Functions and Closures

Functions are fundamental building blocks in Swift. This lesson covers function declaration, parameters, return types, closures, and higher-order functions.

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Function Basics

func greet(person: String) -> String {
    return "Hello, \(person)!"
}

let message = greet(person: "Alice")  // "Hello, Alice!"

Functions Without Return Values

func printGreeting(for name: String) {
    print("Hello, \(name)!")
}

Multiple Return Values with Tuples

func minMax(array: [Int]) -> (min: Int, max: Int)? {
    guard !array.isEmpty else { return nil }
    var currentMin = array[0]
    var currentMax = array[0]
    for value in array[1...] {
        if value < currentMin { currentMin = value }
        if value > currentMax { currentMax = value }
    }
    return (min: currentMin, max: currentMax)
}

if let result = minMax(array: [3, 1, 4, 1, 5, 9]) {
    print("Min: \(result.min), Max: \(result.max)")
}

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Argument Labels and Parameter Names

Swift functions have both an argument label (used when calling) and a parameter name (used inside the function):

func move(from source: String, to destination: String) {
    print("Moving from \(source) to \(destination)")
}
move(from: "London", to: "Paris")

Omitting the Argument Label

Use _ to allow callers to omit the label:

func multiply(_ a: Int, _ b: Int) -> Int {
    return a * b
}
let result = multiply(3, 4)  // no argument labels needed

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Default Parameter Values

func greet(person: String, greeting: String = "Hello") -> String {
    return "\(greeting), \(person)!"
}

greet(person: "Alice")                   // "Hello, Alice!"
greet(person: "Alice", greeting: "Hi")   // "Hi, Alice!"

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Variadic Parameters

func average(_ numbers: Double...) -> Double {
    let total = numbers.reduce(0, +)
    return total / Double(numbers.count)
}

let avg = average(1.0, 2.0, 3.0, 4.0)  // 2.5

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In-Out Parameters

By default, function parameters are constants. Use inout to allow modification:

func swapValues(_ a: inout Int, _ b: inout Int) {
    let temp = a
    a = b
    b = temp
}

var x = 10
var y = 20
swapValues(&x, &y)
print("x: \(x), y: \(y)")  // x: 20, y: 10

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Function Types

Functions in Swift are first-class citizens with their own types:

func add(_ a: Int, _ b: Int) -> Int { return a + b }
func subtract(_ a: Int, _ b: Int) -> Int { return a - b }

// Function type: (Int, Int) -> Int
var operation: (Int, Int) -> Int = add
print(operation(5, 3))  // 8