精通Swift（3）- 流程控制

一 if关键词

let age = 4

if age >= 22 {

    print("Get married")

} else if age >= 18 {

    print("Being a adult")

} else if age >= 7 {

    print("Go to school")

} else {

    print("Just a child")

}

if后面的条件可以省略小括号 

条件后面的大括号不可以省略

if后面的条件只能是Bool类型

二 while

var num = 5

while num > 0 {

    print("num is \(num)")

    num -= 1 } // 打印了5次

while-repeat

var num = -1

repeat {

    print("num is \(num)") } while num > 0 // 打印了1次

repeat-while相当于C语言中的do-while

这里不用num--，是因为从Swift3开始，去除了自增(++)、自减(--)运算符

for关键字

闭区间运算符:a...b， a <= 取值 <= b

let names = ["Anna", "Alex", "Brian", "Jack"]

for i in 0...3 {

    print(names[i])

} // Anna Alex Brian Jack

---------------------------

// i默认是let，有需要时可以声明为var 

for var i in 1...3 {

i += 5

    print(i)

} // 6 7 8

------------------------

for _ in 1...3 {

    print("for")

} // 打印了3次

-----------------------

let range = 1...3

for i in range {

    print(names[i])

} // Alex Brian Jack

------------------------

let a = 1

var b = 2

for i in a...b {

    print(names[i])

} // Alex Brian

----------------------

for i in a...3 {

    print(names[i])

} // Alex Brian Jack

半开区间运算符:a..

for i in 1..<5 {

    print(i)

} // 1 2 3 4

for – 区间运算符用在数组上

let names = ["Anna", "Alex", "Brian", "Jack"]

for name in names[0...3] {

    print(name)

} // Anna Alex Brian Jack

单侧区间:让区间朝一个方向尽可能的远

for name in names[2...] {

    print(name)

} // Brian Jack

for name in names[...2] {

    print(name)

} // Anna Alex Brian

for name in names[..<2] {

    print(name)

} // Anna Alex

---------------------------

let range = ...5

range.contains(7) // false

range.contains(4) // true

range.contains(-3) // true

区间类型

let range1: ClosedRange = 1...3

let range2: Range = 1..<3

let range3: PartialRangeThrough = ...5

字符、字符串也能使用区间运算符，但默认不能用在for-in中

let stringRange1 = "cc"..."ff" // ClosedRange

stringRange1.contains("cb") // false

stringRange1.contains("dz") // true

stringRange1.contains("fg") // false

let stringRange2 = "a"..."f"

stringRange2.contains("d") // true

stringRange2.contains("h") // false

// \0到~囊括了所有可能要用到的ASCII字符

let characterRange: ClosedRange = "\0"..."~" 

characterRange.contains("G") // true

带间隔的区间值

let hours = 11

let hourInterval = 2

// tickMark的取值:从4开始，累加2，不超过11

for tickMark in stride(from: 4, through: hours, by: hourInterval) {

    print(tickMark)

} // 4 6 8 10

switch

case、default后面不能写大括号{}

var number = 1

switch number {

case 1:

    print("number is 1")

break case 2:

    print("number is 2")

    break

default:

    print("number is other")

break

} // number is 1

默认可以不写break，并不会贯穿到后面的条件

var number = 1

switch number {

case 1:

    print("number is 1")

case 2:

    print("number is 2")

default:

    print("number is other")

} // number is 1

fallthrough

使用fallthrough可以实现贯穿效果

var number = 1

switch number {

case 1:

    print("number is 1")

    fallthrough

case 2:

    print("number is 2")

default:

    print("number is other")

}

// number is 1

// number is 2

switch注意点

switch必须要保证能处理所有情况

var number = 1

switch number {

case 1:

    print("number is 1")

case 2:

    print("number is 2")

}

case、default后面至少要有一条语句,如果不想做任何事，加个break即可

var number = 1

switch number {

case 1:

    print("number is 1")

case 2:

    print("number is 2")

default:

break

}

如果能保证已处理所有情况，也可以不必使用default

enum Answer { case right, wrong }

let answer = Answer.right

switch answer {

case Answer.right:

    print("right")

case Answer.wrong:

    print("wrong")

}

// 由于已确定answer是Ansewer类型，因此可以省略Answer 

switch answer {

case .right:

    print("right")

case .wrong:

    print("wrong")

}

复合条件

switch也支持Character、String类型

let string = "Jack"

switch string {

case "Jack":

    fallthrough

case "Rose":

    print("Right person")

default:

break

} // Right person

--------------------------

let character: Character = "a"

switch character {

case "a", "A":

    print("The letter A")

default:

    print("Not the letter A")

} // The letter A

-------------------------

switch string {

case "Jack", "Rose":

    print("Right person")

default:

break

} // Right person

区间匹配、元组匹配

let count = 62

switch count {

case 0:

    print("none")

case 1..<5:

    print("a few")

case 5..<12:

    print("several")

case 12..<100:

    print("dozens of")

case 100..<1000:

    print("hundreds of")

default:

    print("many")

} // dozens of

-----------------------

let point = (1, 1)

switch point {

case (0, 0):

    print("the origin")

case (_, 0):

    print("on the x-axis")

case (0, _):

    print("on the y-axis")

case (-2...2, -2...2):

    print("inside the box")

default:

print("outside of the box") } // inside the box

可以使用下划线 _ 忽略某个值

关于case匹配问题，属于模式匹配(Pattern Matching)的范畴，以后会再次详细展开讲解

值绑定 必要时let也可以改为var

let point = (2, 0)

switch point {

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))")

} // on the x-axis with an x value of 2

where

let point = (1, -1)

switch point {

case let (x, y) where x == y:

    print("on the line x == y")

case let (x, y) where x == -y:

    print("on the line x == -y")

case let (x, y):

    print("(\(x), \(y)) is just some arbitrary point")

} // on the line x == -y

// 将所有正数加起来

var numbers = [10, 20, -10, -20, 30, -30]

var sum = 0

for num in numbers where num > 0 { // 使用where来过滤num

sum += num }

print(sum) // 60

标签语句

outer: for i in 1...4 {

    for k in 1...4 {

        if k == 3 {

            continue outer

        }

        if i == 3 {

            break outer 

        }

        print("i == \(i), k == \(k)")

    }

}