定义和调用函数

定义:
func greet(person: String) -> String {
    let greeting = "Hello, " + person + "!"
    return greeting
}
调用:
print(greet(person: "Anna"))
// Prints "Hello, Anna!"
print(greet(person: "Brian"))
// Prints "Hello, Brian!"

没有参数的函数

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

Functions With Multiple Parameters

多参函数

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!"

无返回值函数

func greet(person: String) {
    print("Hello, \(person)!")
}
greet(person: "Dave")
// Prints "Hello, Dave!"

函数的返回值可以忽略

func printAndCount(string: String) -> Int {
    print(string)
    return string.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

多个返回值

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)
}

Optional Tuple Return Types

如果返回值为空的话,再后面加"?"

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)
}

Function Argument Labels and Parameter Names

每一个函数都有函数参数标签和参数名,在调用函数的时候回用到参数标签;默认情况下,参数使用参数名作为参数标签.

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(firstParameterName: 1, secondParameterName: 2)

指定一个函数标签Specifying Argument Labels

from 就是指定的标签,用空格隔开
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."

省略参数标签

使用下划线:
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)

默认参数值

func someFunction(parameterWithoutDefault: Int, parameterWithDefault: Int = 12) {
    // If you omit the second argument when calling this function, then
    // the value of parameterWithDefault is 12 inside the function body.
}
someFunction(parameterWithoutDefault: 3, parameterWithDefault: 6) // parameterWithDefault is 6
someFunction(parameterWithoutDefault: 4) // parameterWithDefault is 12

可变参数 Variadic Parameters

一个可变形参可接受零个或多个指定类型的值,可通过在形参的类型名后边插入三个点符号（...）来编写可变形参。
一个函数最多有一个可变参数;

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

In-Out Parameters

使用 inout 关键字可以再函数内改变变量的值;
inout 参数不能有默认值,并且可变参数不能标记为 inout

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Function Types

每一个函数都有特定的函数类型，由函数的形参类型和返回类型组成。

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

使用函数类型Using Function Types

声明:
var mathFunction: (Int, Int) -> Int = addTwoInts
使用:
print("Result: \(mathFunction(2, 3))")
// Prints "Result: 5"


let anotherMathFunction = addTwoInts
// anotherMathFunction is inferred to be of type (Int, Int) -> Int

函数类型作为参数类型Function Types as Parameter Types

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Function Types as Return Types

函数:
func stepForward(_ input: Int) -> Int {
    return input + 1
}
func stepBackward(_ input: Int) -> Int {
    return input - 1
}

使用函数作为返回类型,返回类型是(Int) -> Int.
func chooseStepFunction(backward: Bool) -> (Int) -> Int {
    return backward ? stepBackward : stepForward
}

调用:
var currentValue = 3
let moveNearerToZero = chooseStepFunction(backward: currentValue > 0)
// moveNearerToZero now refers to the stepBackward() function