Swift 语言之函数学习笔记

函数的定义与调用

函数定义:

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

函数调用:

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println(sayHello("Anna"))
// prints "Hello, Anna!"
println(sayHello("Brian"))
// prints "Hello, Brian!"

函数参数和返回值

多个输入形参:

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func halfOpenRangeLength(start: Int, end: Int) -> Int {
return end - start
}
println(halfOpenRangeLength(1, 10))
// prints "9"

无参数函数:

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func sayHelloWorld() -> String {
return "hello, world"
}
println(sayHelloWorld())
// prints "hello, world"

无返回值的函数:

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

多返回值函数:

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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([8, -6, 2, 109, 3, 71])
println("min is \(bounds.min) and max is \(bounds.max)")
// prints "min is -6 and max is 109"

返回类型之可选元组:

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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)
}

if let bounds = minMax([8, -6, 2, 109, 3, 71]) {
println("min is \(bounds.min) and max is \(bounds.max)")
}
// prints "min is -6 and max is 109"

函数参数名称

外部参数名称:

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

join(string: "hello", toString: "world", withJoiner: ", ")
// returns "hello, world"

简写外部参数名称:

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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")
// containsAVee equals true, because "aardvark" contains a "v"

参数默认值:

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

join(string: "hello", toString: "world", withJoiner: "-")
// returns "hello-world"

join(string: "hello", toString: "world")
// returns "hello world"

带默认值的外部参数名(自动提供外部参数名):

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func join(s1: String, s2: String, joiner: String = " ") -> String {
return s1 + joiner + s2
}

join("hello", "world", joiner: "-")
// returns "hello-world"

可变参数:

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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

常量形参和变量形参(var定义变量参数):

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func alignRight(var string: String, count: Int, pad: Character) -> String {
let amountToPad = count - count(string)
if amountToPad < 1 {
return string
}
let padString = String(pad)
for _ in 1...amountToPad {
string = padString + string
}
return string
}

let originalString = "hello"
let paddedString = alignRight(originalString, 10, "-")
// paddedString is equal to "-----hello"
// originalString is still equal to "hello"

输入输出形参:

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

var someInt = 3
var anotherInt = 107
swapTwoInts(&someInt, &anotherInt)
println("someInt is now \(someInt), and anotherInt is now \(anotherInt)")
// prints "someInt is now 107, and anotherInt is now 3"

函数类型

使用函数类型:

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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 = addTwoInts
println("Result: \(mathFunction(2, 3))")
// prints "Result: 5"

mathFunction = multiplyTwoInts
println("Result: \(mathFunction(2, 3))")
// prints "Result: 6"

函数类型作为函数形参:

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func printMathResult(mathFunction: (Int, Int) -> Int, a: Int, b: Int) {
println("Result: \(mathFunction(a, b))")
}
printMathResult(addTwoInts, 3, 5)
// prints "Result: 8"

函数类型作为函数返回值:

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

嵌套函数

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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
}
var currentValue = -4
let moveNearerToZero = chooseStepFunction(currentValue > 0)
// moveNearerToZero now refers to the nested stepForward() function
while currentValue != 0 {
println("\(currentValue)... ")
currentValue = moveNearerToZero(currentValue)
}
println("zero!")
// -4...
// -3...
// -2...
// -1...
// zero!
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