Welcome to Swift

About Swift

Swift is a new programming language for iOS and OS X 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 and OS X 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:

println("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.

NOTE

For the best experience, open this chapter as a playground in Xcode. Playgrounds allow you to edit the code listings and see the result immediately.

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.

var myVariable = 42 myVariable = 50 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 a 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.

let implicitInteger = 70 let implicitDouble = 70.0 let explicitDouble: Double = 70

EXPERIMENT

Create a constant with an explicit type of Float and a value of 4.

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.

let label = "The width is " let width = 94 let widthLabel = label + String(width)

EXPERIMENT

Try removing the conversion to String from the last line. What error do you get?

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:

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

EXPERIMENT

Use \() to include a floating-point calculation in a string and to include someone’s name in a greeting.

Create arrays and dictionaries using brackets ([]), and access their elements by writing the index or key in brackets.

var shoppingList = ["catfish", "water", "tulips", "blue paint"] shoppingList[1] = "bottle of water"

var occupations = [

"Malcolm": "Captain",

"Kaylee": "Mechanic",

] occupations["Jayne"] = "Public Relations"

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

let emptyArray = String[]() let emptyDictionary = Dictionary<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.

shoppingList = [] // Went shopping and bought everything.

Control Flow

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

let individualScores = [75, 43, 103, 87, 12] var teamScore = 0 for score in individualScores { if score > 50 { teamScore += 3

} else {

teamScore += 1

}

}

Score

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 the value is missing. Write a question mark (?) after the type of a value to mark the value as optional.

var optionalString: String? = "Hello" optionalString == nil

var optionalName: String? = "John Appleseed" var greeting = "Hello!" if let name = optionalName { greeting = "Hello, \(name)"

}

EXPERIMENT

Change optionalName to nil. What greeting do you get? Add an else clause that sets a different greeting if optionalName is nil.

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.

let vegetable = "red pepper" switch vegetable { case "celery":

let vegetableComment = "Add some raisins and make ants on a log." case "cucumber", "watercress":

let vegetableComment = "That would make a good tea sandwich." case let x where x.hasSuffix("pepper"):

let vegetableComment = "Is it a spicy \(x)?" default: vegetableComment = "Everything tastes good in soup."

EXPERIMENT

Try removing the default case. What error do you get?

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.

let interestingNumbers = [

"Prime": [2, 3, 5, 7, 11, 13],

"Fibonacci": [1, 1, 2, 3, 5, 8],

"Square": [1, 4, 9, 16, 25],

] var largest = 0 for (kind, numbers) in interestingNumbers { for number in numbers { if number > largest { largest = number

}

t

EXPERIMENT

Add another variable to keep track of which kind of number was the largest, as well as what that largest number was.

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.

var n = 2 while n < 100 { n = n * 2

} n

var m = 2

do { m = m * 2 e m < 100

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:

var firstForLoop = 0 for i in 0..3 { firstForLoop += i

} firstForLoop

var secondForLoop = 0 for var i = 0; i < 3; ++i { secondForLoop += 1

ndForLoop

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.

func greet(name: String, day: String) -> String { return "Hello \(name), today is \(day)."

} greet("Bob", "Tuesday")

EXPERIMENT

Remove the day parameter. Add a parameter to include today’s lunch special in the greeting.

Use a tuple to return multiple values from a function.

func getGasPrices() -> (Double, Double, Double) { return (3.59, 3.69, 3.79)

} getGasPrices()

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

func sumOf(numbers: Int...) -> Int { var sum = 0 for number in numbers { sum += number

} return sum

} sumOf() sumOf(42, 597, 12)

EXPERIMENT

Write a function that calculates the average of its arguments.

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.

func returnFifteen() -> Int { var y = 10 func add() { y += 5

} add() return y

} returnFifteen()

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

func makeIncrementer() -> (Int -> Int) { func addOne(number: Int) -> Int { return 1 + number

} return addOne

} var increment = makeIncrementer()

increment(7)

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

func hasAnyMatches(list: Int[], condition: Int -> Bool) -> Bool { for item in list { if condition(item) { return true

} } return false

} func lessThanTen(number: Int) -> Bool { turn number < 10

umbers = [20, 19, 7, 12] nyMatches(numbers, lessThanTen)

Functions are actually a special case of closures. 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.

numbers.map({

(number: Int) -> Int in let result = 3 * number return result

})

EXPERIMENT

Rewrite the closure to return zero for all odd numbers.

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. numbers.map({ number in 3 * number })

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.

sort([1, 5, 3, 12, 2]) { $0 > $1 }

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.

class Shape { var numberOfSides = 0 func simpleDescription() -> String { return "A shape with \(numberOfSides) sides." }

}

EXPERIMENT

Add a constant property with let, and add another method that takes an argument.

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.

var shape = Shape() shape.numberOfSides = 7 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.

class NamedShape { var numberOfSides: Int = 0 var name: String

init(name: String) { self.name = name

}

func simpleDescription() -> String { return "A shape with \(numberOfSides) sides."

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.

class Square: NamedShape { var sideLength: Double

init(sideLength: Double, name: String) { self.sideLength = sideLength

super.init(name: name) numberOfSides = 4

}

nc area() -> Double { return sideLength * sideLength erride func simpleDescription() -> String { return "A square with sides of length \(sideLength)." st = Square(sideLength: 5.2, name: "my test square") rea() impleDescription()

EXPERIMENT

Make another subclass of NamedShape called Circle that takes a radius and a name as arguments to its initializer. Implement an area and a describe method on the Circle class.

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

class EquilateralTriangle: NamedShape { var sideLength: Double = 0.0

init(sideLength: Double, name: String) { self.sideLength = sideLength super.init(name: name) numberOfSides = 3

}

r perimeter: Double { t { return 3.0 * sideLength

t { sideLength = newValue / 3.0 erride func simpleDescription() -> String { return "An equilateral triagle with sides of length \(sideLength)." iangle = EquilateralTriangle(sideLength: 3.1, name: "a triangle") le.perimeter le.perimeter = 9.9 le.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.

class TriangleAndSquare { var triangle: EquilateralTriangle { willSet {

square.sideLength = newValue.sideLength

}

} var square: Square {

willSet { triangle.sideLength = newValue.sideLength

(size: Double, name: String) { square = Square(sideLength: size, name: name) triangle = EquilateralTriangle(sideLength: size, name: name) iangleAndSquare = TriangleAndSquare(size: 10, name: "another test shape") leAndSquare.square.sideLength leAndSquare.triangle.sideLength leAndSquare.square = Square(sideLength: 50, name: "larger square") leAndSquare.triangle.sideLength

Methods on classes have one important difference from functions. Parameter names in functions are used only within the function, but parameters names in methods are also used when you call the method (except for the first parameter). By default, a method has the same name for its parameters when you call it and within the method itself. You can specify a second name, which is used inside the method.

class Counter { var count: Int = 0 func incrementBy(amount: Int, numberOfTimes times: Int) { count += amount * times

}

} var counter = Counter() counter.incrementBy(2, numberOfTimes: 7)

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.

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

enum Rank: Int { case Ace = 1 case Two, Three, Four, Five, Six, Seven, Eight, Nine, Ten case Jack, Queen, King func simpleDescription() -> String { switch self { case .Ace:

return "ace" case .Jack: return "jack" case .Queen:

return "queen" case .King:

return "king" default:

return String(self.toRaw())

}

e = Rank.Ace eRawValue = ace.toRaw()

EXPERIMENT

Write a function that compares two Rank values by comparing their raw values.

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 toRaw and fromRaw functions to convert between the raw value and the enumeration value.

if let convertedRank = Rank.fromRaw(3) { let threeDescription = convertedRank.simpleDescription()

}

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.

enum Suit { case Spades, Hearts, Diamonds, Clubs func simpleDescription() -> String { switch self { case .Spades:

return "spades" case .Hearts:

return "hearts" case .Diamonds: return "diamonds" case .Clubs:

return "clubs"

}

arts = Suit.Hearts artsDescription = hearts.simpleDescription()

EXPERIMENT

Add a color method to Suit that returns “black” for spades and clubs, and returns “red” for hearts and diamonds.

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 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.

struct Card { var rank: Rank var suit: Suit func simpleDescription() -> String { return "The \(rank.simpleDescription()) of \(suit.simpleDescription())" }

} let threeOfSpades = Card(rank: .Three, suit: .Spades) let threeOfSpadesDescription = threeOfSpades.simpleDescription()

EXPERIMENT

Add a method to Card that creates a full deck of cards, with one card of each combination of rank and suit.

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.

enum ServerResponse {

case Result(String, String) case Error(String)

}

let success = ServerResponse.Result("6:00 am", "8:09 pm") let failure = ServerResponse.Error("Out of cheese.")

switch success { let .Result(sunrise, sunset):

serverResponse = "Sunrise is at \(sunrise) and sunset is at \(sunset)." let .Error(error): serverResponse = "Failure... \(error)"

EXPERIMENT

Add a third case to ServerResponse and to the switch.

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.

protocol ExampleProtocol { var simpleDescription: String { get } mutating func adjust()

}

Classes, enumerations, and structs can all adopt protocols.

class SimpleClass: ExampleProtocol { var simpleDescription: String = "A very simple class." var anotherProperty: Int = 69105 func adjust() { simpleDescription += " Now 100% adjusted."

}

}

var a = SimpleClass()

a.adjust()

Description = a.simpleDescription

SimpleStructure: ExampleProtocol { r simpleDescription: String = "A simple structure" utating func adjust() { simpleDescription += " (adjusted)"

= SimpleStructure() ust()

Description = b.simpleDescription

EXPERIMENT

Write an enumeration that conforms to this protocol.

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.

extension Int: ExampleProtocol { var simpleDescription: String { return "The number \(self)"

} mutating func adjust() { self += 42

}

}

7.simpleDescription

EXPERIMENT

Write an extension for the Double type that adds an absoluteValue property.

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.

let protocolValue: ExampleProtocol = a protocolValue.simpleDescription

// 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.

func repeat<ItemType>(item: ItemType, times: Int) -> ItemType[] { var result = ItemType[]() for i in 0..times { result += item

} return result

} repeat("knock", 4)

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

// Reimplement the Swift standard library's optional type enum OptionalValue<T> { case None case Some(T)

} var possibleInteger: OptionalValue<Int> = .None 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.

func anyCommonElements <T, U where T: Sequence, U: Sequence, T.GeneratorType.Element: Equatable,

T.GeneratorType.Element == U.GeneratorType.Element> (lhs: T, rhs: U) -> Bool {

for lhsItem in lhs { for rhsItem in rhs { if lhsItem == rhsItem { return true

}

}

} return false

ommonElements([1, 2, 3], [3])

EXPERIMENT

Modify the anyCommonElements function to make a function that returns an array of the elements that any two sequences have in common.

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>.

Language Guide

The Basics

Swift is a new programming language for iOS and OS X 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 two primary collection types, Array 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. These include tuples, which enable you to create and pass around groupings of values. Tuples can 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 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:

let maximumNumberOfLoginAttempts = 10 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:

var x = 0.0, y = 0.0, z = 0.0

NOTE

If a stored value in your code is not going to change, always declare it as a constant with the let keyword. Use variables only for storing values that need to be able to change.

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:

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:

welcomeMessage = "Hello"

NOTE

It is rare that you need to write type annotations in practice. If you provide an initial value for a constant or variable at the point that it is defined, Swift can almost always infer the type to be used for that constant or variable, as described in Type Safety and Type Inference. In the welcomeMessage example above, no initial value is provided, and so the type of the welcomeMessage variable is specified with a type annotation rather than being inferred from an initial value.

Naming Constants and Variables

You can use almost any character you like for constant and variable names, including Unicode characters:

let π = 3.14159

let 你好 = "你好世界" let = "dogcow"

Constant and variable names cannot contain 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.

NOTE

If you need to give a constant or variable the same name as a reserved Swift keyword, you can do so by surrounding the keyword with back ticks (`) when using it as a name. However, you should avoid using keywords as names unless you have absolutely no choice.

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!":

var friendlyWelcome = "Hello!" friendlyWelcome = "Bonjour!"

// 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:

let languageName = "Swift" languageName = "Swift++"

// 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 println function:

println(friendlyWelcome)

// prints "Bonjour!"

println is a global function that prints a value, followed by a line break, to an appropriate output. If you are working in Xcode, for example, println prints its output in Xcode’s

“console” pane. (A second function, print, performs the same task without appending a line break to the end of the value to be printed.)

The println function prints any String value you pass to it:

println("This is a string")

// prints "This is a string"

The println function can print more complex logging messages, in a similar manner to Cocoa’s NSLog function. 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:

println("The current value of friendlyWelcome is \(friendlyWelcome)") // prints "The current value of friendlyWelcome is Bonjour!"

NOTE

All options you can use with string interpolation are described in String Interpolation.

Comments

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 (//):

// this is a comment

You can also write multiline comments, which start with a forward-slash followed by an asterisk (/*) and end with an asterisk followed by a forward-slash (*/):

/* this is also a comment, 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:

/* this is the start of the first multiline comment /* this is the second, nested multiline comment */ 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:

let cat = " "; println(cat)

// 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:

let minValue = UInt8.min // minValue is equal to 0, and is of type UInt8 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:

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:

NOTE

Use UInt only when you specifically need an unsigned integer type with the same size as the platform’s native word size. If this is not the case, Int is preferred, even when the values to be stored are known to be nonnegative. A consistent use of Int for integer values aids code interoperability, avoids the need to convert between different number types, and matches integer type inference, as described in Type Safety and Type Inference.

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:

NOTE

Double has a precision of at least 15 decimal digits, whereas the precision of Float can be as little as 6 decimal digits. The appropriate floating-point type to use depends on the nature and range of values you need to work with in your code.

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:

let meaningOfLife = 42

// 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:

let pi = 3.14159

// 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:

let anotherPi = 3 + 0.14159

// 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:

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

let decimalInteger = 17 let binaryInteger = 0b10001 // 17 in binary notation let octalInteger = 0o21 // 17 in octal notation 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 10^exp:

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

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

let decimalDouble = 12.1875 let exponentDouble = 1.21875e1 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:

let paddedDouble = 000123.456 let oneMillion = 1_000_000 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 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:

let cannotBeNegative: UInt8 = -1

// UInt8 cannot store negative numbers, and so this will report an error let tooBig: Int8 = Int8.max + 1

// Int8 cannot store a number larger than its maximum value,

// 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:

let twoThousand: UInt16 = 2_000 let one: UInt8 = 1 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:

let three = 3 let pointOneFourOneFiveNine = 0.14159 let pi = Double(three) + pointOneFourOneFiveNine

// 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.

The reverse is also true for floating-point to integer conversion, in that an integer type can be initialized with a Double or Float value:

let integerPi = Int(pi)

// 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.

NOTE

The rules for combining numeric constants and variables are different from the rules for numeric literals. The literal value 3 can be added directly to the literal value 0.14159, because number literals do not have an explicit type in and of themselves. Their type is inferred only at the point that they are evaluated by the compiler.

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:

typealias AudioSample = UInt16

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

var maxAmplitudeFound = AudioSample.min

// 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:

let orangesAreOrange = true 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:

if turnipsAreDelicious { println("Mmm, tasty turnips!")

} else { println("Eww, turnips are horrible.")

}

// 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 be substituted for Bool. The following example reports a compile-time error:

let i = 1

if i {

// this example will not compile, and will report an error

}

However, the alternative example below is valid:

let i = 1 if i == 1 {

// this example will compile successfully

}

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.

let http404Error = (404, "Not Found")

// 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:

let (statusCode, statusMessage) = http404Error println("The status code is \(statusCode)")

// prints "The status code is 404" println("The status message is \(statusMessage)")

// 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:

let (justTheStatusCode, _) = http404Error println("The status code is \(justTheStatusCode)")

// prints "The status code is 404"

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

println("The status code is \(http404Error.0)")

// prints "The status code is 404" println("The status message is \(http404Error.1)")

// prints "The status message is Not Found"

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

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:

println("The status code is \(http200Status.statusCode)")

// prints "The status code is 200" println("The status message is \(http200Status.description)") // 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.

NOTE

Tuples are useful for temporary groups of related values. They are not suited to the creation of complex data structures. If your data structure is likely to persist beyond a temporary scope, model it as a class or structure, rather than as a tuple. For more information, see Classes and Structures.

Optionals

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

NOTE

The concept of optionals doesn’t exist in C or Objective-C. The nearest thing in Objective-C is the ability to return nil from a method that would otherwise return an object, with nil meaning “the absence of a valid object.” However, this only works for objects—it doesn’t work for structs, basic C types, or enumeration values. For these types, Objective-C methods typically return a special value (such as NSNotFound) to indicate the absence of a value. This approach assumes that the method’s caller knows there is a special value to test against and remembers to check for it. Swift’s optionals let you indicate the absence of a value for any type at all, without the need for special constants.

Here’s an example. Swift’s String type has a method called toInt, 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 toInt method to try to convert a String into an Int:

let possibleNumber = "123" let convertedNumber = possibleNumber.toInt()

// convertedNumber is inferred to be of type "Int?", or "optional Int"

Because the toInt method 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.)

If Statements and Forced Unwrapping

You can use an if statement to find out whether an optional contains a value. If an optional does have a value, it evaluates to true; if it has no value at all, it evaluates to false.

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:

if convertedNumber { println("\(possibleNumber) has an integer value of \(convertedNumber!)")

} else { println("\(possibleNumber) could not be converted to an integer") }

// prints "123 has an integer value of 123"

For more on the if statement, see Control Flow.

NOTE

Trying to use ! to access a non-existent optional value triggers a runtime error. Always make sure that an optional contains a non-nil value before using ! to force-unwrap its value.

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 optional bindings for the if statement as follows:

}

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

if let actualNumber = possibleNumber.toInt() { println("\(possibleNumber) has an integer value of \(actualNumber)")

} else { println("\(possibleNumber) could not be converted to an integer") }

// prints "123 has an integer value of 123"

This can be read as:

“If the optional Int returned by possibleNumber.toInt 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.

nil

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

var serverResponseCode: Int? = 404

// serverResponseCode contains an actual Int value of 404 serverResponseCode = nil

// serverResponseCode now contains no value

NOTE

nil cannot be used with non-optional constants and variables. If a constant or variable in your code needs to be able to cope with the absence of a value under certain conditions, always declare it as an optional value of the appropriate type.

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

var surveyAnswer: String?

// surveyAnswer is automatically set to nil

NOTE

Swift’s nil is not the same as nil in Objective-C. In Objective-C, nil is a pointer to a non-existent object. In Swift, nil is not a pointer—it is the absence of a value of a certain type. Optionals of any type can be set to nil, not just object types.

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:

let possibleString: String? = "An optional string." println(possibleString!) // requires an exclamation mark to access its value // prints "An optional string."

let assumedString: String! = "An implicitly unwrapped optional string." println(assumedString) // no exclamation mark is needed to access its value

// prints "An implicitly unwrapped optional string."

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.

NOTE

If you try to access an implicitly unwrapped optional when it does not contain a value, you will trigger a runtime error. The result is exactly the same as if you place an exclamation mark after a normal optional that does not contain a value.

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

if assumedString {

println(assumedString)

}

// 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:

if let definiteString = assumedString { println(definiteString)

}

// prints "An implicitly unwrapped optional string."

NOTE

Implicitly unwrapped optionals should not be used when there is a possibility of a variable becoming nil at a later point. Always use a normal optional type if you need to check for a nil value during the lifetime of a variable.

Assertions

Optionals enable you to check for values that may or may not exist, and to write code that copes gracefully with the absence of a value. In some cases, however, it is simply not possible for your code to continue execution if a value does not exist, or if a provided value does not satisfy certain conditions. 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:

let age = -3 assert(age >= 0, "A person's age cannot be less than zero")

// 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.

Assertion messages cannot use string interpolation. The assertion message can be omitted if desired, as in the following example:

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:

See also Subscripts and Functions.

NOTE

Assertions cause your app to terminate and are not a substitute for designing your code in such a way that invalid conditions are unlikely to arise. Nonetheless, in situations where invalid conditions are possible, an assertion is an effective way to ensure that such conditions are highlighted and noticed during development,

before your app is published.

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:

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++).

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

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:

let b = 10 var a = 5 a = b

// 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:

let (x, y) = (1, 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:

if x = y {

// this is not valid, because x = y does not return a value

}

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 + 2 // equals 3

5 - 3 // equals 2

2 * 3 // equals 6

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:

"hello, " + "world" // equals "hello, world"

Two Character values, or one Character value and one String value, can be added together to make a new String value:

let dog: Character = " " let cow: Character = " " let dogCow = dog + cow

// dogCow is equal to " "

See also Concatenating Strings and Characters.

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).

NOTE

The remainder operator (%) is also known as a modulo operator in other languages. However, its behavior in Swift for negative numbers means that it is, strictly speaking, a remainder rather than a modulo operation.

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:

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 × 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 × 2) + 1

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

-9 % 4 // equals -1

Inserting -9 and 4 into the equation yields:

-9 = (4 × -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:

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.

var i = 0

++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:

For example:

var a = 0 let b = ++a

// a and b are now both equal to 1 let c = a++

// 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 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:

let three = 3 let minusThree = -three // minusThree equals -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:

let minusSix = -6 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 (+=):

var a = 1

a += 2

// 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.

NOTE

The compound assignment operators do not return a value. You cannot write let b = a += 2, for example.

This behavior is different from the increment and decrement operators mentioned above.

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

Comparison Operators

Swift supports all standard C comparison operators:

NOTE

Swift also provides two identity operators (=== and !==), which you use to test whether two object references both refer to the same object instance. For more information, see Classes and Structures.

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

1 == 1 // true, because 1 is equal to 1

2 != 1 // true, because 2 is not equal to 1

2 > 1 // true, because 2 is greater than 1

1 < 2 // true, because 1 is less than 2

1 >= 1 // true, because 1 is greater than or equal to 1

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:

let name = "world" if name == "world" {

println("hello, world")

} else { println("I'm sorry \(name), but I don't recognize you")

}

// 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:

if question { answer1 } else { answer2

}

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

let contentHeight = 40

let hasHeader = true let rowHeight = contentHeight + (hasHeader ? 50 : 20)

// rowHeight is equal to 90

The preceding example is shorthand for the code below:

let contentHeight = 40 let hasHeader = true var rowHeight = contentHeight if hasHeader { rowHeight = rowHeight + 50

} else { rowHeight = rowHeight + 20

}

// 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.

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 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:

for index in 1...5 { println("\(index) times 5 is \(index * 5)")

}

// 1 times 5 is 5

// 2 times 5 is 10

// 3 times 5 is 15

// 4 times 5 is 20

// 5 times 5 is 25

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

Half-Closed Range Operator

The half-closed range operator (a..b) defines a range that runs from a to b, but does not include b. It is said to be half-closed because it contains its first value, but not its final value.

Half-closed 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:

let names = ["Anna", "Alex", "Brian", "Jack"] let count = names.count for i in 0..count { println("Person \(i + 1) is called \(names[i])")

}

// Person 1 is called Anna

// Person 2 is called Alex

// Person 3 is called Brian

// 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-closed 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:

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:

let allowedEntry = false if !allowedEntry { println("ACCESS DENIED")

}

// 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:

let enteredDoorCode = true let passedRetinaScan = false if enteredDoorCode && passedRetinaScan { println("Welcome!")

} else { println("ACCESS DENIED")

}

// 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:

let hasDoorKey = false let knowsOverridePassword = true if hasDoorKey || knowsOverridePassword { println("Welcome!")

} else { println("ACCESS DENIED")

}

// prints "Welcome!"

Combining Logical Operators

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

if enteredDoorCode && passedRetinaScan || hasDoorKey || knowsOverridePassword { println("Welcome!")

} else { println("ACCESS DENIED")

}

// 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. It 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 miniexpressions 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:

if (enteredDoorCode && passedRetinaScan) || hasDoorKey || knowsOverridePassword { println("Welcome!")

} else { println("ACCESS DENIED")

}

// 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 similar syntax to C strings. 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.

Strings can also be used 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.

NOTE

Swift’s String type is bridged seamlessly to Foundation’s NSString class. If you are working with the Foundation framework in Cocoa or Cocoa Touch, the entire NSString API is available to call on any String value you create, in addition to the String features described in this chapter. You can also use a String value with any API that requires an NSString instance.

For more information about using String with Foundation and Cocoa, see Using Swift with Cocoa and ObjectiveC.

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 ("").

A string literal can be used to provide an initial value for a constant or variable:

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.

String literals can include the following special characters:

The code below shows an example of each kind of special character. The wiseWords constant contains two escaped double quote characters. The dollarSign, blackHeart, and sparklingHeart constants demonstrate the three different Unicode scalar character formats:

let wiseWords = "\"Imagination is more important than knowledge\" - Einstein"

// "Imagination is more important than knowledge" - Einstein let dollarSign = "\x24" // $, Unicode scalar U+0024 let blackHeart = "\u2665" // ♥, Unicode scalar U+2665 let sparklingHeart = "\U0001F496" // , Unicode scalar U+1F496

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:

var emptyString = "" // empty string literal var anotherEmptyString = String() // initializer syntax

// these two strings are both empty, and are equivalent to each other

You can find out whether a String value is empty by checking its Boolean isEmpty property:

if emptyString.isEmpty { println("Nothing to see here")

}

// 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):

var variableString = "Horse" variableString += " and carriage"

// variableString is now "Horse and carriage"

let constantString = "Highlander" constantString += " and another Highlander"

// this reports a compile-time error - a constant string cannot be modified

NOTE

This approach is different from string mutation in Objective-C and Cocoa, where you choose between two classes (NSString and NSMutableString) to indicate whether a string can be mutated.

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.

NOTE

This behavior differs from that of NSString in Cocoa. When you create an NSString instance in Cocoa, and pass it to a function or method or assign it to a variable, you are always passing or assigning a reference to the same single NSString. No copying of the string takes place, unless you specifically request it.

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

Swift’s String type represents a collection of Character values in a specified order. Each

Character value represents a single Unicode character. You can access the individual Character values in a string by iterating over that string with a for-in loop:

for character in "Dog! " { println(character)

}

// D

// o

// g // ! //

The for-in loop is described in For Loops.

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

let yenSign: Character = "¥"

Counting Characters

To retrieve a count of the characters in a string, call the global countElements function and pass in a string as the function’s sole parameter:

let unusualMenagerie = "Koala , Snail , Penguin , Dromedary "

println("unusualMenagerie has \(countElements(unusualMenagerie)) characters") // prints "unusualMenagerie has 40 characters"

NOTE

Different Unicode characters and different representations of the same Unicode character can require different amounts of memory to store. Because of this, characters in Swift do not each take up the same amount of memory within a string’s representation. As a result, the length of a string cannot be calculated without iterating through the string to consider each of its characters in turn. If you are working with particularly long string values, be aware that the countElements function must iterate over the characters within a string in order to calculate an accurate character count for that string.

Note also that the character count returned by countElements is not always the same as the length property of an NSString that contains the same characters. The length of an NSString is based on the number of 16-bit code units within the string’s UTF-16 representation and not the number of Unicode characters within the string. To reflect this fact, the length property from NSString is called utf16count when it is accessed on a Swift String value.

Concatenating Strings and Characters

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

let string1 = "hello" let string2 = " there" let character1: Character = "!" let character2: Character = "?"

let stringPlusCharacter = string1 + character1 // equals "hello!" let stringPlusString = string1 + string2 // equals "hello there" let characterPlusString = character1 + string1 // equals "!hello" let characterPlusCharacter = character1 + character2 // equals "!?"

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

var instruction = "look over" instruction += string2

// instruction now equals "look over there"

var welcome = "good morning" welcome += character1

// welcome now equals "good morning!"

NOTE

You can’t append a String or Character to an existing Character variable, because a Character value must contain a single character only.

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:

let multiplier = 3 let message = "\(multiplier) times 2.5 is \(Double(multiplier) * 2.5)"

// 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.

NOTE

The expressions you write inside parentheses within an interpolated string cannot contain an unescaped double quote (") or backslash (\), and cannot contain a carriage return or line feed.

Comparing Strings

Swift provides three ways to compare String values: string equality, prefix equality, and suffix equality.

String Equality

Two String values are considered equal if they contain exactly the same characters in the same order:

let quotation = "We're a lot alike, you and I." let sameQuotation = "We're a lot alike, you and I." if quotation == sameQuotation { println("These two strings are considered equal")

}

// prints "These two strings are considered equal"

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. Both methods perform a character-by-character comparison between the base string and the prefix or suffix string.

The examples below consider an array of strings representing the scene locations from the first two acts of Shakespeare’s Romeo and Juliet:

let romeoAndJuliet = [

"Act 1 Scene 1: Verona, A public place",

"Act 1 Scene 2: Capulet's mansion",

"Act 1 Scene 3: A room in Capulet's mansion",

"Act 1 Scene 4: A street outside Capulet's mansion",

"Act 1 Scene 5: The Great Hall in Capulet's mansion",

"Act 2 Scene 1: Outside Capulet's mansion",

"Act 2 Scene 2: Capulet's orchard",

"Act 2 Scene 3: Outside Friar Lawrence's cell", ct 2 Scene 4: A street in Verona", ct 2 Scene 5: Capulet's mansion", ct 2 Scene 6: Friar Lawrence's cell"

You can use the hasPrefix method with the romeoAndJuliet array to count the number of scenes in Act 1 of the play:

var act1SceneCount = 0 for scene in romeoAndJuliet { if scene.hasPrefix("Act 1 ") {

++act1SceneCount

}

} println("There are \(act1SceneCount) scenes in Act 1")

// 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:

var mansionCount = 0 var cellCount = 0 for scene in romeoAndJuliet { if scene.hasSuffix("Capulet's mansion") {

++mansionCount

} else if scene.hasSuffix("Friar Lawrence's cell") {

++cellCount

}

}

("\(mansionCount) mansion scenes; \(cellCount) cell scenes")

nts "6 mansion scenes; 2 cell scenes"

Uppercase and Lowercase Strings

You can access an uppercase or lowercase version of a string with its uppercaseString and lowercaseString properties:

let normal = "Could you help me, please?" let shouty = normal.uppercaseString

// shouty is equal to "COULD YOU HELP ME, PLEASE?" let whispered = normal.lowercaseString

// whispered is equal to "could you help me, please?"

Unicode

Unicode is an international standard for encoding and representing text. 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. They support a number of different Unicode encodings, as described below.

Unicode Terminology

Every character in Unicode can be represented by one or more unicode scalars. A unicode scalar is a unique 21-bit number (and name) for a character or modifier, such as U+0061 for LOWERCASE LATIN LETTER A ("a"), or U+1F425 for FRONT-FACING BABY CHICK (" ").

When a Unicode string is written to a text file or some other storage, these unicode scalars are encoded in one of several Unicode-defined formats. Each format encodes the string in small chunks known as code units. These include the UTF-8 format (which encodes a string as 8-bit code units) and the UTF-16 format (which encodes a string as 16-bit code units).

Unicode Representations of Strings

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 characters. This process is described in Working with Characters.

Alternatively, access a String value in one of three other Unicode-compliant representations:

Each example below shows a different representation of the following string, which is made up of the characters D, o, g, !, and the character ( DOG FACE, or Unicode scalar U+1F436):

let dogString = "Dog! "

UTF-8

You can access a UTF-8 representation of a String by iterating over its utf8 property. This property is of type UTF8View, which is a collection of unsigned 8-bit (UInt8) values, one for each byte in the string’s UTF-8 representation:

for codeUnit in dogString.utf8 { print("\(codeUnit) ")

} print("\n")

// 68 111 103 33 240 159 144 182

In the example above, the first four decimal codeUnit values (68, 111, 103, 33) represent the characters D, o, g, and !, whose UTF-8 representation is the same as their ASCII representation. The last four codeUnit values (240, 159, 144, 182) are a four-byte UTF-8 representation of the DOG FACE character.

UTF-16

You can access a UTF-16 representation of a String by iterating over its utf16 property. This property is of type 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:

for codeUnit in dogString.utf16 { print("\(codeUnit) ")

} print("\n")

// 68 111 103 33 55357 56374

Again, the first four codeUnit values (68, 111, 103, 33) represent the characters D, o, g, and !, whose UTF-16 code units have the same values as in the string’s UTF-8 representation.

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 lead surrogate value of

U+D83D (decimal value 55357) and a trail surrogate value of U+DC36 (decimal value 56374).

Unicode Scalars

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. A Unicode scalar is any 21-bit Unicode code point that is not a lead surrogate or trail surrogate code point.

Each UnicodeScalar has a value property that returns the scalar’s 21-bit value, represented within a UInt32 value:

for scalar in dogString.unicodeScalars { print("\(scalar.value) ")

} print("\n")

// 68 111 103 33 128054

The value properties for the first four UnicodeScalar values (68, 111, 103, 33) once again represent the characters D, o, g, and !. The value property of the fifth and final UnicodeScalar, 128054, is a decimal equivalent of the hexadecimal value 1F436, which is equivalent to 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:

for scalar in dogString.unicodeScalars { println("\(scalar) ")

}

// D

// o

// g // !

//

Collection Types

Swift provides two collection types, known as arrays and dictionaries, for storing collections of values. Arrays store ordered lists of values of the same type. Dictionaries store unordered collections of values of the same type, which can be referenced and looked up through a unique identifier (also known as a key).

Arrays 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 an array or dictionary by mistake. It also means you can be confident about the types of values you will retrieve from an array or dictionary. Swift’s use of explicitly typed collections ensures that your code is always clear about the types of values it can work with and enables you to catch any type mismatches early in your code’s development.

NOTE

Swift’s Array type exhibits different behavior to other types when assigned to a constant or variable, or when passed to a function or method. For more information, see Mutability of Collections and Assignment and Copy Behavior for Collection Types.

Arrays

An array stores multiple values of the same type in an ordered list. The same value can appear in an array multiple times at different positions.

Swift arrays are specific about the kinds of values they can store. They differ from

Objective-C’s NSArray and NSMutableArray classes, which can store any kind of object and do not provide any information about the nature of the objects they return. In Swift, the type of values that a particular array can store is always made clear, either through an explicit type annotation, or through type inference, and does not have to be a class type. If you create an array of Int values, for example, you can’t insert any value other than Int values into that array. Swift arrays are type safe, and are always clear about what they may contain.

Array Type Shorthand Syntax

The type of a Swift array is written in full as Array<SomeType>, where SomeType is the type that the array is allowed to store. You can also write the type of an array in shorthand form as SomeType[]. 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.

Array Literals

You can 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:

[ value 1 , value 2 , value 3 ]

The example below creates an array called shoppingList to store String values:

var shoppingList: String[] = ["Eggs", "Milk"]

// 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 only allowed to store String values. Here, the shoppingList array is initialized with two String values ("Eggs" and "Milk"), written within an array literal.

NOTE

The shoppingList array is declared as a variable (with the var introducer) and not a constant (with the let introducer) because more items are added to the shopping list in the examples below.

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:

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:

println("The shopping list contains \(shoppingList.count) items.")

// 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:

if shoppingList.isEmpty { println("The shopping list is empty.")

} else { println("The shopping list is not empty.")

}

// 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:

shoppingList.append("Flour")

// shoppingList now contains 3 items, and someone is making pancakes

Alternatively, add a new item to the end of an array with the addition assignment operator (+=):

shoppingList += "Baking Powder"

// shoppingList now contains 4 items

You can also append an array of compatible items with the addition assignment operator (+=):

shoppingList += ["Chocolate Spread", "Cheese", "Butter"]

// 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:

var firstItem = shoppingList[0]

// firstItem is equal to "Eggs"

Note that the first item in the array has an index of 0, not 1. Arrays in Swift are always zero-indexed.

You can use subscript syntax to change an existing value at a given index:

shoppingList[0] = "Six eggs"

// 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":

shoppingList[4...6] = ["Bananas", "Apples"]

// shoppingList now contains 6 items

NOTE

You can’t use subscript syntax to append a new item to the end of an array. If you try to use subscript syntax to retrieve or set a value for an index that is outside of an array’s existing bounds, you will trigger a runtime error. However, you can check that an index is valid before using it, by comparing it to the array’s count property. Except when count is 0 (meaning the array is empty), the largest valid index in an array will always be count - 1, because arrays are indexed from zero.

To insert an item into the array at a specified index, call the array’s insert(atIndex:) method:

shoppingList.insert("Maple Syrup", atIndex: 0)

// shoppingList now contains 7 items

// "Maple Syrup" is now the first item in the list

This call to the insert 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):

let mapleSyrup = shoppingList.removeAtIndex(0)

// the item that was at index 0 has just been removed

// shoppingList now contains 6 items, and no Maple Syrup

// 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":

firstItem = shoppingList[0]

// 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:

let apples = shoppingList.removeLast()

// the last item in the array has just been removed

// shoppingList now contains 5 items, and no cheese

// 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:

for item in shoppingList { println(item)

}

// Six eggs

// Milk

// Flour

// Baking Powder

// Bananas

If you need the integer index of each item as well as its value, use the global enumerate function to iterate over the array instead. The enumerate function 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:

for (index, value) in enumerate(shoppingList) { println("Item \(index + 1): \(value)")

}

// Item 1: Six eggs

// Item 2: Milk

// Item 3: Flour

// Item 4: Baking Powder

// Item 5: Bananas

For more about the for-in loop, see For Loops.

Creating and Initializing an Array

You can create an empty array of a certain type (without setting any initial values) using initializer syntax:

var someInts = Int[]() println("someInts is of type Int[] with \(someInts.count) items.")

// prints "someInts is of type Int[] with 0 items."

Note that the type of the someInts variable is inferred to be Int[], because it is set to the output of an Int[] 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):

someInts.append(3)

// someInts now contains 1 value of type Int someInts = []

// someInts is now an empty array, but is still of type Int[]

Swift’s Array type also provides an initializer for creating an array of a certain size with all of its values set to a provided 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):

var threeDoubles = Double[](count: 3, repeatedValue: 0.0)

// threeDoubles is of type Double[], and equals [0.0, 0.0, 0.0]

Thanks to type inference, you don’t need to specify the type to be stored in the array when using this initializer, because it can be inferred from the default value:

var anotherThreeDoubles = Array(count: 3, repeatedValue: 2.5)

// anotherThreeDoubles is inferred as Double[], and equals [2.5, 2.5, 2.5]

Finally, you can create a new array by adding together two existing arrays of compatible type with the addition operator (+). The new array’s type is inferred from the type of the two arrays you add together:

var sixDoubles = threeDoubles + anotherThreeDoubles

// sixDoubles is inferred as Double[], and equals [0.0, 0.0, 0.0, 2.5, 2.5, 2.5]

Dictionaries

A dictionary is a container that stores multiple values of the same type. 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.

Swift dictionaries are specific about the types of keys and values they can store. They differ from Objective-C’s NSDictionary and NSMutableDictionary classes, which can use any kind of object as their keys and values and do not provide any information about the nature of these objects. In Swift, the type of keys and values that a particular dictionary can store is always made clear, either through an explicit type annotation or through type inference.

Swift’s dictionary type is written as Dictionary<KeyType, ValueType>, where KeyType is the type of value that can be used as a dictionary key, and ValueType is the type of value that the dictionary stores for those keys.

The only restriction is that KeyType must be hashable—that is, it must provide a way to make itself uniquely representable. All of Swift’s basic types (such as String, Int, Double, and Bool) are hashable by default, and all of these types can be used as the keys of a dictionary. Enumeration member values without associated values (as described in Enumerations) are also hashable by default.

Dictionary Literals

You can initialize a dictionary with 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:

[ 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:

var airports: Dictionary<String, String> = ["TYO": "Tokyo", "DUB": "Dublin"]

The airports dictionary is declared as having a type of Dictionary<String, String>, which means “a Dictionary whose keys are of type String, and whose values are also of type String”.

NOTE

The airports dictionary is declared as a variable (with the var introducer), and not a constant (with the let introducer), because more airports will be added to the dictionary in the examples below.

The airports dictionary is initialized with a dictionary literal containing two key-value pairs. The first pair has a key of "TYO" and a value of "Tokyo". The second pair has a key of "DUB" and a value of "Dublin".

This dictionary literal contains two String: String pairs. This 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 be written in a shorter form instead:

var airports = ["TYO": "Tokyo", "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 Dictionary<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 can find out the number of items in a Dictionary by checking its read-only count property:

println("The dictionary of airports contains \(airports.count) items.")

// prints "The dictionary of airports contains 2 items."

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:

airports["LHR"] = "London"

// the airports dictionary now contains 3 items

You can also use subscript syntax to change the value associated with a particular key:

airports["LHR"] = "London Heathrow"

// 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:

if let oldValue = airports.updateValue("Dublin International", forKey: "DUB") { println("The old value for DUB was \(oldValue).")

}

// 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:

if let airportName = airports["DUB"] { println("The name of the airport is \(airportName).")

} else { println("That airport is not in the airports dictionary.")

}

// prints "The name of the airport is Dublin International."

You can use subscript syntax to remove a key-value pair from a dictionary by assigning a value of nil for that key:

airports["APL"] = "Apple International"

// "Apple International" is not the real airport for APL, so delete it airports["APL"] = nil

// 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:

if let removedValue = airports.removeValueForKey("DUB") { println("The removed airport's name is \(removedValue).")

} else { println("The airports dictionary does not contain a value for DUB.") }

// prints "The removed airport's name is Dublin International."

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:

for (airportCode, airportName) in airports { println("\(airportCode): \(airportName)")

}

// TYO: Tokyo

// LHR: London Heathrow

For more about the for-in loop, see For Loops.

You can also retrieve an iteratable collection of a dictionary’s keys or values by accessing its keys and values properties:

for airportCode in airports.keys { println("Airport code: \(airportCode)")

}

// Airport code: TYO

// Airport code: LHR

for airportName in airports.values { println("Airport name: \(airportName)")

} port name: Tokyo port 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:

let airportCodes = Array(airports.keys)

// airportCodes is ["TYO", "LHR"]

let airportNames = Array(airports.values)

// airportNames is ["Tokyo", "London Heathrow"]

NOTE

Swift’s Dictionary type is an unordered collection. The order in which keys, values, and key-value pairs are retrieved when iterating over a dictionary is not specified.

Creating an Empty Dictionary

As with arrays, you can create an empty Dictionary of a certain type by using initializer syntax:

var namesOfIntegers = Dictionary<Int, String>()

// namesOfIntegers is an empty Dictionary<Int, String>

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, create an empty dictionary with an empty dictionary literal, which is written as [:] (a colon inside a pair of square brackets):

namesOfIntegers[16] = "sixteen"

// namesOfIntegers now contains 1 key-value pair namesOfIntegers = [:]

// namesOfIntegers is once again an empty dictionary of type Int, String

NOTE

Behind the scenes, Swift’s array and dictionary types are implemented as generic collections. For more on generic types and collections, see Generics.

Mutability of Collections

Arrays and dictionaries store multiple values together in a single collection. If you create an array 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 size of the collection after it is created by adding more items to the collection, or by removing existing items from the ones it already contains. Conversely, if you assign an array or a dictionary to a constant, that array or dictionary is immutable, and its size cannot be changed.

For dictionaries, immutability also means that you cannot replace the value for an existing key in the dictionary. An immutable dictionary’s contents cannot be changed once they are set.

Immutability has a slightly different meaning for arrays, however. You are still not allowed to perform any action that has the potential to change the size of an immutable array, but you are allowed to set a new value for an existing index in the array. This enables Swift’s Array type to provide optimal performance for array operations when the size of an array is fixed.

The mutability behavior of Swift’s Array type also affects how array instances are assigned and modified. For more information, see Assignment and Copy Behavior for Collection Types.

NOTE

It is good practice to create immutable collections in all cases where the collection’s size does not need to change. Doing so enables the Swift compiler to optimize the performance of the collections you create.

Control Flow

Swift provides all the familiar control flow constructs of C-like languages. These include for and while loops to perform a task multiple times; if 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-condition-increment 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 types of pattern, including range 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

A for loop performs a set of statements a certain number of times. Swift provides two kinds of for loop:

for-in performs a set of statements for each item in a range, sequence, collection, or progression.

for-condition-increment 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 collections of items, 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:

for index in 1...5 {

println("\(index) times 5 is \(index * 5)")

}

// 1 times 5 is 5

// 2 times 5 is 10

// 3 times 5 is 15

// 4 times 5 is 20

// 5 times 5 is 25

The collection of items being iterated is a closed 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-timestable 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 println 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.

NOTE

The index constant exists only within the scope of the loop. If you want to check the value of index after the loop completes, or if you want to work with its value as a variable rather than a constant, you must declare it yourself before its use in the loop.

If you don’t need each value from the range, you can ignore the values by using an underscore in place of a variable name:

let base = 3 let power = 10 var answer = 1 for _ in 1...power { answer *= base

} println("\(base) to the power of \(power) is \(answer)") // 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 half-closed loop that starts with 0 and ends with 9. 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 the for-in loop with an array to iterate over its items:

let names = ["Anna", "Alex", "Brian", "Jack"] for name in names { println("Hello, \(name)!")

}

// Hello, Anna!

// Hello, Alex!

// Hello, Brian!

// 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 in 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:

let numberOfLegs = ["spider": 8, "ant": 6, "cat": 4] for (animalName, legCount) in numberOfLegs { println("\(animalName)s have \(legCount) legs")

}

// spiders have 8 legs

// ants have 6 legs

// cats have 4 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.)

In addition to arrays and dictionaries, you can also use the for-in loop to iterate over the Character values in a string:

for character in "Hello" { println(character)

}

// H

// e

// l

// l

// o

For-Condition-Increment

In addition to for-in loops, Swift supports traditional C-style for loops with a condition and an incrementer:

for var index = 0; index < 3; ++index { println("index is \(index)")

}

// index is 0

// index is 1

// index is 2

Here’s the general form of this loop format:

}

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.

The loop format and execution process described above is shorthand for (and equivalent to) the outline below:

initialization

while condition { statements increment

}

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:

var index: Int for index = 0; index < 3; ++index { println("index is \(index)")

}

// index is 0

// index is 1 // index is 2 println("The loop statements were executed \(index) times")

// 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:

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:

while condition { statements

}

This example plays a simple game of Snakes and Ladders (also known as Chutes and Ladders):

The rules of the game are as follows:

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 indices 0 through 25 inclusive):

let finalSquare = 25 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:

board[03] = +08; board[06] = +11; board[09] = +09; board[10] = +02 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:

var square = 0 var diceRoll = 0 while square < finalSquare {

// roll the dice if ++diceRoll == 7 { diceRoll = 1 } // move by the rolled amount square += diceRoll if square < board.count {

// if we're still on the board, move up or down for a snake or a ladder square += board[square]

("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.

Do-While

The other variation of the while loop, known as the do-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 do-while loop:

do { statements

} while condition

Here’s the Snakes and Ladders example again, written as a do-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:

let finalSquare = 25 var board = Int[](count: finalSquare + 1, repeatedValue: 0) board[03] = +08; board[06] = +11; board[09] = +09; board[10] = +02 board[14] = -10; board[19] = -11; board[22] = -02; board[24] = -08 var square = 0 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:

do {

// move up or down for a snake or ladder

square += board[square]

// roll the dice if ++diceRoll == 7 { diceRoll = 1 } // move by the rolled amount square += diceRoll } while square < finalSquare println("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 do-while loop is better suited to this game than the while loop in the previous example. In the do-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:

var temperatureInFahrenheit = 30

if temperatureInFahrenheit <= 32 { println("It's very cold. Consider wearing a scarf.")

}

// 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:

temperatureInFahrenheit = 40 if temperatureInFahrenheit <= 32 { println("It's very cold. Consider wearing a scarf.")

} else { println("It's not that cold. Wear a t-shirt.")

}

// 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:

temperatureInFahrenheit = 90 if temperatureInFahrenheit <= 32 { println("It's very cold. Consider wearing a scarf.")

} else if temperatureInFahrenheit >= 86 { println("It's really warm. Don't forget to wear sunscreen.")

} else { println("It's not that cold. Wear a t-shirt.")

}

// 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:

temperatureInFahrenheit = 72 if temperatureInFahrenheit <= 32 { println("It's very cold. Consider wearing a scarf.")

} else if temperatureInFahrenheit >= 86 { println("It's really warm. Don't forget to wear sunscreen.")

}

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:

otherwise, do something else

}

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:

let someCharacter: Character = "e" switch someCharacter { case "a", "e", "i", "o", "u":

println("\(someCharacter) is a vowel") case "b", "c", "d", "f", "g", "h", "j", "k", "l", "m", "n", "p", "q", "r", "s", "t", "v", "w", "x", "y", "z":

println("\(someCharacter) is a consonant") default: println("\(someCharacter) is not a vowel or a consonant")

nts "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.

NOTE

You can still break out of a matched switch case before that case has completed its execution if you need to.

See Break in a Switch Statement for details.

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:

let anotherCharacter: Character = "a" switch anotherCharacter { case "a": case "A": println("The letter A") default:

println("Not the letter A")

}

// 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:

}

NOTE

To opt in to fallthrough behavior for a particular switch case, use the fallthrough keyword, as described in Fallthrough.

Range Matching

Values in switch cases can be checked for their inclusion in a range. This example uses number ranges to provide a natural-language count for numbers of any size:

let count = 3_000_000_000_000 let countedThings = "stars in the Milky Way" var naturalCount: String switch count { case 0:

naturalCount = "no" case 1...3:

naturalCount = "a few" case 4...9: turalCount = "several" 10...99: turalCount = "tens of"

100...999: turalCount = "hundreds of" 1000...999_999:

turalCount = "thousands of"

lt:

turalCount = "millions and millions of"

("There are \(naturalCount) \(countedThings).")

nts "There are millions and millions of stars in the Milky Way."

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 range 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:

let somePoint = (1, 1) switch somePoint { case (0, 0): println("(0, 0) is at the origin") case (_, 0):

println("(\(somePoint.0), 0) is on the x-axis") case (0, _):

println("(0, \(somePoint.1)) is on the y-axis") case (-2...2, -2...2):

ntln("(\(somePoint.0), \(somePoint.1)) is inside the box")

lt:

ntln("(\(somePoint.0), \(somePoint.1)) is outside of the box")

nts "(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:

let anotherPoint = (2, 0) switch anotherPoint { case (let x, 0):

println("on the x-axis with an x value of \(x)") case (0, let y):

println("on the y-axis with a y value of \(y)") case let (x, y):

println("somewhere else at (\(x), \(y))")

} nts "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 println 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:

let yetAnotherPoint = (1, -1) switch yetAnotherPoint { case let (x, y) where x == y:

println("(\(x), \(y)) is on the line x == y") case let (x, y) where x == -y:

println("(\(x), \(y)) is on the line x == -y") case let (x, y):

println("(\(x), \(y)) is just some arbitrary point")

} nts "(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 point. 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 four control transfer statements:

The control, break and fallthrough statements are described below. The return statement is described in Functions.

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.

NOTE

In a for-condition-increment loop, the incrementer is still evaluated after calling the continue statement. The loop itself continues to work as usual; only the code within the loop’s body is skipped.

The following example removes all vowels and spaces from a lowercase string to create a cryptic puzzle phrase:

let puzzleInput = "great minds think alike" var puzzleOutput = "" for character in puzzleInput { switch character { case "a", "e", "i", "o", "u", " ":

continue default:

puzzleOutput += character

}

(puzzleOutput) nts "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.

NOTE

A switch case that only contains a comment is reported as a compile-time error. Comments are not statements and do not cause a switch case to be ignored. Always use a break statement to ignore a switch case.

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:

let numberSymbol: Character = "三" // Simplified Chinese for the number 3 var possibleIntegerValue: Int?

switch numberSymbol { case "1", "١", "一", "๑":

possibleIntegerValue = 1 case "2", "٢", "二", "๒":

possibleIntegerValue = 2 case "3", "٣", "三", "๓":

possibleIntegerValue = 3

"4", "٤", "四", "๔": ssibleIntegerValue = 4

lt:

eak

ntegerValue = possibleIntegerValue { ntln("The integer value of \(numberSymbol) is \(integerValue).")

{ ntln("An integer value could not be found for \(numberSymbol).")

nts "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 statement 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 caseby-case basis with the fallthrough keyword. The example below uses fallthrough to create a textual description of a number:

let integerToDescribe = 5 var description = "The number \(integerToDescribe) is" switch integerToDescribe { case 2, 3, 5, 7, 11, 13, 17, 19:

description += " a prime number, and also" fallthrough default:

description += " an integer."

}

(description) nts "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 println function. In this example, the number 5 is correctly identified as a prime number.

NOTE

The fallthrough keyword does not check the case conditions for the switch case that it causes execution to fall into. The fallthrough keyword simply causes code execution to move directly to the statements inside the next case (or default case) block, as in C’s standard switch statement behavior.

Labeled Statements

You can nest loops and switch statements inside other loops and switch statements in Swift to create complex control flow structures. However, loops and switch statements can both use the break statement to end their execution prematurely. Therefore, it is sometimes useful to be explicit about which loop or switch 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 switch statement with a statement label, and use this label with the break statement 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:

label name : while condition { statements

}

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:

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:

let finalSquare = 25 var board = Int[](count: finalSquare + 1, repeatedValue: 0) board[03] = +08; board[06] = +11; board[09] = +09; board[10] = +02 board[14] = -10; board[19] = -11; board[22] = -02; board[24] = -08 var square = 0 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:

gameLoop: while square != finalSquare {

if ++diceRoll == 7 { diceRoll = 1 } switch square + diceRoll { case finalSquare:

// diceRoll will move us to the final square, so the game is over break gameLoop case let newSquare where newSquare > finalSquare:

// diceRoll will move us beyond the final square, so roll again continue gameLoop fault:

// this is a valid move, so find out its effect square += diceRoll square += board[square]

("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:

NOTE

If the break statement above did not use the gameLoop label, it would break out of the switch statement, not the while statement. Using the gameLoop label makes it clear which control statement should be terminated.

Note also that it is not strictly necessary to use the gameLoop label when calling continue gameLoop to jump

to the next iteration of the loop. There is only one loop in the game, and so there is no ambiguity as to which loop the continue statement will affect. However, there is no harm in using the gameLoop label with the continue statement. Doing so is consistent with the label’s use alongside the break statement, and helps make the game’s logic clearer to read and understand.

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 greetingForPerson, 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:

func sayHello(personName: String) -> String { let greeting = "Hello, " + personName + "!" return greeting

}

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 elsewhere in your code in a clear and unambiguous way:

println(sayHello("Anna")) // prints "Hello, Anna!" println(sayHello("Brian"))

// 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 println 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:

func sayHelloAgain(personName: String) -> String { return "Hello again, " + personName + "!"

} println(sayHelloAgain("Anna"))

// 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:

func halfOpenRangeLength(start: Int, end: Int) -> Int { return end - start

} println(halfOpenRangeLength(1, 10))

// 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:

func sayHelloWorld() -> String { return "hello, world"

} println(sayHelloWorld())

// 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 Without Return Values

Functions are not required to define a return type. Here’s a version of the sayHello function, called waveGoodbye, which prints its own String value rather than returning it:

func sayGoodbye(personName: String) { println("Goodbye, \(personName)!")

} sayGoodbye("Dave")

// 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.

NOTE

Strictly speaking, the sayGoodbye function does still return a value, even though no return value is defined. Functions without a defined return type return a special value of type Void. This is simply an empty tuple, in effect a tuple with zero elements, which can be written as ().

The return value of a function can be ignored when it is called:

func printAndCount(stringToPrint: String) -> Int { println(stringToPrint) return countElements(stringToPrint)

} func printWithoutCounting(stringToPrint: String) { printAndCount(stringToPrint)

} printAndCount("hello, world")

// prints "hello, world" and returns a value of 12

WithoutCounting("hello, world") nts "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.

NOTE

Return values can be ignored, but a function that says it will return a value must always do so. A function with a defined return type cannot allow control to fall out of the bottom of the function without returning a value, and attempting to do so will result in a compile-time error.

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 count, which counts the number of vowels, consonants, and other characters in a string, based on the standard set of vowels and consonants used in American English:

func count(string: String) -> (vowels: Int, consonants: Int, others: Int) { var vowels = 0, consonants = 0, others = 0 for character in string { switch String(character).lowercaseString { case "a", "e", "i", "o", "u":

++vowels case "b", "c", "d", "f", "g", "h", "j", "k", "l", "m",

"n", "p", "q", "r", "s", "t", "v", "w", "x", "y", "z":

++consonants default:

++others

}

turn (vowels, consonants, others)

You can use this count function to count the characters in an arbitrary string, and to retrieve the counted totals as a tuple of three named Int values:

let total = count("some arbitrary string!") println("\(total.vowels) vowels and \(total.consonants) consonants") // prints "6 vowels and 13 consonants"

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.

Function Parameter Names

All of the above functions define parameter names for their parameters:

func someFunction(parameterName: Int) {

// function body goes here, and can use parameterName

// to refer to the argument value for that parameter

}

However, these parameter names are only used within the body of the function itself, and cannot be used when calling the function. These kinds of parameter names are known as local parameter names, because they are only available for use within the function’s body.

External Parameter Names

Sometimes it’s useful to name each parameter when you call a function, to indicate the purpose of each argument you pass to the function.

If you want users of your function to provide parameter names when they call your function, define an external parameter name for each parameter, in addition to the local parameter name. You write an external parameter name before the local parameter name it supports, separated by a space:

func someFunction(externalParameterName localParameterName: Int) {

// function body goes here, and can use localParameterName

// to refer to the argument value for that parameter

}

NOTE

If you provide an external parameter name for a parameter, that external name must always be used when calling the function.

As an example, consider the following function, which joins two strings by inserting a third “joiner” string between them:

func join(s1: String, s2: String, joiner: String) -> String { return s1 + joiner + s2

}

When you call this function, the purpose of the three strings that you pass to the function is unclear:

join("hello", "world", ", ")

// returns "hello, world"

To make the purpose of these String values clearer, define external parameter names for each join function parameter:

func join(string s1: String, toString s2: String, withJoiner joiner: String)

-> String { return s1 + joiner + s2

}

In this version of the join function, the first parameter has an external name of string and a local name of s1; the second parameter has an external name of toString and a local name of s2; and the third parameter has an external name of withJoiner and a local name of joiner.

You can now use these external parameter names to call the function in a clear and unambiguous way:

join(string: "hello", toString: "world", withJoiner: ", ")

// returns "hello, world"

The use of external parameter names enables this second version of the join function to be called in an expressive, sentence-like manner by users of the function, while still providing a function body that is readable and clear in intent.

NOTE

Consider using external parameter names whenever the purpose of a function’s arguments would be unclear to someone reading your code for the first time. You do not need to specify external parameter names if the purpose of each parameter is clear and unambiguous when the function is called.

Shorthand External Parameter Names

If you want to provide an external parameter name for a function parameter, and the local parameter name is already an appropriate name to use, you do not need to write the same name twice for that parameter. Instead, write the name once, and prefix the name with a hash symbol (#). This tells Swift to use that name as both the local parameter name and the external parameter name.

This example defines a function called containsCharacter, which defines external parameter names for both of its parameters by placing a hash symbol before their local parameter names:

func containsCharacter(#string: String, #characterToFind: Character) -> Bool { for character in string { if character == characterToFind { return true

} } return false

}

This function’s choice of parameter names makes for a clear, readable function body, while also enabling the function to be called without ambiguity:

let containsAVee = containsCharacter(string: "aardvark", characterToFind: "v")

// containsAVee equals true, because "aardvark" contains a "v"

Default Parameter Values

You can define a default value for any parameter as part of a function’s definition. If a default value is defined, you can omit that parameter when calling the function.

NOTE

Place parameters with default values at the end of a function’s parameter list. This ensures that all calls to the function use the same order for their non-default arguments, and makes it clear that the same function is being called in each case.

Here’s a version of the join function from earlier, which provides a default value for its joiner parameter:

func join(string s1: String, toString s2: String, withJoiner joiner: String = " ") -> String { return s1 + joiner + s2

}

If a string value for joiner is provided when the join function is called, that string value is used to join the two strings together, as before:

join(string: "hello", toString: "world", withJoiner: "-")

// returns "hello-world"

However, if no value of joiner is provided when the function is called, the default value of a single space (" ") is used instead:

join(string: "hello", toString: "world")

// returns "hello world"

External Names for Parameters with Default Values

In most cases, it is useful to provide (and therefore require) an external name for any parameter with a default value. This ensures that the argument for that parameter is clear in purpose if a value is provided when the function is called.

To make this process easier, Swift provides an automatic external name for any defaulted parameter you define, if you do not provide an external name yourself. The automatic external name is the same as the local name, as if you had written a hash symbol before the local name in your code.

Here’s a version of the join function from earlier, which does not provide external names for any of its parameters, but still provides a default value for its joiner parameter:

func join(s1: String, s2: String, joiner: String = " ") -> String { return s1 + joiner + s2

}

In this case, Swift automatically provides an external parameter name of joiner for the defaulted parameter. The external name must therefore be provided when calling the function, making the parameter’s purpose clear and unambiguous:

join("hello", "world", joiner: "-")

// returns "hello-world"

NOTE

You can opt out of this behavior by writing an underscore (_) instead of an explicit external name when you define the parameter. However, external names for defaulted parameters are always preferred where appropriate.

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:

func arithmeticMean(numbers: Double...) -> Double { var total: Double = 0 for number in numbers { total += number

}

return total / Double(numbers.count)

} arithmeticMean(1, 2, 3, 4, 5)

// returns 3.0, which is the arithmetic mean of these five numbers meticMean(3, 8, 19) urns 10.0, which is the arithmetic mean of these three numbers

NOTE

A function may have at most one variadic parameter, and it must always appear last in the parameter list, to avoid ambiguity when calling the function with multiple parameters.

If your function has one or more parameters with a default value, and also has a variadic parameter, place the variadic parameter after all the defaulted parameters at the very end of the list.

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:

func alignRight(var string: String, count: Int, pad: Character) -> String { let amountToPad = count - countElements(string) for _ in 1...amountToPad { string = pad + string

} return string

}

let originalString = "hello" let paddedString = alignRight(originalString, 10, "-") ddedString is equal to "-----hello" ginalString is still equal to "hello"

This example defines a new function called alignRight, 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 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. The function then adds amountToPad copies of the pad character to the left of the existing string and returns the result. It uses the string variable parameter for all its string manipulation.

NOTE

The changes you make to a variable parameter do not persist beyond the end of each call to the function, and are not visible outside the function’s body. The variable parameter only exists for the lifetime of that function call.

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.

NOTE

In-out parameters cannot have default values, and variadic parameters cannot be marked as inout. If you mark a parameter as inout, it cannot also be marked as var or let.

Here’s an example of a function called swapTwoInts, which has two in-out integer parameters called a and b:

func swapTwoInts(inout a: Int, inout b: Int) { let temporaryA = a a = b b = temporaryA

}

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:

var someInt = 3 var anotherInt = 107 swapTwoInts(&someInt, &anotherInt) println("someInt is now \(someInt), and anotherInt is now \(anotherInt)")

// 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.

NOTE

In-out parameters are not the same as returning a value from a function. The swapTwoInts example above does not define a return type or return a value, but it still modifies the values of someInt and anotherInt. Inout parameters are an alternative way for a function to have an effect outside of the scope of its function body.

Function Types

Every function has a specific function type, made up of the parameter types and the return type of the function.

For example:

func addTwoInts(a: Int, b: Int) -> Int { return a + b

} func multiplyTwoInts(a: Int, b: Int) -> Int { return a * b

}

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:

func printHelloWorld() { println("hello, world")

}

The type of this function is () -> (), or “a function that has no parameters, and returns Void.” Functions that don’t specify a return value always return Void, which is equivalent to an empty tuple in Swift, shown as ().

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:

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:

println("Result: \(mathFunction(2, 3))")

// 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:

mathFunction = multiplyTwoInts println("Result: \(mathFunction(2, 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:

let anotherMathFunction = addTwoInts

// 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:

func printMathResult(mathFunction: (Int, Int) -> Int, a: Int, b: Int) { println("Result: \(mathFunction(a, b))")

}

printMathResult(addTwoInts, 3, 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:

func stepForward(input: Int) -> Int { return input + 1

} func stepBackward(input: Int) -> Int { return input - 1

}

Here’s a function called chooseStepFunction, whose return type is “a function of type (Int) -> Int”.

chooseStepFunction returns the stepForward function or the stepBackward function based on a Boolean parameter called backwards:

func chooseStepFunction(backwards: Bool) -> (Int) -> Int {

return backwards ? stepBackward : stepForward

}

You can now use chooseStepFunction to obtain a function that will step in one direction or the other:

var currentValue = 3 let moveNearerToZero = chooseStepFunction(currentValue > 0)

// moveNearerToZero now refers to the stepBackward() function

The preceding example works out 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:

println("Counting to zero:") // Counting to zero:

while currentValue != 0 { println("\(currentValue)... ") currentValue = moveNearerToZero(currentValue)

}

println("zero!")

// 3...

// 2...

.

o!

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:

func chooseStepFunction(backwards: Bool) -> (Int) -> Int { func stepForward(input: Int) -> Int { return input + 1 } func stepBackward(input: Int) -> Int { return input - 1 } return backwards ? stepBackward : stepForward