Swift语法 Swift5 【02 - 流程控制】

作者: Liwx
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iOS Swift 语法 底层原理 与 内存管理分析专题：【iOS Swift5语法】

00 - 汇编
01 - 基础语法
02 - 流程控制
03 - 函数
04 - 枚举
05 - 可选项
06 - 结构体和类
07 - 闭包
08 - 属性
09 - 方法
10 - 下标
11 - 继承
12 - 初始化器init
13 - 可选项

01-if-else

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
与操作可以用逗号或者&&, 或操作可以用||
逗号可以用来可选项绑定

let age = 28
if age >= 22, age < 30 {
    print("Gat married")
} else if age >= 18 {
    print("Being a adult")
} else if age >= 7 {
    print("Go to school")
} else {
    print("Just a child")
}

条件后面的大括号不可以省略

// 报错: Expected '{' after 'if' condition
if age > 100
    print("age > 100")

if后面的条件只能是Bool类型

if age {    // error: cannot convert value of type 'Int' to expected condition type 'Bool'

}

02-while

Swift3开始,去除了自增(++), 自减(--)运算符

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

repeat-while相当于C语言中的do-while
- repeat-while至少执行1次

var num2 = -1
repeat {
    print("num2 is \(num2)")
} while num2 > 0

03-for

闭区间运算符...

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

print("----------")

let range = 1...3
for i in range {
    print(names[i])
}  // Alex Brian Jack

print("----------")

let a = 1
var b = 2
for i in a...b {
    print(names[i])
}  // Alex Brian

print("----------")
for i in  a...3 {
    print(names[i])
}  // Alex Brian Jack

for 接收参数默认声明为let,可以声明为var

// 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

使用下划线_忽略

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

半开区间运算符

// - 半开区间运算符: a..



04-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


判断是否在区间内

let range = ...5
range.contains(7)   // false
range.contains(4)   // true
range.contains(-3)  // true


05-区间类型

闭区间类型ClosedRange 

// 闭区间
let range1: ClosedRange = 1...3


区间类型Range 

let range2: Range = 1..<3


半开区间类型PartialRangeThrough 

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"    // ClosedRange
stringRange2.contains("d")  // true
stringRange2.contains("h")  // false


字符区间必须指明Character类型, 否则编译器自动推导为String类型

// \0到~包括了所有可能要用到的ASCII字符
// 定义字符区间必须指明Character类型,否则编译器自动推导为String类型
let characterRange: ClosedRange = "\0"..."~"
characterRange.contains("G")    // true


06-带间隔的区间值

 stride定义

///   - start: 从start值开始
///   - through: 依次累加through
///   - stride: 到stride结束,不超过stride
public func stride(from start: T, through end: T, by stride: T.Stride) -> StrideThrough where T : Strideable

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


07-switch

case default后面不能写大括号{}
default后面至少有一条语句,不能写分号,因为分号不是语句。通常没有其他处理就用break

var number = 2
switch number {
case 1:
    print("number is 1")
    break
case 2:
    print("number is 2")
    break
case 3:
    print("number is 3")
    break
default:
    break   // default后面至少有一条语句,不能写分号,因为分号不是语句
    //    print("number is \(number)")
}   // number is 2



 case默认可以不写break, 并不会贯穿到后面的条件

var number1 = 1
switch number1 {
case 1:
    print("number1 is 1")
case 2:
    print("number1 is 2")
default:
    print("number1 is other")
} // number1 is 1


08-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:
    break
}
// number is 1
// number is 2


09-switch注意点

switch必须要保证能处理所有情况

// error: switch must be exhaustive
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Answer类型,因此可以省略Answer
switch answer {
case .right:
    print("right")
case .wrong:
    print("wrong")
}


10-复合条件

 switch也支持Character、String类型

let string = "Jack"
switch string {
case "Jack":
    fallthrough    // 贯穿
case "Rose":
    print("Right person")
default:
    break
} // Right person

// 等价于上面
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


11-区间匹配、元组匹配

swift区间条件匹配 

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_忽略某个值

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


12-值绑定

值绑定就是将相应的值 绑定到常量/变量。值绑定默认为let, 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, let y):
    print("somewhere else at (\(x), \(y))")
}
// on the x-axis with an x value of 2


13-where

 where 条件过滤


 where在switch中使用

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


 where在for-in中使用

// 将所有正数加起来
var numbers = [10, 20, -10, -20, 30, -30]
var sum = 0
for number in numbers where number > 0 {
    sum += number
}
print(sum)  // 60


14-标签语句

使用continue, break 进行标签跳转

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)")
    }
}
// i == 1, k == 1
// i == 1, k == 2
// i == 2, k == 1
// i == 2, k == 2



iOS Swift 语法 底层原理 与 内存管理分析 专题：【iOS Swift5语法】

下一篇: 03 - 函数
 上一篇: 01 - 基础语法

Swift语法 Swift5 【02 - 流程控制】

目录

01-if-else

02-while

03-for

04-for-区间运算符用在数组上

05-区间类型

06-带间隔的区间值

07-switch

08-fallthrough

09-switch注意点

10-复合条件

11-区间匹配、元组匹配

12-值绑定

13-where

14-标签语句

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