Swift控制流

概览

　　Swift 的条件语句包含if和switch，循环语句包含for-in、for、while和do-while，循环/判断条件不需要括号，但循环/判断体（body）必需括号：

let individualScores = [75, 43, 103, 87, 12]
var teamScore = 0 for score in individualScores {
    if score > 50 {
        teamScore += 3
    } else {
        teamScore += 1
    }
}

可空类型

　　结合if和let，可以方便的处理可空变量（nullable variable）。对于空值，需要在类型声明后添加?显式标明该类型可空。

var optionalString: String? = "Hello"
optionalString == nil

var optionalName: String? = "John Appleseed" var gretting = "Hello!" if let name = optionalName {
    gretting = "Hello, \(name)"
}

灵活的 switch

　　Swift 中的switch支持各种各样的比较操作：

let vegetable = "red pepper" switch vegetable {
case "celery":
    let vegetableComment = "Add some raisins and make ants on a log." case "cucumber", "watercress":
    let vegetableComment = "That would make a good tea sandwich." case let x where x.hasSuffix ("pepper"):
    let vegetableComment = "Is it a spicy \(x)?" default:
    let vegetableComment = "Everything tastes good in soup."
}

其它循环

　　for-in除了遍历数组也可以用来遍历字典：

let interestingNumbers = [
    "Prime": [2, 3, 5, 7, 11, 13],
    "Fibonacci": [1, 1, 2, 3, 5, 8],
    "Square": [1, 4, 9, 16, 25],
]
var largest = 0 for (kind, numbers) in interestingNumbers {
    for number in numbers {
        if number > largest {
            largest = number
        }
    }
}
largest

while循环和do-while循环：

var n = 2 while n < 100 {
    n = n * 2
}
n

var m = 2 do {
    m = m * 2
} while m < 100
m

Swift 支持传统的for循环，此外也可以通过结合..（生成一个区间）和for-in实现同样的逻辑。

var firstForLoop = 0 for i in 0..3 {
    firstForLoop += i
}
firstForLoop

var secondForLoop = 0 for var i = 0; i < 3; ++i {
    secondForLoop += 1
}
secondForLoop

注意：Swift 除了..还有...：..生成前闭后开的区间，而...生成前闭后闭的区间。