Swift 流程控制

一.for循环

for－in

实例1：

       for index in 1...5{
            print("\(index)times 5 is \(index * 5)")
        }

结果：

1times 5 is 5
2times 5 is 10
3times 5 is 15
4times 5 is 20
5times 5 is 25

1...5表示：［1，5］大于等于1且小于等于5

如果括号内不使用index，可以用_代替：

 var a = 5
        for _ in 1...5{
            a += 1
        }
        print("\(a)")

结果：

遍历数组：

 let nameArr = ["Lili","Marry","xiaoming"]
        for name in nameArr {
            print("Hello,\(name)!")
        }

结果：

Hello,Lili!
Hello,Marry!
Hello,xiaoming!

遍历字典：

  let ageDict = ["Lili":9,"Marry":10,"xiaoming":12]
        for (name,age) in ageDict {
            print("\(name) is \(age)!")
        }

结果：

Marry is 10!
Lili is 9!
xiaoming is 12!

由于字典是无序的，所以结果不一定按顺序输出。

二。while循环

语法：

    while condition {

        statements

    }

举例：

   //唉，举例无能
        var a = 10
        while a>0 {
            a = a-2
        }
        print(a)

结果：0

原来的do...while循环在此处替换成了repeat...while循环。（不举例了）

三。switch

实例：

        let someCharacter: Character = "z"
        switch someCharacter {
        case "a":
            print("The first letter of the alphabet")
        case "z":
            print("The last letter of the alphabet")
        default:
            print("Some other character")
        }

结果：

The first letter of the alphabet

＊注意：与C和Objective_C不同，3.0的swift中switch语句的case中不再需要使用break跳出switch语句，匹配条件后即跳出Switch语句，同时，switch中的default是必须的。

虽然break在switch中不再是必须的，但是由于switch中的case／default中语句不允许有空语句，此时，我们可以使用break跳出语句。

例如：

    let anotherCharacter: Character = "a"
        switch anotherCharacter {
        case "a": // Invalid,the case has an empty body
        case "A":
            print("The letter A")
        default:
            print("Not the letter A")
        }
        if let integerValue = possibleIntegerValue {
            print("The integer value of \(numberSymbol) is \(integerValue).")
        } else {
            print("An integer value Could not be found for \(numberSymbol).")
        }

case "a":是无效的，这将导致程序报错： 'case' label in a 'switch' should have at least one executable statement。

再看下面这个例子

  let numberSymbol: Character = "三"  // Chinese symbol for the number 3
        var possibleIntegerValue: Int?
        switch numberSymbol {
        case "1","١","一","๑":
            possibleIntegerValue = 1
        case "2","٢","二","๒":
            possibleIntegerValue = 2
        case "3","٣","三","๓":
            possibleIntegerValue = 3
        case "4","٤","四","๔":
            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).")
        }
        // Prints "The integer value of 三 is 3."

此时，default中的可能性很多，我们可能不再需要设置一个possibleIntegerValue的值，此时就可以使用break跳出switch语句。

间隔匹配：

例如：

 let grade = 67
        switch grade {
        case 0...59:
            print("不及格")
        case 60...79:
            print("及格")
        case 80...89:
            print("良好")
        case 90...100:
            print("优秀")
        default:
            break
        }
//输出：及格

元祖（Tuples）匹配：

    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")
    }
    // Prints "(1,1) is inside the Box"

下划线（_）也称为通配符，可以匹配任何可能的值。

值绑定：

    let anotherPoint = (2,0)
    switch anotherPoint {
    case (let x,0):
        print("on the x-axis with an x value of \(x)")
    case (0,let y):
        print("on the y-axis with a y value of \(y)")
    case let (x,y):
        print("somewhere else at (\(x),\(y))")
    }
    // Prints "on the x-axis with an x value of 2"

where：Switch中还可以使用where来设置附加条件

例如：

    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")
    }
    // Prints "(1,-1) is on the line x == -y"

控制转移语句：

1.continue；

2.break（不再重复说明）;

3.fallthrough;

4.return.

5.throw。

continue:跳出当前循环，执行下一次循环。

例如：

    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)
    // Prints "grtmndsthnklk"

fallthrough:条件匹配成功后，继续执行后面的语句。

例如：

    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)
    // Prints "The number 5 is a prime number,and also an integer."

return：跳出整个循环与语句。

Swift 流程控制

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