Defining and Calling Functions
func greetAgain(person: String) -> String {
return "Hello again, " + person + "!"
}
print(greetAgain(person: "Anna"))
// Prints "Hello again, Anna!
Function Parameters and Return Values
Functions Without Parameters
func sayHelloWorld() -> String {
return "hello, world"
}
print(sayHelloWorld())
// Prints "hello, world
Functions With Multiple Parameters
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!
Functions Without Return Values
func greet(person: String) {
print("Hello, \(person)!")
}
greet(person: "Dave")
// Prints "Hello, Dave!
实际上，Swift还是返回了一个Void，它是一个空元组，写作()
Functions with Multiple Return Values
使用元组作为返回就可以实现函数返回多个值了。
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
Optional Tuple Return Types
检查是否有值返回
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)
}
Functions With an Implicit Return
函数只有一行的时候，可以不用return，函数会默认返回该行。
func greeting(for person: String) -> String {
"Hello, " + person + "!" // 注意它是没有return的
}
print(greeting(for: "Dave"))
// Prints "Hello, Dave!"
func anotherGreeting(for person: String) -> String {
return "Hello, " + person + "!"
}
print(anotherGreeting(for: "Dave"))
// Prints "Hello, Dave!
Function Argument Labels and Parameter Names
argument label是调用函数的时候使用的。
argument name是实现函数的时候使用的。
默认地，name可以被当作label使用。
Specifying Argument Labels
label在前，name在后。
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.
使用label可以让函数读起来更通顺。
Omitting Argument Labels
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)
有label的时候，那么调用函数时一定要使用它。
Default Parameter Values
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
Variadic Parameters
数量可变的参数。在类型后面添加...表示，也就说接收一个数组。
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
函数的参数默认是常量，是不能被更改的。
但是你可以在类型声明前添加关键字inout表示该参数可以更改，并且在函数外也是有效的。
调用函数的时候，inout参数要前加&。
In-out参数不能有默认值，Variadic不能被标记为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
Function Types
函数的类型由它的参数类型和返回类型决定。
下面两个函数的类型都是(Int, Int) -> Int
func addTwoInts(_ a: Int, _ b: Int) -> Int {
return a + b
}
func multiplyTwoInts(_ a: Int, _ b: Int) -> Int {
return a * b
}
无参数和无返回值的函数类型是() -> Void
Using Function Types
var mathFunction: (Int, Int) -> Int = addTwoInts
print("Result: \(mathFunction(2, 3))")
// Prints "Result: 5
mathFunction = multiplyTwoInts
print("Result: \(mathFunction(2, 3))")
// Prints "Result: 6
let anotherMathFunction = addTwoInts
// anotherMathFunction is inferred to be of type (Int, Int) -> Int
Function Types as Parameter Types
也就是将某一函数作为另一函数的参数。
func printMathResult(_ mathFunction: (Int, Int) -> Int, _ a: Int, _ b: Int) {
print("Result: \(mathFunction(a, b))")
}
printMathResult(addTwoInts, 3, 5)
// Prints "Result: 8
Function Types as Return Types
将函数作为另一函数的返回值
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)
// 现在moveNearerToZero就是相当于函数stepBackward()了
print("Counting to zero:")
// Counting to zero:
while currentValue != 0 {
print("\(currentValue)... ")
currentValue = moveNearerToZero(currentValue)
}
print("zero!")
// 3...
// 2...
// 1...
// zero!
Nested Functions
函数嵌套
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!
一个函数可以返回它的嵌套函数供函数之外使用（是不是晕了？）
