Information in this document is taken (often word-for-word) from Apple's The Swift Programming Language.

Swift code doesn't have a main() function; code written at global scope is used as the entry point for the program.

print("Hello, World!")


Use let to make a constant and var to make a variable.

var someVariable = 42
someVariable = 50
let someConstant = 42

Swift infers the type. If the initial value doesn't provide enough information, specify the type by writing it after the variable, separated by a colon.

let explicitDouble: Double = 70

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


Strings can include values with a backslash before a pair of parentheses.

let apples = 3
let appleSummary = "I have \(apples) apples."

Use three double quotes (""") for strings that take up multiple lines. Indentation at the start of each quotes line is removed, as long as it matches the indentation of the closing quote.


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

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 = [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.

Control Flow

Use if and switch to make conditionals, and use for-in, while, and repeat-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


An optional value either contains a value or contains nil to indicate that a value is missing. Write a question mark (?) after the type of a value to mark the value as optional.

var optionalName: String? = "John Appleseed"

You can use if and let together to work with values that might be missing.

if let name = optionalName {
    print("Hello, \(name)")

Another way to handle optional values is to provide a default value using the ?? operator. If the optional value is missing, the default value is used instead.

let nickName: String? = nil
print("Hi \(nickName ?? "there")!")


let vegetable = "red pepper"
switch vegetable {
case "celery":
    print("Add some raisins and make ants on a log.")
case "cucumber", "watercress":
    print("That would make a good tea sandwich.")
case let x where x.hasSuffix("pepper"):
    print("Is it a spicy \(x)?")
    print("Everything tastes good in soup.")

Notice how let can be used in a pattern to assign the value that matches the pattern to a constant. There's implicit breaks after every case.


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


You can keep an index in a loop by using ..< to make a range of indexes.

var total = 0
for i in 0..<4 {
    total += i

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

Functions and Closures

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

By default, functions use their parameter names as labels for their arguments. Write a custom argument label before the parameter name, or write _ to use no argument label.

func greet(_ person: String, on day: String) -> String {
    return "Hello \(person), today is \(day)."
greet("John", on: "Wednesday")

Note that functions can be nested.


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

func calculateStatistics(scored: [Int]) -> (min: Int, max: Int, sum: Int) {
    var min = scores[0]
    var max = scores[0]
    var sum = 0

    for score in scores {
        if score > max {
            max = score
        } else if score < min {
            min = score
        sum += score

    return (min, max, sum)
let statistics = calculateStatistics(scores: [5, 3, 100, 3, 9])

First-Class Types

Functions are a first-class type. This means that a function can return another function as its value and/or take another function as one of its arguments.

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

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

If a closure's type is already known, you can omit the type of its parameters, its return type, or both. Single statement closures implicitly return the value of their only statement.

let mappedNumbers = 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. When a closure is the only argument to a function, you can omit the parentheses entirely.

let sortedNumbers = numbers.sorted { $0 > $1 }

Objects and Classes

class Shape {
    var numberOfSides = 0 // property declaration
    var name: String

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

    func simpleDescription() -> String {
        return "A shape with \(numberOfSides) sides."
var shape = Shape("Triangle")
shape.numberOfSides = 3
var shapeDescription = shape.simpleDescription()

Notice how self is used to distinguish the name property from the name argument to the initializer. 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. Methods on a subclass that override the superclass's implementation are marked with override (just before func).

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

class EquilateralTriangle: Shape {
    var sideLength: Double = 0.0

    init(sideLength: Double) {
        self.sideLength = sideLength
        super.init(name: "Triangle")
        numberOfSides = 3

    var perimeter: Double {
        get {
            return numberOfSides * sideLength
        set {
            sideLength = newValue / numberOfSides

    override func simpleDescription() -> String {
        return "An equilateral triangle with sides of length \(sideLength)."
var triangle = EquilateralTriangle(sideLength: 3.1)
triangle.perimeter = 9.9

In the setter for perimeter, the new value has the implicit name newValue. You can provide an explicit name in parentheses after set. Properties can also take willSet and didSet blocks. The code you provide is run any time the value changes outside of an initializer.

When working with optional values, you can write ? before operations like methods, properties, and subscripting. If the value before ? 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 optionalShape: Shape? = Shape(name: "Circle")
let description = optionalShape?.simpleDescription()

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 .gueen:
            return "aueen"
        case .king:
            return "king"
            return String(self.rawValue)
let ace = Rank.ace

By default, Swift assigns the raw values starting at zero and incrementing by one each time, but you can change this behavior by explicitly specifying values. In the example above, Ace is explicitly given a raw value of 1, and 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 init?(rawValue:) initializer to make an instance of an enumeration from a raw value. It returns either the enumeration case matching the raw value or nil if there is no matching Rank.

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

The case 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 right after enum Name. Note that values can also be associated with the case.

Use struct to create a structure. Structures support many of the same behaviors as classes, including methods and initializers. One of the most important different 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 threeOfSpaces = Card(rank: .three, suit: .spades)
let threeOfSpacesDescription = threeOfSpades.simpleDescription()

Protocols and Extensions

Use protocol to declare a protocol. Classes, enumerations, and structs can all adopt protocols.

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

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

Error Handling

You represent errors using any type that adopts the Error protocol.

enum PrinterError: Error {
    case outOfPaper
    case noToner
    case onFire

Use throw to throw an error and throws to mark a function that can throw an error.

func send(job: Int, toPrinter printerName: String) throws -> String {
    if printerName == "Never Has Toner" {
        throw PrinterError.noToner
    return "Job sent"

A do-catch can handle errors. Inside the do block, you mark code that can throw an error by writing try in front of it. Inside the catch block, the error is automatically given the name error unless you give it a different name.

do {
    let printerResponse = try send(job: 1040, toPrinter: "Bi Sheng")
} catch {

You can provide multiple catch blocks that handle specific errors. You write a pattern after catch just as you do after case in a switch.

Another way to handle errors is to use try? to convert the result to an optional. If the function throws an error, the specific error is discarded and the result is nil. Otherwise, the result is an optional containing the value that the function returned.

let printerSuccess = try? send(job: 1884, toPrinter: "Mergenthaler")

Use defer to write a block of code that is executed after all other code in the function, just before the function returns. The code is executed regardless of whether the function throws an error.


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

func makeArray<Item>(repeating item: Item, numberOfTimes: Int) -> [Item] {
    var result = [Item]()
    for _ in 0..<numberOfTimes {
    return result
makeArray(repeating: "knock", numberOfTimes: 4)

Use where right before the body 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.