Functions
函数的参数和返回值
没有参数的函数
func sayHelloWorld() -> String {
return "hello, world"
}
print(sayHelloWorld())
// Prints "hello, world"
多个参数的函数
func greet(person: String, alreadyGreeted: Bool) -> String {
if alreadyGreeted {
return greetAgain(person: person)
} else {
return greet(person: person)
}
}
print(greet(person: "Tim", alreadyGreeted: true))
没有返回值的函数
func greet(person: String) {
print("Hello, \(person)!")
}
greet(person: "Dave")
// Prints "Hello, Dave!"
多个返回值的函数
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)
}
返回optional
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)
}
函数的参数标签和参数名
参数标签名，默认为参数名
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)
自定义参数标签名
func someFunction(argumentLabel parameterName: Int) {
// In the function body, parameterName refers to the argument value
// for that parameter.
}
someFunction(argumentLabel: 1)
忽略标签名
使用“_”
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
与Java类似,类型后面加"..."
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
把值类型参数当作引用类型参数使用，inout不能有默认值，也不能在可变参数上使用。
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
func addTwoInts(_ a: Int, _ b: Int) -> Int {
return a + b
}
func multiplyTwoInts(_ a: Int, _ b: Int) -> Int {
return a * b
}
函数addTwoInts和multiplyTwoInts的类型相同的，都是(Int, Int) -> Int
func printHelloWorld() {
print("hello, world")
}
函数printHelloWorld的类型为() -> Void
使用函数类型
在变量或常量中使用
声明(Int, Int) -> Int类型的变量
var mathFunction: (Int, Int) -> Int = addTwoInts
print("Result: \(mathFunction(2, 3))")
// Prints "Result: 5"
mathFunction = multiplyTwoInts
print("Result: \(mathFunction(2, 3))")
// Prints "Result: 6"
声明(Int, Int) -> Int类型的常量
let anotherMathFunction = addTwoInts
// anotherMathFunction is inferred to be of type (Int, Int) -> Int
作为函数的参数使用
声明带(Int, Int) -> Int类型参数的函数或方法
func printMathResult(_ mathFunction: (Int, Int) -> Int, _ a: Int, _ b: Int) {
print("Result: \(mathFunction(a, b))")
}
printMathResult(addTwoInts, 3, 5)
// Prints "Result: 8"
作为函数的返回类型使用
声明返回(Int) -> Int类型的函数或方法
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
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
}