers—either by inheriting them as per rule 1, or by providing a custom implementation as part of its definition—then it automatically inherits all of the superclass convenience initializers.

### Designated and Convenience Initializers in Action

The following example shows designated initializers, convenience initializers, and automatic initializer inheritance in action. This example defines a hierarchy of three classes called Food, RecipeIngredient, and ShoppingListItem, and demonstrates how their initializers interact.

The base class in the hierarchy is called Food, which is a simple class to encapsulate the name of a foodstuff. The Food class introduces a single String property called name and provides two initializers for creating Food instances:

1. class Food {
2.  var name: String
3.  init(name: String) {
4.  self.name = name
5.  }
6.  convenience init() {
7.  self.init(name: "[Unnamed]")
8.  }
9. }

The figure below shows the initializer chain for the Food class:

Classes do not have a default memberwise initializer, and so the Food class provides a designated initializer that takes a single argument called name. This initializer can be used to create a new Food instance with a specific name:

1. let namedMeat = Food(name: "Bacon")
2. // namedMeat's name is "Bacon"

The init(name: String) initializer from the Food class is provided as a designated initializer, because it ensures that all stored properties of a new Food instance are fully initialized. The Food class does not have a superclass, and so the init(name: String) initializer does not need to call super.init() to complete its initialization.

The Food class also provides a convenience initializer, init(), with no arguments. The init() initializer provides a default placeholder name for a new food by delegating across to the Food class’s init(name: String) with a name value of [Unnamed]:

1. let mysteryMeat = Food()
2. // mysteryMeat's name is "[Unnamed]"

The second class in the hierarchy is a subclass of Food called RecipeIngredient. The RecipeIngredient class models an ingredient in a cooking recipe. It introduces an Int property called quantity (in addition to the name property it inherits from Food) and defines two initializers for creating RecipeIngredient instances:

1. class RecipeIngredient: Food {
2.  var quantity: Int
3.  init(name: String, quantity: Int) {
4.  self.quantity = quantity
5.  super.init(name: name)
6.  }
7.  override convenience init(name: String) {
8.  self.init(name: name, quantity: 1)
9.  }
10. }

The figure below shows the initializer chain for the RecipeIngredient class:

The RecipeIngredient class has a single designated initializer, init(name: String, quantity: Int), which can be used to populate all of the properties of a new RecipeIngredient instance. This initializer starts by assigning the passed quantity argument to the quantity property, which is the only new property introduced by RecipeIngredient. After doing so, the initializer delegates up to the init(name: String) initializer of the Food class. This process satisfies safety check 1 from Two-Phase Initialization above.

RecipeIngredient also defines a convenience initializer, init(name: String), which is used to create a RecipeIngredient instance by name alone. This convenience initializer assumes a quantity of 1 for any RecipeIngredient instance that is created without an explicit quantity. The definition of this convenience initializer makes RecipeIngredient instances quicker and more convenient to create, and avoids code duplication when creating several single-quantity RecipeIngredient instances. This convenience initializer simply delegates across to the class’s designated initializer, passing in a quantity value of 1.

The init(name: String) convenience initializer provided by RecipeIngredient takes the same parameters as the init(name: String) designated initializer from Food. Because this convenience initializer overrides a designated initializer from its superclass, it must be marked with the override modifier (as described in Initializer Inheritance and Overriding).

Even though RecipeIngredient provides the init(name: String) initializer as a convenience initializer, RecipeIngredient has nonetheless provided an implementation of all of its superclass’s designated initializers. Therefore, RecipeIngredient automatically inherits all of its superclass’s convenience initializers too.

In this example, the superclass for RecipeIngredient is Food, which has a single convenience initializer called init(). This initializer is therefore inherited by RecipeIngredient. The inherited version of init() functions in exactly the same way as the Food version, except that it delegates to the RecipeIngredient version of init(name: String) rather than the Food version.

All three of these initializers can be used to create new RecipeIngredient instances:

1. let oneMysteryItem = RecipeIngredient()
2. let oneBacon = RecipeIngredient(name: "Bacon")
3. let sixEggs = RecipeIngredient(name: "Eggs", quantity: 6)

The third and final class in the hierarchy is a subclass of RecipeIngredient called ShoppingListItem. The ShoppingListItem class models a recipe ingredient as it appears in a shopping list.

Every item in the shopping list starts out as “unpurchased”. To represent this fact, ShoppingListItem introduces a Boolean property called purchased, with a default value of false. ShoppingListItem also adds a computed description property, which provides a textual description of a ShoppingListItem instance:

1. class ShoppingListItem: RecipeIngredient {
2.  var purchased = false
3.  var description: String {
4.  var output = "\(quantity) x \(name)"
5.  output += purchased ? " ✔" : " ✘"
6.  return output
7.  }
8. }

Because it provides a default value for all of the properties it introduces and does not define any initializers itself, ShoppingListItem automatically inherits all of the designated and convenience initializers from its superclass.

The figure below shows the overall initializer chain for all three classes:

You can use all three of the inherited initializers to create a new ShoppingListItem instance:

1. var breakfastList = [
2.  ShoppingListItem(),
3.  ShoppingListItem(name: "Bacon"),
4.  ShoppingListItem(name: "Eggs", quantity: 6),
5. ]
6. breakfastList[0].name = "Orange juice"
7. breakfastList[0].purchased = true
8. for item in breakfastList {
9.  print(item.description)
10. }
11. // 1 x Orange juice ✔
12. // 1 x Bacon ✘
13. // 6 x Eggs ✘

Here, a new array called breakfastList is created from an array literal containing three new ShoppingListItem instances. The type of the array is inferred to be [ShoppingListItem]. After the array is created, the name of the ShoppingListItem at the start of the array is changed from "[Unnamed]" to "Orange juice" and it is marked as having been purchased. Printing the description of each item in the array shows that their default states have been set as expected.

## Failable Initializers

It is sometimes useful to define a class, structure, or enumeration for which initialization can fail. This failure might be triggered by invalid initialization parameter values, the absence of a required external resource, or some other condition that prevents initialization from succeeding.

To cope with initialization conditions that can fail, define one or more failable initializers as part of a class, structure, or enumeration definition. You write a failable initializer by placing a question mark after the init keyword (init?).

A failable initializer creates an optional value of the type it initializes. You write return nil within a failable initializer to indicate a point at which initialization failure can be triggered.

The example below defines a structure called Animal, with a constant String property called species. The Animal structure also defines a failable initializer with a single parameter called species. This initializer checks if the species value passed to the initializer is an empty string. If an empty string is found, an initialization failure is triggered. Otherwise, the species property’s value is set, and initialization succeeds:

1. struct Animal {
2.  let species: String
3.  init?(species: String) {
4.  if species.isEmpty { return nil }
5.  self.species = species
6.  }
7. }

You can use this failable initializer to try to initialize a new Animal instance and to check if initialization succeeded:

1. let someCreature = Animal(species: "Giraffe")
2. // someCreature is of type Animal?, not Animal
3. 
4. if let giraffe = someCreature {
5.  print("An animal was initialized with a species of \(giraffe.species)")
6. }
7. // prints "An animal was initialized with a species of Giraffe"

If you pass an empty string value to the failable initializer’s species parameter, the initializer triggers an initialization failure:

1. let anonymousCreature = Animal(species: "")
2. // anonymousCreature is of type Animal?, not Animal
3. 
4. if anonymousCreature == nil {
5.  print("The anonymous creature could not be initialized")
6. }
7. // prints "The anonymous creature could not be initialized"

### Failable Initializers for Enumerations

You can use a failable initializer to select an appropriate enumeration member based on one or more parameters. The initializer can then fail if the provided parameters do not match an appropriate enumeration member.

The example below defines an enumeration called TemperatureUnit, with three possible states (Kelvin, Celsius, and Fahrenheit). A failable initializer is used to find an appropriate enumeration member for a Character value representing a temperature symbol:

1. enum TemperatureUnit {
2.  case Kelvin, Celsius, Fahrenheit
3.  init?(symbol: Character) {
4.  switch symbol {
5.  case "K":
6.  self = .Kelvin
7.  case "C":
8.  self = .Celsius
9.  case "F":
10.  self = .Fahrenheit
11.  default:
12.  return nil
13.  }
14.  }
15. }

You can use this failable initializer to choose an appropriate enumeration member for the three possible states and to cause initialization to fail if the parameter does not match one of these states:

1. let fahrenheitUnit = TemperatureUnit(symbol: "F")
2. if fahrenheitUnit != nil {
3.  print("This is a defined temperature unit, so initialization succeeded.")
4. }
5. // prints "This is a defined temperature unit, so initialization succeeded."
6. 
7. let unknownUnit = TemperatureUnit(symbol: "X")
8. if unknownUnit == nil {
9.  print("This is not a defined temperature unit, so initialization failed.")
10. }
11. // prints "This is not a defined temperature unit, so initialization failed."

### Failable Initializers for Enumerations with Raw Values

Enumerations with raw values automatically receive a failable initializer, init?(rawValue:), that takes a parameter called rawValue of the appropriate raw-value type and selects a matching enumeration member if one is found, or triggers an initialization failure if no matching value exists.

You can rewrite the TemperatureUnit example from above to use raw values of type Character and to take advantage of the init?(rawValue:) initializer:

1. enum TemperatureUnit: Character {
2.  case Kelvin = "K", Celsius = "C", Fahrenheit = "F"
3. }
4. 
5. let fahrenheitUnit = TemperatureUnit(rawValue: "F")
6. if fahrenheitUnit != nil {
7.  print("This is a defined temperature unit, so initialization succeeded.")
8. }
9. // prints "This is a defined temperature unit, so initialization succeeded."
10. 
11. let unknownUnit = TemperatureUnit(rawValue: "X")
12. if unknownUnit == nil {
13.  print("This is not a defined temperature unit, so initialization failed.")
14. }
15. // prints "This is not a defined temperature unit, so initialization failed."

### Failable Initializers for Classes

A failable initializer for a value type (that is, a structure or enumeration) can trigger an initialization failure at any point within its initializer implementation. In the Animal structure example above, the initializer triggers an initialization failure at the very start of its implementation, before the species property has been set.

For classes, however, a failable initializer can trigger an initialization failure only after all stored properties introduced by that class have been set to an initial value and any initializer delegation has taken place.

The example below shows how you can use an implicitly unwrapped optional property to satisfy this requirement within a failable class initializer:

1. class Product {
2.  let name: String!
3.  init?(name: String) {
4.  self.name = name
5.  if name.isEmpty { return nil }
6.  }
7. }

The Product class defined above is very similar to the Animal structure seen earlier. The Product class has a constant name property that must not be allowed to take an empty string value. To enforce this requirement, the Product class uses a failable initializer to ensure that the property’s value is non-empty before allowing initialization to succeed.

However, Product is a class, not a structure. This means that unlike Animal, any failable initializer for the Product class must provide an initial value for the name property before triggering an initialization failure.

In the example above, the name property of the Product class is defined as having an implicitly unwrapped optional string type (String!). Because it is of an optional type, this means that the name property has a default value of nil before it is assigned a specific value during initialization. This default value of nil in turn means that all of the properties introduced by the Product class have a valid initial value. As a result, the failable initializer for Product can trigger an initialization failure at the start of the initializer if it is passed an empty string, before assigning a specific value to the name property within the initializer.

Because the name property is a constant, you can be confident that it will always contain a non-nil value if initialization succeeds. Even though it is defined with an implicitly unwrapped optional type, you can always access its implicitly unwrapped value with confidence, without needing to check for a value of nil:

1. if let bowTie = Product(name: "bow tie") {
2.  // no need to check if bowTie.name == nil
3.  print("The product's name is \(bowTie.name)")
4. }
5. // prints "The product's name is bow tie"

### Propagation of Initialization Failure

A failable initializer of a class, structure, or enumeration can delegate across to another failable initializer from the same class, structure, or enumeration. Similarly, a subclass failable initializer can delegate up to a superclass failable initializer.

In either case, if you delegate to another initializer that causes initialization to fail, the entire initialization process fails immediately, and no further initialization code is executed.

The example below defines a subclass of Product called CartItem. The CartItem class models an item in an online shopping cart. CartItem introduces a stored constant property called quantity and ensures that this property always has a value of at least 1:

1. class CartItem: Product {
2.  let quantity: Int!
3.  init?(name: String, quantity: Int) {
4.  self.quantity = quantity
5.  super.init(name: name)
6.  if quantity < 1 { return nil }
7.  }
8. }

The quantity property has an implicitly unwrapped integer type (Int!). As with the name property of the Product class, this means that the quantity property has a default value of nil before it is assigned a specific value during initialization.

The failable initializer for CartItem starts by delegating up to the init(name:) initializer from its superclass, Product. This satisfies the requirement that a failable initializer must always perform initializer delegation before triggering an initialization failure.

If the superclass initialization fails because of an empty name value, the entire initialization process fails immediately and no further initialization code is executed. If the superclass initialization succeeds, the CartItem initializer validates that it has received a quantity value of 1 or more.

If you create a CartItem instance with a non-empty name and a quantity of 1 or more, initialization succeeds:

1. if let twoSocks = CartItem(name: "sock", quantity: 2) {
2.  print("Item: \(twoSocks.name), quantity: \(twoSocks.quantity)")
3. }
4. // prints "Item: sock, quantity: 2"

If you try to create a CartItem instance with a quantity value of 0, the CartItem initializer causes initialization to fail:

1. if let zeroShirts = CartItem(name: "shirt", quantity: 0) {
2.  print("Item: \(zeroShirts.name), quantity: \(zeroShirts.quantity)")
3. } else {
4.  print("Unable to initialize zero shirts")
5. }
6. // prints "Unable to initialize zero shirts"

Similarly, if you try to create a CartItem instance with an empty name value, the superclass Product initializer causes initialization to fail:

1. if let oneUnnamed = CartItem(name: "", quantity: 1) {
2.  print("Item: \(oneUnnamed.name), quantity: \(oneUnnamed.quantity)")
3. } else {
4.  print("Unable to initialize one unnamed product")
5. }
6. // prints "Unable to initialize one unnamed product"

### Overriding a Failable Initializer

You can override a superclass failable initializer in a subclass, just like any other initializer. Alternatively, you can override a superclass failable initializer with a subclass non-failable initializer. This enables you to define a subclass for which initialization cannot fail, even though initialization of the superclass is allowed to fail.

Note that if you override a failable superclass initializer with a nonfailable subclass initializer, the subclass initializer cannot delegate up to the superclass initializer. A nonfailable initializer can never delegate to a failable initializer.

The example below defines a class called Document. This class models a document that can be initialized with a name property that is either a non-empty string value or nil, but cannot be an empty string:

1. class Document {
2.  var name: String?
3.  // this initializer creates a document with a nil name value
4.  init() {}
5.  // this initializer creates a document with a non-empty name value
6.  init?(name: String) {
7.  self.name = name
8.  if name.isEmpty { return nil }
9.  }
10. }

The next example defines a subclass of Document called AutomaticallyNamedDocument. The AutomaticallyNamedDocument subclass overrides both of the designated initializers introduced by Document. These overrides ensure that an AutomaticallyNamedDocument instance has an initial name value of "[Untitled]" if the instance is initialized without a name, or if an empty string is passed to the init(name:) initializer:

1. class AutomaticallyNamedDocument: Document {
2.  override init() {
3.  super.init()
4.  self.name = "[Untitled]"
5.  }
6.  override init(name: String) {
7.  super.init()
8.  if name.isEmpty {
9.  self.name = "[Untitled]"
10.  } else {
11.  self.name = name
12.  }
13.  }
14. }

The AutomaticallyNamedDocument overrides its superclass’s failable init?(name:) initializer with a nonfailable init(name:) initializer. Because AutomaticallyNamedDocument copes with the empty string case in a different way than its superclass, its initializer does not need to fail, and so it provides a nonfailable version of the initializer instead.

### The init! Failable Initializer

You typically define a failable initializer that creates an optional instance of the appropriate type by placing a question mark after the init keyword (init?). Alternatively, you can define a failable initializer that creates an implicitly unwrapped optional instance of the appropriate type. Do this by placing an exclamation mark after the init keyword (init!) instead of a question mark.

You can delegate from init? to init! and vice versa, and you can override init? with init! and vice versa. You can also delegate from init to init!, although doing so will trigger an assertion if the init! initializer causes initialization to fail.

## Required Initializers

Write the required modifier before the definition of a class initializer to indicate that every subclass of the class must implement that initializer:

1. class SomeClass {
2.  required init() {
3.  // initializer implementation goes here
4.  }
5. }

You must also write the required modifier before every subclass implementation of a required initializer, to indicate that the initializer requirement applies to further subclasses in the chain. You do not write the override modifier when overriding a required designated initializer:

1. class SomeSubclass: SomeClass {
2.  required init() {
3.  // subclass implementation of the required initializer goes here
4.  }
5. }

## Setting a Default Property Value with a Closure or Function

If a stored property’s default value requires some customization or setup, you can use a closure or global function to provide a customized default value for that property. Whenever a new instance of the type that the property belongs to is initialized, the closure or function is called, and its return value is assigned as the property’s default value.

These kinds of closures or functions typically create a temporary value of the same type as the property, tailor that value to represent the desired initial state, and then return that temporary value to be used as the property’s default value.

Here’s a skeleton outline of how a closure can be used to provide a default property value:

1. class SomeClass {
2.  let someProperty: SomeType = {
3.  // create a default value for someProperty inside this closure
4.  // someValue must be of the same type as SomeType
5.  return someValue
6.  }()
7. }

Note that the closure’s end curly brace is followed by an empty pair of parentheses. This tells Swift to execute the closure immediately. If you omit these parentheses, you are trying to assign the closure itself to the property, and not the return value of the closure.

The example below defines a structure called Checkerboard, which models a board for the game of Checkers (also known as Draughts):

The game of Checkers is played on a ten-by-ten board, with alternating black and white squares. To represent this game board, the Checkerboard structure has a single property called boardColors, which is an array of 100 Bool values. A value of true in the array represents a black square and a value of false represents a white square. The first item in the array represents the top left square on the board and the last item in the array represents the bottom right square on the board.

The boardColors array is initialized with a closure to set up its color values:

1. struct Checkerboard {
2.  let boardColors: [Bool] = {
3.  var temporaryBoard = [Bool]()
4.  var isBlack = false
5.  for i in 1...10 {
6.  for j in 1...10 {
7.  temporaryBoard.append(isBlack)
8.  isBlack = !isBlack
9.  }
10.  isBlack = !isBlack
11.  }
12.  return temporaryBoard
13.  }()
14.  func squareIsBlackAtRow(row: Int, column: Int) -> Bool {
15.  return boardColors[(row * 10) + column]
16.  }
17. }

Whenever a new Checkerboard instance is created, the closure is executed, and the default value of boardColors is calculated and returned. The closure in the example above calculates and sets the appropriate color for each square on the board in a temporary array called temporaryBoard, and returns this temporary array as the closure’s return value once its setup is complete. The returned array value is stored in boardColors and can be queried with the squareIsBlackAtRow utility function:

1. let board = Checkerboard()
2. print(board.squareIsBlackAtRow(0, column: 1))
3. // prints "true"
4. print(board.squareIsBlackAtRow(9, column: 9))
5. // prints "false"

# Deinitialization

A deinitializer is called immediately before a class instance is deallocated. You write deinitializers with the deinit keyword, similar to how initializers are written with the init keyword. Deinitializers are only available on class types.

## How Deinitialization Works

Swift automatically deallocates your instances when they are no longer needed, to free up resources. Swift handles the memory management of instances through automatic reference counting (ARC), as described in Automatic Reference Counting. Typically you don’t need to perform manual clean-up when your instances are deallocated. However, when you are working with your own resources, you might need to perform some additional clean-up yourself. For example, if you create a custom class to open a file and write some data to it, you might need to close the file before the class instance is deallocated.

Class definitions can have at most one deinitializer per class. The deinitializer does not take any parameters and is written without parentheses:

1. deinit {
2.  // perform the deinitialization
3. }

Deinitializers are called automatically, just before instance deallocation takes place. You are not allowed to call a deinitializer yourself. Superclass deinitializers are inherited by their subclasses, and the superclass deinitializer is called automatically at the end of a subclass deinitializer implementation. Superclass deinitializers are always called, even if a subclass does not provide its own deinitializer.

Because an instance is not deallocated until after its deinitializer is called, a deinitializer can access all properties of the instance it is called on and can modify its behavior based on those properties (such as looking up the name of a file that needs to be closed).

## Deinitializers in Action

Here’s an example of a deinitializer in action. This example defines two new types, Bank and Player, for a simple game. The Bank class manages a made-up currency, which can never have more than 10,000 coins in circulation. There can only ever be one Bank in the game, and so the Bank is implemented as a class with type properties and methods to store and manage its current state:

1. class Bank {
2.  static var coinsInBank = 10_000
3.  static func vendCoins(var numberOfCoinsToVend: Int) -> Int {
4.  numberOfCoinsToVend = min(numberOfCoinsToVend, coinsInBank)
5.  coinsInBank -= numberOfCoinsToVend
6.  return numberOfCoinsToVend
7.  }
8.  static func receiveCoins(coins: Int) {
9.  coinsInBank += coins
10.  }
11. }

Bank keeps track of the current number of coins it holds with its coinsInBank property. It also offers two methods—vendCoins(_:) and receiveCoins(_:)—to handle the distribution and collection of coins.

vendCoins(_:) checks that there are enough coins in the bank before distributing them. If there are not enough coins, Bank returns a smaller number than the number that was requested (and returns zero if no coins are left in the bank). vendCoins(_:) declares numberOfCoinsToVend as a variable parameter, so that the number can be modified within the method’s body without the need to declare a new variable. It returns an integer value to indicate the actual number of coins that were provided.

The receiveCoins(_:) method simply adds the received number of coins back into the bank’s coin store.

The Player class describes a player in the game. Each player has a certain number of coins stored in their purse at any time. This is represented by the player’s coinsInPurse property:

1. class Player {
2.  var coinsInPurse: Int
3.  init(coins: Int) {
4.  coinsInPurse = Bank.vendCoins(coins)
5.  }
6.  func winCoins(coins: Int) {
7.  coinsInPurse += Bank.vendCoins(coins)
8.  }
9.  deinit {
10.  Bank.receiveCoins(coinsInPurse)
11.  }
12. }

Each Player instance is initialized with a starting allowance of a specified number of coins from the bank during initialization, although a Player instance may receive fewer than that number if not enough coins are available.

The Player class defines a winCoins(_:) method, which retrieves a certain number of coins from the bank and adds them to the player’s purse. The Player class also implements a deinitializer, which is called just before a Player instance is deallocated. Here, the deinitializer simply returns all of the player’s coins to the bank:

1. var playerOne: Player? = Player(coins: 100)
2. print("A new player has joined the game with \(playerOne!.coinsInPurse) coins")
3. // prints "A new player has joined the game with 100 coins"
4. print("There are now \(Bank.coinsInBank) coins left in the bank")
5. // prints "There are now 9900 coins left in the bank"

A new Player instance is created, with a request for 100 coins if they are available. This Player instance is stored in an optional Player variable called playerOne. An optional variable is used here, because players can leave the game at any point. The optional lets you track whether there is currently a player in the game.

Because playerOne is an optional, it is qualified with an exclamation mark (!) when its coinsInPurse property is accessed to print its default number of coins, and whenever its winCoins(_:) method is called:

1. playerOne!.winCoins(2_000)
2. print("PlayerOne won 2000 coins & now has \(playerOne!.coinsInPurse) coins")
3. // prints "PlayerOne won 2000 coins & now has 2100 coins"
4. print("The bank now only has \(Bank.coinsInBank) coins left")
5. // prints "The bank now only has 7900 coins left"

Here, the player has won 2,000 coins. The player’s purse now contains 2,100 coins, and the bank has only 7,900 coins left.

1. playerOne = nil
2. print("PlayerOne has left the game")
3. // prints "PlayerOne has left the game"
4. print("The bank now has \(Bank.coinsInBank) coins")
5. // prints "The bank now has 10000 coins"

The player has now left the game. This is indicated by setting the optional playerOne variable to nil, meaning “no Player instance.” At the point that this happens, the playerOne variable’s reference to the Player instance is broken. No other properties or variables are still referring to the Player instance, and so it is deallocated in order to free up its memory. Just before this happens, its deinitializer is called automatically, and its coins are returned to the bank.

# Automatic Reference Counting

Swift uses Automatic Reference Counting (ARC) to track and manage your app’s memory usage. In most cases, this means that memory management “just works” in Swift, and you do not need to think about memory management yourself. ARC automatically frees up the memory used by class instances when those instances are no longer needed.

However, in a few cases ARC requires more information about the relationships between parts of your code in order to manage memory for you. This chapter describes those situations and shows how you enable ARC to manage all of your app’s memory.

## How ARC Works

Every time you create a new instance of a class, ARC allocates a chunk of memory to store information about that instance. This memory holds information about the type of the instance, together with the values of any stored properties associated with that instance.

Additionally, when an instance is no longer needed, ARC frees up the memory used by that instance so that the memory can be used for other purposes instead. This ensures that class instances do not take up space in memory when they are no longer needed.

However, if ARC were to deallocate an instance that was still in use, it would no longer be possible to access that instance’s properties, or call that instance’s methods. Indeed, if you tried to access the instance, your app would most likely crash.

To make sure that instances don’t disappear while they are still needed, ARC tracks how many properties, constants, and variables are currently referring to each class instance. ARC will not deallocate an instance as long as at least one active reference to that instance still exists.

To make this possible, whenever you assign a class instance to a property, constant, or variable, that property, constant, or variable makes a strong reference to the instance. The reference is called a “strong“ reference because it keeps a firm hold on that instance, and does not allow it to be deallocated for as long as that strong reference remains.

## ARC in Action

Here’s an example of how Automatic Reference Counting works. This example starts with a simple class called Person, which defines a stored constant property called name:

1. class Person {
2.  let name: String
3.  init(name: String) {
4.  self.name = name
5.  print("\(name) is being initialized")
6.  }
7.  deinit {
8.  print("\(name) is being deinitialized")
9.  }
10. }

The Person class has an initializer that sets the instance’s name property and prints a message to indicate that initialization is underway. The Person class also has a deinitializer that prints a message when an instance of the class is deallocated.

The next code snippet defines three variables of type Person?, which are used to set up multiple references to a new Person instance in subsequent code snippets. Because these variables are of an optional type (Person?, not Person), they are automatically initialized with a value of nil, and do not currently reference a Person instance.

1. var reference1: Person?
2. var reference2: Person?
3. var reference3: Person?

You can now create a new Person instance and assign it to one of these three variables:

1. reference1 = Person(name: "John Appleseed")
2. // prints "John Appleseed is being initialized"

Note that the message "John Appleseed is being initialized" is printed at the point that you call the Person class’s initializer. This confirms that initialization has taken place.

Because the new Person instance has been assigned to the reference1 variable, there is now a strong reference from reference1 to the new Person instance. Because there is at least one strong reference, ARC makes sure that this Person is kept in memory and is not deallocated.

If you assign the same Person instance to two more variables, two more strong references to that instance are established:

1. reference2 = reference1
2. reference3 = reference1

There are now three strong references to this single Person instance.

If you break two of these strong references (including the original reference) by assigning nil to two of the variables, a single strong reference remains, and the Person instance is not deallocated:

1. reference1 = nil
2. reference2 = nil

ARC does not deallocate the Person instance until the third and final strong reference is broken, at which point it is clear that you are no longer using the Person instance:

1. reference3 = nil
2. // prints "John Appleseed is being deinitialized"

## Strong Reference Cycles Between Class Instances

In the examples above, ARC is able to track the number of references to the new Person instance you create and to deallocate that Person instance when it is no longer needed.

However, it is possible to write code in which an instance of a class never gets to a point where it has zero strong references. This can happen if two class instances hold a strong reference to each other, such that each instance keeps the other alive. This is known as a strong reference cycle.

You resolve strong reference cycles by defining some of the relationships between classes as weak or unowned references instead of as strong references. This process is described in Resolving Strong Reference Cycles Between Class Instances. However, before you learn how to resolve a strong reference cycle, it is useful to understand how such a cycle is caused.

Here’s an example of how a strong reference cycle can be created by accident. This example defines two classes called Person and Apartment, which model a block of apartments and its residents:

1. class Person {
2.  let name: String
3.  init(name: String) { self.name = name }
4.  var apartment: Apartment?
5.  deinit { print("\(name) is being deinitialized") }
6. }
7. 
8. class Apartment {
9.  let unit: String
10.  init(unit: String) { self.unit = unit }
11.  var tenant: Person?
12.  deinit { print("Apartment \(unit) is being deinitialized") }
13. }

Every Person instance has a name property of type String and an optional apartment property that is initially nil. The apartment property is optional, because a person may not always have an apartment.

Similarly, every Apartment instance has a unit property of type String and has an optional tenant property that is initially nil. The tenant property is optional because an apartment may not always have a tenant.

Both of these classes also define a deinitializer, which prints the fact that an instance of that class is being deinitialized. This enables you to see whether instances of Person and Apartment are being deallocated as expected.

This next code snippet defines two variables of optional type called john and unit4A, which will be set to a specific Apartment and Person instance below. Both of these variables have an initial value of nil, by virtue of being optional:

1. var john: Person?
2. var unit4A: Apartment?

You can now create a specific Person instance and Apartment instance and assign these new instances to the john and unit4A variables:

1. john = Person(name: "John Appleseed")
2. unit4A = Apartment(unit: "4A")

Here’s how the strong references look after creating and assigning these two instances. The john variable now has a strong reference to the new Person instance, and the unit4A variable has a strong reference to the new Apartment instance:

You can now link the two instances together so that the person has an apartment, and the apartment has a tenant. Note that an exclamation mark (!) is used to unwrap and access the instances stored inside the john and unit4A optional variables, so that the properties of those instances can be set:

1. john!.apartment = unit4A
2. unit4A!.tenant = john

Here’s how the strong references look after you link the two instances together:

Unfortunately, linking these two instances creates a strong reference cycle between them. The Person instance now has a strong reference to the Apartment instance, and the Apartment instance has a strong reference to the Person instance. Therefore, when you break the strong references held by the john and unit4A variables, the reference counts do not drop to zero, and the instances are not deallocated by ARC:

1. john = nil
2. unit4A = nil

Note that neither deinitializer was called when you set these two variables to nil. The strong reference cycle prevents the Person and Apartment instances from ever being deallocated, causing a memory leak in your app.

Here’s how the strong references look after you set the john and unit4A variables to nil:

The strong references between the Person instance and the Apartment instance remain and cannot be broken.

## Resolving Strong Reference Cycles Between Class Instances

Swift provides two ways to resolve strong reference cycles when you work with properties of class type: weak references and unowned references.

Weak and unowned references enable one instance in a reference cycle to refer to the other instance without keeping a strong hold on it. The instances can then refer to each other without creating a strong reference cycle.

Use a weak reference whenever it is valid for that reference to become nil at some point during its lifetime. Conversely, use an unowned reference when you know that the reference will never be nil once it has been set during initialization.

### Weak References

A weak reference is a reference that does not keep a strong hold on the instance it refers to, and so does not stop ARC from disposing of the referenced instance. This behavior prevents the reference from becoming part of a strong reference cycle. You indicate a weak reference by placing the weak keyword before a property or variable declaration.

Use a weak reference to avoid reference cycles whenever it is possible for that reference to have “no value” at some point in its life. If the reference will always have a value, use an unowned reference instead, as described in Unowned References. In the Apartment example above, it is appropriate for an apartment to be able to have “no tenant” at some point in its lifetime, and so a weak reference is an appropriate way to break the reference cycle in this case.

Because weak references are allowed to have “no value”, you must declare every weak reference as having an optional type. Optional types are the preferred way to represent the possibility for “no value” in Swift.

Because a weak reference does not keep a strong hold on the instance it refers to, it is possible for that instance to be deallocated while the weak reference is still referring to it. Therefore, ARC automatically sets a weak reference to nil when the instance that it refers to is deallocated. You can check for the existence of a value in the weak reference, just like any other optional value, and you will never end up with a reference to an invalid instance that no longer exists.

The example below is identical to the Person and Apartment example from above, with one important difference. This time around, the Apartment type’s tenant property is declared as a weak reference:

1. class Person {
2.  let name: String
3.  init(name: String) { self.name = name }
4.  var apartment: Apartment?
5.  deinit { print("\(name) is being deinitialized") }
6. }
7. 
8. class Apartment {
9.  let unit: String
10.  init(unit: String) { self.unit = unit }
11.  weak var tenant: Person?
12.  deinit { print("Apartment \(unit) is being deinitialized") }
13. }

The strong references from the two variables (john and unit4A) and the links between the two instances are created as before:

1. var john: Person?
2. var unit4A: Apartment?
3. 
4. john = Person(name: "John Appleseed")
5. unit4A = Apartment(unit: "4A")
6. 
7. john!.apartment = unit4A
8. unit4A!.tenant = john

Here’s how the references look now that you’ve linked the two instances together:

The Person instance still has a strong reference to the Apartment instance, but the Apartment instance now has a weak reference to the Person instance. This means that when you break the strong reference held by the john variables, there are no more strong references to the Person instance:

Because there are no more strong references to the Person instance, it is deallocated:

1. john = nil
2. // prints "John Appleseed is being deinitialized"

The only remaining strong reference to the Apartment instance is from the unit4A variable. If you break that strong reference, there are no more strong references to the Apartment instance:

Because there are no more strong references to the Apartment instance, it too is deallocated:

1. unit4A = nil
2. // prints "Apartment 4A is being deinitialized"

The final two code snippets above show that the deinitializers for the Person instance and Apartment instance print their “deinitialized” messages after the john and unit4A variables are set to nil. This proves that the reference cycle has been broken.

### Unowned References

Like weak references, an unowned reference does not keep a strong hold on the instance it refers to. Unlike a weak reference, however, an unowned reference is assumed to always have a value. Because of this, an unowned reference is always defined as a non-optional type. You indicate an unowned reference by placing the unowned keyword before a property or variable declaration.

Because an unowned reference is non-optional, you don’t need to unwrap the unowned reference each time it is used. An unowned reference can always be accessed directly. However, ARC cannot set the reference to nil when the instance it refers to is deallocated, because variables of a non-optional type cannot be set to nil.

The following example defines two classes, Customer and CreditCard, which model a bank customer and a possible credit card for that customer. These two classes each store an instance of the other class as a property. This relationship has the potential to create a strong reference cycle.

The relationship between Customer and CreditCard is slightly different from the relationship between Apartment and Person seen in the weak reference example above. In this data model, a customer may or may not have a credit card, but a credit card will always be associated with a customer. To represent this, the Customer class has an optional card property, but the CreditCard class has a non-optional customer property.

Furthermore, a new CreditCard instance can only be created by passing a number value and a customer instance to a custom CreditCard initializer. This ensures that a CreditCard instance always has a customer instance associated with it when the CreditCard instance is created.

Because a credit card will always have a customer, you define its customer property as an unowned reference, to avoid a strong reference cycle:

1. class Customer {
2.  let name: String
3.  var card: CreditCard?
4.  init(name: String) {
5.  self.name = name
6.  }
7.  deinit { print("\(name) is being deinitialized") }
8. }
9. 
10. class CreditCard {
11.  let number: UInt64
12.  unowned let customer: Customer
13.  init(number: UInt64, customer: Customer) {
14.  self.number = number
15.  self.customer = customer
16.  }
17.  deinit { print("Card #\(number) is being deinitialized") }
18. }

This next code snippet defines an optional Customer variable called john, which will be used to store a reference to a specific customer. This variable has an initial value of nil, by virtue of being optional:

1. var john: Customer?

You can now create a Customer instance, and use it to initialize and assign a new CreditCard instance as that customer’s card property:

1. john = Customer(name: "John Appleseed")
2. john!.card = CreditCard(number: 1234_5678_9012_3456, customer: john!)

Here’s how the references look, now that you’ve linked the two instances:

The Customer instance now has a strong reference to the CreditCard instance, and the CreditCard instance has an unowned reference to the Customer instance.

Because of the unowned customer reference, when you break the strong reference held by the john variable, there are no more strong references to the Customer instance:

Because there are no more strong references to the Customer instance, it is deallocated. After this happens, there are no more strong references to the CreditCard instance, and it too is deallocated:

1. john = nil
2. // prints "John Appleseed is being deinitialized"
3. // prints "Card #1234567890123456 is being deinitialized"

The final code snippet above shows that the deinitializers for the Customer instance and CreditCard instance both print their “deinitialized” messages after the john variable is set to nil.

### Unowned References and Implicitly Unwrapped Optional Properties

The examples for weak and unowned references above cover two of the more common scenarios in which it is necessary to break a strong reference cycle.

The Person and Apartment example shows a situation where two properties, both of which are allowed to be nil, have the potential to cause a strong reference cycle. This scenario is best resolved with a weak reference.

The Customer and CreditCard example shows a situation where one property that is allowed to be nil and another property that cannot be nil have the potential to cause a strong reference cycle. This scenario is best resolved with an unowned reference.

However, there is a third scenario, in which both properties should always have a value, and neither property should ever be nil once initialization is complete. In this scenario, it is useful to combine an unowned property on one class with an implicitly unwrapped optional property on the other class.

This enables both properties to be accessed directly (without optional unwrapping) once initialization is complete, while still avoiding a reference cycle. This section shows you how to set up such a relationship.

The example below defines two classes, Country and City, each of which stores an instance of the other class as a property. In this data model, every country must always have a capital city, and every city must always belong to a country. To represent this, the Country class has a capitalCity property, and the City class has a country property:

1. class Country {
2.  let name: String
3.  var capitalCity: City!
4.  init(name: String, capitalName: String) {
5.  self.name = name
6.  self.capitalCity = City(name: capitalName, country: self)
7.  }
8. }
9. 
10. class City {
11.  let name: String
12.  unowned let country: Country
13.  init(name: String, country: Country) {
14.  self.name = name
15.  self.country = country
16.  }
17. }

To set up the interdependency between the two classes, the initializer for City takes a Country instance, and stores this instance in its country property.

The initializer for City is called from within the initializer for Country. However, the initializer for Country cannot pass self to the City initializer until a new Country instance is fully initialized, as described in Two-Phase Initialization.

To cope with this requirement, you declare the capitalCity property of Country as an implicitly unwrapped optional property, indicated by the exclamation mark at the end of its type annotation (City!). This means that the capitalCity property has a default value of nil, like any other optional, but can be accessed without the need to unwrap its value as described in Implicitly Unwrapped Optionals.

Because capitalCity has a default nil value, a new Country instance is considered fully initialized as soon as the Country instance sets its name property within its initializer. This means that the Country initializer can start to reference and pass around the implicit self property as soon as the name property is set. The Country initializer can therefore pass self as one of the parameters for the City initializer when the Country initializer is setting its own capitalCity property.

All of this means that you can create the Country and City instances in a single statement, without creating a strong reference cycle, and the capitalCity property can be accessed directly, without needing to use an exclamation mark to unwrap its optional value:

1. var country = Country(name: "Canada", capitalName: "Ottawa")
2. print("\(country.name)'s capital city is called \(country.capitalCity.name)")
3. // prints "Canada's capital city is called Ottawa"

In the example above, the use of an implicitly unwrapped optional means that all of the two-phase class initializer requirements are satisfied. The capitalCity property can be used and accessed like a non-optional value once initialization is complete, while still avoiding a strong reference cycle.

## Strong Reference Cycles for Closures

You saw above how a strong reference cycle can be created when two class instance properties hold a strong reference to each other. You also saw how to use weak and unowned references to break these strong reference cycles.

A strong reference cycle can also occur if you assign a closure to a property of a class instance, and the body of that closure captures the instance. This capture might occur because the closure’s body accesses a property of the instance, such as self.someProperty, or because the closure calls a method on the instance, such as self.someMethod(). In either case, these accesses cause the closure to “capture” self, creating a strong reference cycle.

This strong reference cycle occurs because closures, like classes, are reference types. When you assign a closure to a property, you are assigning a reference to that closure. In essence, it’s the same problem as above—two strong references are keeping each other alive. However, rather than two class instances, this time it’s a class instance and a closure that are keeping each other alive.

Swift provides an elegant solution to this problem, known as a closure capture list. However, before you learn how to break a strong reference cycle with a closure capture list, it is useful to understand how such a cycle can be caused.

The example below shows how you can create a strong reference cycle when using a closure that references self. This example defines a class called HTMLElement, which provides a simple model for an individual element within an HTML document:

1. class HTMLElement {
2. 
3.  let name: String
4.  let text: String?
5. 
6.  lazy var asHTML: Void -> String = {
7.  if let text = self.text {
8.  return "<\(self.name)>\(text)</\(self.name)>"
9.  } else {
10.  return "<\(self.name) />"
11.  }
12.  }
13. 
14.  init(name: String, text: String? = nil) {
15.  self.name = name
16.  self.text = text
17.  }
18. 
19.  deinit {
20.  print("\(name) is being deinitialized")
21.  }
22. 
23. }

The HTMLElement class defines a name property, which indicates the name of the element, such as "p" for a paragraph element, or "br" for a line break element. HTMLElement also defines an optional text property, which you can set to a string that represents the text to be rendered within that HTML element.

In addition to these two simple properties, the HTMLElement class defines a lazy property called asHTML. This property references a closure that combines name and text into an HTML string fragment. The asHTML property is of type () -> String, or “a function that takes no parameters, and returns a String value”.

By default, the asHTML property is assigned a closure that returns a string representation of an HTML tag. This tag contains the optional text value if it exists, or no text content if text does not exist. For a paragraph element, the closure would return "<p>some text</p>" or "<p />", depending on whether the text property equals "some text" or nil.

The asHTML property is named and used somewhat like an instance method. However, because asHTML is a closure property rather than an instance method, you can replace the default value of the asHTML property with a custom closure, if you want to change the HTML rendering for a particular HTML element.

The HTMLElement class provides a single initializer, which takes a name argument and (if desired) a text argument to initialize a new element. The class also defines a deinitializer, which prints a message to show when an HTMLElement instance is deallocated.

Here’s how you use the HTMLElement class to create and print a new instance:

1. var paragraph: HTMLElement? = HTMLElement(name: "p", text: "hello, world")
2. print(paragraph!.asHTML())
3. // prints "<p>hello, world</p>"

Unfortunately, the HTMLElement class, as written above, creates a strong reference cycle between an HTMLElement instance and the closure used for its default asHTML value. Here’s how the cycle looks:

The instance’s asHTML property holds a strong reference to its closure. However, because the closure refers to self within its body (as a way to reference self.name and self.text), the closure captures self, which means that it holds a strong reference back to the HTMLElement instance. A strong reference cycle is created between the two. (For more information about capturing values in a closure, see Capturing Values.)

If you set the paragraph variable to nil and break its strong reference to the HTMLElement instance, neither the HTMLElement instance nor its closure are deallocated, because of the strong reference cycle:

1. paragraph = nil

Note that the message in the HTMLElement deinitializer is not printed, which shows that the HTMLElement instance is not deallocated.

## Resolving Strong Reference Cycles for Closures

You resolve a strong reference cycle between a closure and a class instance by defining a capture list as part of the closure’s definition. A capture list defines the rules to use when capturing one or more reference types within the closure’s body. As with strong reference cycles between two class instances, you declare each captured reference to be a weak or unowned reference rather than a strong reference. The appropriate choice of weak or unowned depends on the relationships between the different parts of your code.

### Defining a Capture List

Each item in a capture list is a pairing of the weak or unowned keyword with a reference to a class instance (such as self) or a variable initialized with some value (such as delegate = self.delegate!). These pairings are written within a pair of square braces, separated by commas.

Place the capture list before a closure’s parameter list and return type if they are provided:

1. lazy var someClosure: (Int, String) -> String = {
2.  [unowned self, weak delegate = self.delegate!] (index: Int, stringToProcess: String) -> String in
3.  // closure body goes here
4. }

If a closure does not specify a parameter list or return type because they can be inferred from context, place the capture list at the very start of the closure, followed by the in keyword:

1. lazy var someClosure: Void -> String = {
2.  [unowned self, weak delegate = self.delegate!] in
3.  // closure body goes here
4. }

### Weak and Unowned References

Define a capture in a closure as an unowned reference when the closure and the instance it captures will always refer to each other, and will always be deallocated at the same time.

Conversely, define a capture as a weak reference when the captured reference may become nil at some point in the future. Weak references are always of an optional type, and automatically become nil when the instance they reference is deallocated. This enables you to check for their existence within the closure’s body.

An unowned reference is the appropriate capture method to use to resolve the strong reference cycle in the HTMLElement example from earlier. Here’s how you write the HTMLElement class to avoid the cycle:

1. class HTMLElement {
2. 
3.  let name: String
4.  let text: String?
5. 
6.  lazy var asHTML: Void -> String = {
7.  [unowned self] in
8.  if let text = self.text {
9.  return "<\(self.name)>\(text)</\(self.name)>"
10.  } else {
11.  return "<\(self.name) />"
12.  }
13.  }
14. 
15.  init(name: String, text: String? = nil) {
16.  self.name = name
17.  self.text = text
18.  }
19. 
20.  deinit {
21.  print("\(name) is being deinitialized")
22.  }
23. 
24. }

This implementation of HTMLElement is identical to the previous implementation, apart from the addition of a capture list within the asHTML closure. In this case, the capture list is [unowned self], which means “capture self as an unowned reference rather than a strong reference”.

You can create and print an HTMLElement instance as before:

1. var paragraph: HTMLElement? = HTMLElement(name: "p", text: "hello, world")
2. print(paragraph!.asHTML())
3. // prints "<p>hello, world</p>"

Here’s how the references look with the capture list in place:

This time, the capture of self by the closure is an unowned reference, and does not keep a strong hold on the HTMLElement instance it has captured. If you set the strong reference from the paragraph variable to nil, the HTMLElement instance is deallocated, as can be seen from the printing of its deinitializer message in the example below:

1. paragraph = nil
2. // prints "p is being deinitialized"

# Optional Chaining

Optional chaining is a process for querying and calling properties, methods, and subscripts on an optional that might currently be nil. If the optional contains a value, the property, method, or subscript call succeeds; if the optional is nil, the property, method, or subscript call returns nil. Multiple queries can be chained together, and the entire chain fails gracefully if any link in the chain is nil.

## Optional Chaining as an Alternative to Forced Unwrapping

You specify optional chaining by placing a question mark (?) after the optional value on which you wish to call a property, method or subscript if the optional is non-nil. This is very similar to placing an exclamation mark (!) after an optional value to force the unwrapping of its value. The main difference is that optional chaining fails gracefully when the optional is nil, whereas forced unwrapping triggers a runtime error when the optional is nil.

To reflect the fact that optional chaining can be called on a nil value, the result of an optional chaining call is always an optional value, even if the property, method, or subscript you are querying returns a non-optional value. You can use this optional return value to check whether the optional chaining call was successful (the returned optional contains a value), or did not succeed due to a nil value in the chain (the returned optional value is nil).

Specifically, the result of an optional chaining call is of the same type as the expected return value, but wrapped in an optional. A property that normally returns an Int will return an Int? when accessed through optional chaining.

The next several code snippets demonstrate how optional chaining differs from forced unwrapping and enables you to check for success.

First, two classes called Person and Residence are defined:

1. class Person {
2.  var residence: Residence?
3. }
4. 
5. class Residence {
6.  var numberOfRooms = 1
7. }

Residence instances have a single Int property called numberOfRooms, with a default value of 1. Person instances have an optional residence property of type Residence?.

If you create a new Person instance, its residence property is default initialized to nil, by virtue of being optional. In the code below, john has a residence property value of nil:

1. let john = Person()

If you try to access the numberOfRooms property of this person’s residence, by placing an exclamation mark after residence to force the unwrapping of its value, you trigger a runtime error, because there is no residence value to unwrap:

1. let roomCount = john.residence!.numberOfRooms
2. // this triggers a runtime error

The code above succeeds when john.residence has a non-nil value and will set roomCount to an Int value containing the appropriate number of rooms. However, this code always triggers a runtime error when residence is nil, as illustrated above.

Optional chaining provides an alternative way to access the value of numberOfRooms. To use optional chaining, use a question mark in place of the exclamation mark:

1. if let roomCount = john.residence?.numberOfRooms {
2.  print("John's residence has \(roomCount) room(s).")
3. } else {
4.  print("Unable to retrieve the number of rooms.")
5. }
6. // prints "Unable to retrieve the number of rooms."

This tells Swift to “chain” on the optional residence property and to retrieve the value of numberOfRooms if residence exists.

Because the attempt to access numberOfRooms has the potential to fail, the optional chaining attempt returns a value of type Int?, or “optional Int”. When residence is nil, as in the example above, this optional Int will also be nil, to reflect the fact that it was not possible to access numberOfRooms.

Note that this is true even though numberOfRooms is a non-optional Int. The fact that it is queried through an optional chain means that the call to numberOfRooms will always return an Int? instead of an Int.

You can assign a Residence instance to john.residence, so that it no longer has a nil value:

1. john.residence = Residence()

john.residence now contains an actual Residence instance, rather than nil. If you try to access numberOfRooms with the same optional chaining as before, it will now return an Int? that contains the default numberOfRooms value of 1:

1. if let roomCount = john.residence?.numberOfRooms {
2.  print("John's residence has \(roomCount) room(s).")
3. } else {
4.  print("Unable to retrieve the number of rooms.")
5. }
6. // prints "John's residence has 1 room(s)."

## Defining Model Classes for Optional Chaining

You can use optional chaining with calls to properties, methods, and subscripts that are more than one level deep. This enables you to drill down into subproperties within complex models of interrelated types, and to check whether it is possible to access properties, methods, and subscripts on those subproperties.

The code snippets below define four model classes for use in several subsequent examples, including examples of multilevel optional chaining. These classes expand upon the Person and Residence model from above by adding a Room and Address class, with associated properties, methods, and subscripts.

The Person class is defined in the same way as before:

1. class Person {
2.  var residence: Residence?
3. }

The Residence class is more complex than before. This time, the Residence class defines a variable property called rooms, which is initialized with an empty array of type [Room]:

1. class Residence {
2.  var rooms = [Room]()
3.  var numberOfRooms: Int {
4.  return rooms.count
5.  }
6.  subscript(i: Int) -> Room {
7.  get {
8.  return rooms[i]
9.  }
10.  set {
11.  rooms[i] = newValue
12.  }
13.  }
14.  func printNumberOfRooms() {
15.  print("The number of rooms is \(numberOfRooms)")
16.  }
17.  var address: Address?
18. }

Because this version of Residence stores an array of Room instances, its numberOfRooms property is implemented as a computed property, not a stored property. The computed numberOfRooms property simply returns the value of the count property from the rooms array.

As a shortcut to accessing its rooms array, this version of Residence provides a read-write subscript that provides access to the room at the requested index in the rooms array.

This version of Residence also provides a method called printNumberOfRooms, which simply prints the number of rooms in the residence.

Finally, Residence defines an optional property called address, with a type of Address?. The Address class type for this property is defined below.

The Room class used for the rooms array is a simple class with one property called name, and an initializer to set that property to a suitable room name:

1. class Room {
2.  let name: String
3.  init(name: String) { self.name = name }
4. }

The final class in this model is called Address. This class has three optional properties of type String?. The first two properties, buildingName and buildingNumber, are alternative ways to identify a particular building as part of an address. The third property, street, is used to name the street for that address:

1. class Address {
2.  var buildingName: String?
3.  var buildingNumber: String?
4.  var street: String?
5.  func buildingIdentifier() -> String? {
6.  if buildingName != nil {
7.  return buildingName
8.  } else if buildingNumber != nil && street != nil {
9.  return "\(buildingNumber) \(street)"
10.  } else {
11.  return nil
12.  }
13.  }
14. }

The Address class also provides a method called buildingIdentifier(), which has a return type of String?. This method checks the properties of the address and returns buildingName if it has a value, or buildingNumber concatenated with street if both have values, or nil otherwise.

## Accessing Properties Through Optional Chaining

As demonstrated in Optional Chaining as an Alternative to Forced Unwrapping, you can use optional chaining to access a property on an optional value, and to check if that property access is successful.

Use the classes defined above to create a new Person instance, and try to access its numberOfRooms property as before:

1. let john = Person()
2. if let roomCount = john.residence?.numberOfRooms {
3.  print("John's residence has \(roomCount) room(s).")
4. } else {
5.  print("Unable to retrieve the number of rooms.")
6. }
7. // prints "Unable to retrieve the number of rooms."

Because john.residence is nil, this optional chaining call fails in the same way as before.

You can also attempt to set a property’s value through optional chaining:

1. let someAddress = Address()
2. someAddress.buildingNumber = "29"
3. someAddress.street = "Acacia Road"
4. john.residence?.address = someAddress

In this example, the attempt to set the address property of john.residence will fail, because john.residence is currently nil.

## Calling Methods Through Optional Chaining

You can use optional chaining to call a method on an optional value, and to check whether that method call is successful. You can do this even if that method does not define a return value.

The printNumberOfRooms() method on the Residence class prints the current value of numberOfRooms. Here’s how the method looks:

1. func printNumberOfRooms() {
2.  print("The number of rooms is \(numberOfRooms)")
3. <