[Swift5.1] 2-流程控制

一、if-else 语句

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循环语句

var num = 5
while num > 0 {
    print("num is \(num)")
    num -= 1
}  // 打印5次

var num = -1
repeat {
    print("num is \(num)")
}while num > 0  //打印1次
// num is -1

• repeat-while 相当于C语言中的do-while
• 这里不用num-- ,因为从swift3开始,去除自增(++) 、 自减(--) 运算符

三、for循环语句

1)区间运算符: a...b 代表 a <= 取值 <= b

let name = ["Anna", "Alex", "Brian", "Jack"]
for i in 0...3 {
    print(names[i])
} //Anna Alex Brian Jack

• 常量存储遍历范围

let name = ["Anna", "Alex", "Brian", "Jack"]
let range = 1...3  //CloseRang<Int>
for i in range {
    print(names[i])
} //Alex Brian Jack

• 常量和变量定义取值范围

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

• i 默认就是let , 有需要时可以声明为var

for var i in 1...3 {
    i += 5
    print(i)
} // 6 7 8

• 循环中用不到 i 可以用_省略

for _ in 1...3 {
    print("for")
} //打印3次

2)半开区间运算符: a...b , a <= 取值 < b

for i in 1..<5 {
    print(i)
} // 1 2 3 4

3) 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: 让区间朝同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)
} // 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) //ture
range.contains(-2) //ture

4) 区间类型

• 闭区间类型 ClosedRange<Int>
• 半开半闭区间类型 Range<Int>
• 单侧区间类型 PartialRangeThrough<Int>

let range1 : ClosedRange<Int> = 1...3 
let range2 : Range<Int> = 1..<3
let range3 : PartialRangeThrough<Int> = ...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<String>
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~囊过了所有可能要用到的ASCII字符

let characterRange:ClosedRange<Character> = "\0"..."~"
characterRange.contains("G") //true

5) 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, to: hours, by: hourInterval) {
    print(tickMark)
} // 4 6 8 10

四、Switch语句

1)常规用法

• case、default后面不能写大括号

var number = 1
switch number {
case 1:
    print(" ")
    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

switch number {
case 1:
    print(" ")
case 2:
    print("number is 2")
default:
    print("number is other")
} // number is 1

2) 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

3) switch注意点

• switch 必须要保证能处理所有情况

• case 、default要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")
    fallthrough
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")
default:
    print("wrong")
}

• 由于已确定answer是Answer类型, 因此可以省略Answer

enum Answer {case right, wrong}
let answer = Answer.right
switch answer {
case .right:
    print("right")
case .wrong:
    print("wrong")
}

4)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

switch string {
case "Jack", "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

5) 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

• 可以使用下划线或略某个值
• 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

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

6) 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 an y value of \(y)")
case let(x , y):
        print("somewhere else at (\(x), \(y))")
} // on the x-axis with an x value of 2

五、where语句

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过滤num
    sum += num
}
print(sum) //60

六、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)")
    }
}