函数

苹果官方指南

苹果官方指南翻译

• 函数名

  描述函数功能，调用函数时使用。

• 定义和调用函数

  func greetAgain(person: String) -> String {    return "Hello again, " + person + "!"}print(greetAgain(person: "Anna"))// Prints "Hello again, Anna!"

  func 关键字，greetAgain 函数名，person 参数标签，String 参数类型，-> String 返回值及其类型， {} 函数功能代码，"Anna" 实际参数

  函数形式参数和返回值

• 无形式参数的函数

  func sayHelloWorld() -> String {    return "hello, world"}print(sayHelloWorld())// prints "hello, world"

• 多形式参数的函数

  func greet(person: String, alreadyGreeted: Bool) -> String {    if alreadyGreeted {        return greetAgain(person: person)    } else {        return greet(person: person)    }}print(greet(person: "Tim", alreadyGreeted: true))// Prints "Hello again, Tim!"

• 无返回值的函数

  func printAndCount(string: String) -> Int {    print(string)    return string.characters.count}func printWithoutCounting(string: String) {    let _ = printAndCount(string: string)}printAndCount(string: "hello, world")// prints "hello, world" and returns a value of 12printWithoutCounting(string: "hello, world")// prints "hello, world" but does not return a value

  严格来说，无返回值函数返回了特殊值Void，如有返回值，需要处理返回值，不然函数出错。

• 多返回值的函数

  func minMax(array: [Int]) -> (min: Int, max: Int) {    var currentMin = array[0]    var currentMax = array[0]    for value in array[1..
        
          currentMax {            currentMax = value        }    }    return (currentMin, currentMax)}let bounds = minMax(array: [8, -6, 2, 109, 3, 71])print("min is \(bounds.min) and max is \(bounds.max)")// Prints "min is -6 and max is 109"
        

• 可选元组的返回类型

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

  函数实际参数标签和形式参数名

• 指定实际参数标签

  func greet(person: String, from hometown: String) -> String {    return "Hello \(person)!  Glad you could visit from \(hometown)."}print(greet(person: "Bill", from: "Cupertino"))// Prints "Hello Bill!  Glad you could visit from Cupertino."

• 省略实际参数标枪

  func someFunction(_ firstParameterName: Int, secondParameterName: Int) {    // In the function body, firstParameterName and secondParameterName    // refer to the argument values for the first and second parameters.}someFunction(1, secondParameterName: 2)

  利用下划线（ _ ）来为这个形式参数代替显式的实际参数标签

• 默认形式参数值

  func someFunction(parameterWithDefault: Int = 12) {    // In the function body, if no arguments are passed to the function    // call, the value of parameterWithDefault is 12.}someFunction(parameterWithDefault: 6) // parameterWithDefault is 6someFunction() // parameterWithDefault is 12

• 可变的形式参数

  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 numbersarithmeticMean(3, 8.25, 18.75)// returns 10.0, which is the arithmetic mean of these three numbers

  形式参数的类型名称后边插入三个点符号（ ...）来书写可变形式参数

• 输入输出形式参数

  func swapTwoInts(_ a: inout Int, _ b: inout Int) {    let temporaryA = a    a = b    b = temporaryA}var someInt = 3var anotherInt = 107swapTwoInts(&someInt, &anotherInt)print("someInt is now \(someInt), and anotherInt is now \(anotherInt)")// prints "someInt is now 107, and anotherInt is now 3"

  在将变量作为实际参数传递给输入输出形式参数的时候，直接在它前边添加一个和符合 ( &) 来明确可以被函数修改。

  函数类型

• 使用函数类型

  func addTwoInts(_ a: Int, _ b: Int) -> Int {    return a + b}func multiplyTwoInts(_ a: Int, _ b: Int) -> Int {    return a * b}var mathFunction: (Int, Int) -> Int = addTwoIntsprint("Result: \(mathFunction(2, 3))")// prints "Result: 5"mathFunction = multiplyTwoIntsprint("Result: \(mathFunction(2, 3))")// prints "Result: 6"

• 函数类型作为形式参数类型

  func printMathResult(_ mathFunction: (Int, Int) -> Int, _ a: Int, _ b: Int) {    print("Result: \(mathFunction(a, b))")}printMathResult(addTwoInts, 3, 5)// Prints "Result: 8"

• 函数类型作为返回类型

  func stepForward(_ input: Int) -> Int {    return input + 1}func stepBackward(_ input: Int) -> Int {    return input - 1}func chooseStepFunction(backwards: Bool) -> (Int) -> Int {    return backwards ? stepBackward : stepForward}var currentValue = 3let moveNearerToZero = chooseStepFunction(backward: currentValue > 0)// moveNearerToZero now refers to the stepBackward() functionprint("Counting to zero:")// Counting to zero:while currentValue != 0 {    print("\(currentValue)... ")    currentValue = moveNearerToZero(currentValue)}print("zero!")// 3...// 2...// 1...// zero!

• 内嵌函数

  func chooseStepFunction(backward: Bool) -> (Int) -> Int {    func stepForward(input: Int) -> Int { return input + 1 }    func stepBackward(input: Int) -> Int { return input - 1 }    return backward ? stepBackward : stepForward}var currentValue = -4let moveNearerToZero = chooseStepFunction(backward: currentValue > 0)// moveNearerToZero now refers to the nested stepForward() functionwhile currentValue != 0 {    print("\(currentValue)... ")    currentValue = moveNearerToZero(currentValue)}print("zero!")// -4...// -3...// -2...// -1...// zero!