Swift控制语句(if,switch,forin,for,while,dowhile)

在代码业务中，条件判断是必不可少的，控制流程的语句在每种语言中都是差不多的。swift包括了：

if,switch,for-in,for,while,do-while

if 条件语句 (if else) (if... else if ... else)

判断条件为true时执行相关代码。例如：

var a = 0

if a > 0 {

println("a > 0")

}else if a == 0{

println("a = 0")

}else{

println("a < 0")

}

switch switch语句会尝试把某个值与若干个模式（pattern）进行匹配。根据第一个匹配成功的模式，switch语句会执行对应的代码

例如：

 let someCharacter: Character = "e"
        switch someCharacter {
        case "a", "e", "i", "o", "u":
            print("(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":
            print("(someCharacter) is a consonant")
        default:
            print("(someCharacter) is not a vowel or a consonant")
        }

switch有几个注意事项：

1、case语句下必须包含至少一行代码

2、case语句下可以不包含break，它不存在隐式的贯穿

3、case可以进行区间匹配

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"
        }
        print("There are \(naturalCount) \(countedThings).")

      4、元组 你可以使用元组在同一个switch语句中测试多个值。元组中的元素可以是值，也可以是区间。另外，使用下划线（_）来匹配所有可能的值

例如：

let somePoint = (1, 1)
        switch somePoint {
        case (0, 0):
            print("(0, 0) is at the origin")
        case (_, 0):
            print("((somePoint.0), 0) is on the x-axis")
        case (0, _):
            print("(0, (somePoint.1)) is on the y-axis")
        case (-2...2, -2...2):
            print("((somePoint.0), (somePoint.1)) is inside the box")
        default:
            print("((somePoint.0), (somePoint.1)) is outside of the box")
        }

5、where条件判断

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")
        }