Swift4--函数,自学笔记

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函数

  • 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 12
    printWithoutCounting(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..<array.count] {
            if value < currentMin {
                currentMin = value
            } else if value > 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..<array.count] {
            if value < currentMin {
                currentMin = value
            } else if value > 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 6
    someFunction() // 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 numbers
    arithmeticMean(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 = 3
    var anotherInt = 107
    swapTwoInts(&someInt, &anotherInt)
    print("someInt is now \(someInt), and anotherInt is now \(anotherInt)")
    // prints "someInt is now 107, and anotherInt is now 3"

    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 = addTwoInts
    
    print("Result: \(mathFunction(2, 3))")
    // prints "Result: 5"
    
    mathFunction = multiplyTwoInts
    print("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 = 3
    let moveNearerToZero = chooseStepFunction(backward: currentValue > 0)
    // moveNearerToZero now refers to the stepBackward() function
    
    print("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 = -4
    let moveNearerToZero = chooseStepFunction(backward: currentValue > 0)
    // moveNearerToZero now refers to the nested stepForward() function
    while currentValue != 0 {
        print("\(currentValue)... ")
        currentValue = moveNearerToZero(currentValue)
    }
    print("zero!")
    // -4...
    // -3...
    // -2...
    // -1...
    // zero!

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