Swift语法基础：17

在Swift中, 也有控制流, 分别是For, For-In, For条件递增, While, Do-While等等, 让我们一起来探讨一下:

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1.For循环

在Swift中提供两种循环, 一种是For-In, 另一种是For条件递增, 先来看第一种:

for index in 1...5 {
    println("(index) times 5 is (index * 5)")
}
// 打印出来的结果: 
//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

如果你不需要知道范围内每一项的值,你可以使用下划线(_)替代变量名来忽略对值的访问:

let base = 3
let power = 10
var answer = 1

for _ in 1...power {
    answer *= base
}
println("(base) to the power of (power) is (answer)")
// 打印出来的结果:3 to the power of 10 is 59049

在前面数组和字典的遍历就已经举过例子了, 所以这里就不再列举了.

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2.For条件递增

在Swift中, 我们也可以使用使用条件递增, 只不过和OC有一些区别, 现在让我们来看看:

for var index = 0; index < 3; ++index {
    println("indx is (index)")
}
// 打印出来的结果: indx is 0
// indx is 1
// indx is 2

其他的基本上都差不多, 知识在Swift中的声明不太一样.

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3.While循环

While循环其实和OC也是差不多的, 让我们来看看例子:

var square = 0
var diceRoll = 0
let finalSquare = 25
var board = [Int](count: finalSquare + 1, repeatedValue: 0)
while square < finalSquare {
    if ++diceRoll == 7{
        diceRoll = 1
    }
    square += diceRoll
    if square < board.count {
        square += board[square]
    }
}
println("(square), (diceRoll), (finalSquare)")
// 打印出来的结果: 27, 3, 25

原理基本上都差不多只是有一些细节需要注意一下.

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4.Do-While循环

在Swift中的Do-While循环里, 和OC中的Do-While差别也不大, 让我们来看看例子:

var a = 20
var b = 10
do {
if ++b == a{
        a = 0
    }
} while a > b
println("(a), (b)")
// 打印出来的结果: 0, 20

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5.条件语句

在Swift中, 也有判断的条件语句, 那就是if, 用法和OC一样, 没区别, 让我们来看看吧:

var temperatureInFahrenheit = 30
if temperatureInFahrenheit <= 32 {
    println("It's very cold. Consider wearing a scarf.")
}
// 打印出来的结果: It's very cold. Consider wearing a scarf.

和OC一样, 用 if 语句来判断条件, 如果条件成立, 那么就会返回true, 否则就是false.

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6.Switch开关

在Swift中的Switch开关非常的好用, 和OC不一样, 在OC中, 如果Switch忘记写break就会报错, 但是在Swift中就不需要写这个东西, 比如:

let someCharacter: Character = "e"
switch someCharacter {
    case "a", "e", "i", "o", "u":
    println("(someCharacter) is a vowel")

    case "b", "c", "d", "f", "g", "h", "j", "k", "l", "m","n", "p", "q", "r", "s", "t", "v", "w", "x", "y", "z":
    println("(someCharacter) is a consonant")

    default:
    println("(someCharacter) is not a vowel or a consonant")
}
// 打印出来的结果: e is a vowel

在Switch中还可以这么使用:

let anotherCharacter: Character = "a"
switch anotherCharacter {
    case "a":
        break
    case "A":
        println("The letter A")
    default:
        println("Not the letter A")
}
// 打印出来的结果: 空的

PS: 在Swift中不存在隐式贯穿, 在Swift中, 一旦case执行完毕后, 就不会继续往下执行, 所以也就不需要break了, 但如果你的case是空的, 那么就必须得写break了, 因为在Swift中, Switch的开关必须得有一个内容.

同样的, 一个case也可以包含多个模式, 比如:

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7.范围匹配

let count = 3_000_000_000_000
let countedThings = "stars in the Milky Way"
var naturalCount: String
switch count {
    case 0:
        naturalCount = "no"
    case 1...3:
        naturalCount = "a few"
    case 4...9:
        naturalCount = "several"
    case 10...99:
        naturalCount = "tens of"
    case 100...999:
        naturalCount = "hundreds of"
    case 1000...999_999:
        naturalCount = "thousands of"
    default:
        naturalCount = "millions and millions of"
    }
println("There are (naturalCount) (countedThings).")
// 打印出来的结果:There are millions and millions of stars in the Milky Way.

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好了, 这次我们就讲到这里, 下次我们继续~