Swift入门(二)

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函数

函数的定义、参数、返回值

func sayHello(personName: String) -> String {    let greeting = "Hello, " + personName + "!"    return greeting}

可以省略参数和返回值

func sayGoodbye(personName: String) {    println("Goodbye, \(personName)!")}

也可以包含多个参数和多个返回值

func count(string1: String, string2: String) -> (vowels: Int, consonants: Int, others: Int) {    var vowels = 0, consonants = 0, others = 0    for character in string1 {        switch String(character).lowercaseString {        case "a", "e", "i", "o", "u":            ++vowels        case "b", "c", "d", "f", "g", "h", "j", "k", "l", "m",          "n", "p", "q", "r", "s", "t", "v", "w", "x", "y", "z":            ++consonants        default:            ++others        }    }    return (vowels, consonants, others)}

多重返回值返回的是一个元祖,没有返回值时其实返回的是Void,一个空的元祖

参数名

参数名一般只能函数内部使用,但为了使语言表达更清晰,可以定义外部参数名,使用这些外部参数名使代码更易阅读

func join(string s1: String, toString s2: String, withJoiner joiner: String) -> String {    return s1 + joiner + s2}join(string: "hello", toString: "world", withJoiner: ", ")

外部名可以与内部名保持一致

func containsCharacter(#string: String, #characterToFind: Character) -> Bool {    for character in string {        if character == characterToFind {            return true        }    }    return false}let containsAVee = containsCharacter(string: "aardvark", characterToFind: "v")

函数参数可以提供默认值

func join(string s1: String, toString s2: String, withJoiner joiner: String = " ") -> String {    return s1 + joiner + s2}join(string: "hello", toString:"world")// returns "hello world"join(string: "hello", toString: "world", withJoiner: "-")// returns "hello-world"

对提供了默认值的参数,参数名自动表示为内部名和外部名
输入输出参数inout,表示参数可以被函数体修改,并保留给外部继续使用

func swapTwoInts(inout a: Int, inout b: Int) {    let temporaryA = a    a = b    b = temporaryA}

调用时需要在参数前加&

var someInt = 3var anotherInt = 107swapTwoInts(&someInt, &anotherInt)

函数类型

函数类型由参数类型和返回值类型共同决定,类似c语言的函数指针

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 = addTwoIntsprintln("Result: \(mathFunction(2, 3))")mathFunction = multiplyTwoIntsprintln("Result: \(mathFunction(2, 3))")

函数类型与其他类型一样,可以做参数,也可以做返回值

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(currentValue > 0)

函数可以嵌套

func chooseStepFunction(backwards: Bool) -> (Int) -> Int {    func stepForward(input: Int) -> Int { return input + 1 }    func stepBackward(input: Int) -> Int { return input - 1 }    return backwards ? stepBackward : stepForward}

闭包

闭包特性应用在嵌套函数和闭包表达式中,是一种函数式编程特性

闭包表达式

一般形式如下

{ (parameters) -> returnType in    statements}

例如

reversed = sorted(names, { (s1: String, s2: String) -> Bool in    return s1 > s2})// 闭包可以类型推断,所以可以简化为reversed = sorted(names, { s1, s2 in return s1 > s2 } )// 单行表达式可以省略返回的returnreversed = sorted(names, { s1, s2 in s1 > s2 } )// 还可以缩写参数reversed = sorted(names, { $0 > $1 } )

如果闭包函数很长,为了增强可读性,可以讲闭包函数放到()外边紧跟函数调用,

func someFunctionThatTakesAClosure(closure: () -> ()) {    // 函数体部分}// 以下是使用尾随闭包进行函数调用someFunctionThatTakesAClosure() {  // 闭包主体部分}

捕获值

闭包可以在其定义的上下文中捕获常量或变量。 即使定义这些常量和变量的原域已经不存在,闭包仍然可以在闭包函数体内引用和修改这些值。

func makeIncrementor(forIncrement amount: Int) -> () -> Int {    var runningTotal = 0    func incrementor() -> Int {        runningTotal += amount        return runningTotal    }    return incrementor}let incrementByTen = makeIncrementor(forIncrement: 10)incrementByTen()// 返回的值为10incrementByTen()// 返回的值为20incrementByTen()// 返回的值为30
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