控制流

2017-09-20  本文已影响12人  寥寥十一

For-In 循环

输出乘 5 乘法表前面一部分内容:

 for index in 1...5 {
     print("(index) times 5 is (index * 5)")
 }
 // 1 times 5 is 5
 // 2 times 5 is 10
 // 3 times 5 is 15
 // 4 times 5 is 20
 // 5 times 5 is 25

如果你不需要区间序列内每一项的值,你可以使用下划线( _ )替代变量名来忽略这个值:

 let base = 3
 let power = 10
 var answer = 1
 for _ in 1...power {
     answer *= base
 }
print("(base) to the power of (power) is (answer)") 
// 输出 "3 to the power of 10 is 59049"

使用 for-in 遍历一个数组所有元素:

let names = ["Anna", "Alex", "Brian", "Jack"]
for name in names {
    print("Hello, (name)!")
}
// Hello, Anna!
// Hello, Alex!
// Hello, Brian!
// Hello, Jack!

在 for-in 循环中使用显式的常量名称来解读 (key, value) 元组

let numberOfLegs = ["spider": 8, "ant": 6, "cat": 4]
 for (animalName, legCount) in numberOfLegs {
     print("(animalName)s have (legCount) legs")
 }
 // ants have 6 legs
 // cats have 4 legs
 // spiders have 8 legs

While 循环

while 循环的一般格式:

while condition {
statements 
}

repeat-while 循环的一般格式:

repeat {
    statements
} while condition

条件语句

if

temperatureInFahrenheit = 90
if temperatureInFahrenheit <= 32 {
    print("It's very cold. Consider wearing a scarf.")
} else if temperatureInFahrenheit >= 86 {
print("It's really warm. Don't forget to wear sunscreen.")
} else {
     print("It's not that cold. Wear a t-shirt.")
}
// 输出 "It's really warm. Don't forget to wear sunscreen."

Switch

 switch some value to consider {
 case value 1:
     respond to value 1
 case value 2,
value 3:
     respond to value 2 or 3
 default:
     otherwise, do something else
 }

每一个 case 分支都必须包含至少一条语句。
Where
case 分支的模式可以使用 where 语句来判断额外的条件。

let yetAnotherPoint = (1, -1)
 switch yetAnotherPoint {
 case let (x, y) where x == y:
     print("(\(x), \(y)) is on the line x == y")
 case let (x, y) where x == -y:
     print("(\(x), \(y)) is on the line x == -y")
 case let (x, y):
print("(\(x), \(y)) is just some arbitrary point")
}
// 输出 "(1, -1) is on the line x == -y"

控制转移语句

Swift 有五种控制转移语句:

let puzzleInput = "great minds think alike"
var puzzleOutput = ""
for character in puzzleInput.characters {
    switch character {
    case "a", "e", "i", "o", "u", " ":
        continue
    default:
        puzzleOutput.append(character)
    }
}
print(puzzleOutput)
// 输出 "grtmndsthnklk"
let numberSymbol: Character = "三" // 简体中文里的数字 3 var possibleIntegerValue: Int?
switch numberSymbol {
case "1", "?", "一", "?":
possibleIntegerValue = 1 case "2", "?", "二", "?":
possibleIntegerValue = 2 case "3", "?", "三", "?":
possibleIntegerValue = 3 case "4", "?", "四", "?":
     possibleIntegerValue = 4
 default:
break }
 if let integerValue = possibleIntegerValue {
     print("The integer value of \(numberSymbol) is \(integerValue).")
 } else {
     print("An integer value could not be found for \(numberSymbol).")
}
// 输出 "The integer value of 三 is 3."
let integerToDescribe = 5
 var description = "The number \(integerToDescribe) is"
 switch integerToDescribe {
 case 2, 3, 5, 7, 11, 13, 17, 19:
     description += " a prime number, and also"
     fallthrough
 default:
     description += " an integer."
 }
print(description)
// 输出 "The number 5 is a prime number, and also an integer."

提前退出

像 if 语句一样, guard 的执行取决于一个表达式的布尔值。

func greet(person: [String: String]) {
    guard let name = person["name"] else {
return }
    print("Hello \(name)")
    guard let location = person["location"] else {
        print("I hope the weather is nice near you.")
return }
    print("I hope the weather is nice in \(location).")
}
greet(["name": "John"])
// 输出 "Hello John!"
// 输出 "I hope the weather is nice near you." greet(["name": "Jane", "location": "Cupertino"]) // 输出 "Hello Jane!"
// 输出 "I hope the weather is nice in Cupertino."

如果 guard 语句的条件被满足,则继续执行 guard 语句大括号后的代码。将变量或者常量的可选绑定作为guard语句的条件,都可以保护 guard 语句后面的代码。
如果条件不被满足,在 else 分支上的代码就会被执行。

检测 API 可用性

在 if 或 guard 语句中使用 可用性条件(availability condition) 去有条件的执行一段代码,来在运行时判 断调用的API是否可用。编译器使用从可用性条件语句中获取的信息去验证,在这个代码块中调用的 API 是否可 用。

if #available(iOS 10, macOS 10.12, *) {
// 在 iOS 使用 iOS 10 的 API, 在 macOS 使用 macOS 10.12 的 API
} else {
// 使用先前版本的 iOS 和 macOS 的 API
}
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