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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!"
func greetAgain(person: String) -> String {
return "Hello again, " + person + "!"
}
print(greetAgain(person: "Anna"))
// Prints "Hello again, Anna!"
函数参数和返回值
没有参数的函数
返回值为字符串的值
func sayHelloWorld() -> String {
return "hello, world"
}
print(sayHelloWorld())
// Prints "hello, world"
具有多个参数的函数
一个参数名person 类型为字符串，另一个alreadyGreeted 类型为bool 返回一个字符串
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!"
没有返回值的函数
func greet(person: String) {
print("Hello, \(person)!")
}
greet(person: "Dave")
// Prints "Hello, Dave!"
由于该函数的定义不需要返回值，因此它不包含返回箭头（->）或返回类型
调用函数时，可以忽略其返回值：
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
具有多个返回值的函数
返回两个返回值，最大最小
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"
可选的元组返回类型
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)
}
if 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"
隐式返回函数
func greeting(for person: String) -> String {
"Hello, " + person + "!"
}
print(greeting(for: "Dave"))
// Prints "Hello, Dave!"
func anotherGreeting(for person: String) -> String {
return "Hello, " + person + "!"
}
print(anotherGreeting(for: "Dave"))
// Prints "Hello, Dave!"
func greeting(for person: String) -> String {
"Hello, " + person + "!"
}
func greeting1(person: String) -> String {
"Hello, " + person + "!"
}
image.png
函数参数标签和参数名称
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)
指定参数标签
func someFunction(argumentLabel parameterName: Int) {
// In the function body, parameterName refers to the argument value
// for that parameter.
}
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."
image.png
在参数前面加from 和for类似 隐式参数
省略参数标签
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(1, secondParameterName: 2)
在参数名前面加_ 的时候 在函数调用的时候 参数名可以省略
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
如果是参数有默认值 那么，调用的时候可以 默认的参数可以 忽略
someFunction(parameterWithoutDefault: 4)
如果都传了参数 值那么 会按照 最新传入的值进行运算
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
求和，numbers: Double... 写法我现在也没搞明白为嘛这么写
输入输出参数
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"
image.png
如果用inout修饰那么 必须要有 &来获取参数的值
传入一个参数，调用函数后，返回一个 “&参数”的值
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
}
函数类型作为参数类型
var mathFunction: (Int, Int) -> Int = addTwoInts
image.png
定义一个方法，打印出来如上是function
mathFunction = multiplyTwoInts
print("Result: \(mathFunction(2, 3))")
// Prints "Result: 6"
函数类型作为返回类型
func printMathResult(_ mathFunction: (Int, Int) -> Int, _ a: Int, _ b: Int) {
print("Result: \(mathFunction(a, b))")
}
printMathResult(addTwoInts, 3, 5)
// Prints "Result: 8"
image.png
函数作为一个参数，
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(backward: Bool) -> (Int) -> Int {
return backward ? 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!
嵌套函数
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!
image.png
返回的类型是函数类型 函数的的返回值的类型是 整形
OK
本期的 Swift-函数就到这里。