Swift5.1 入门 - 流程控制（2）

1. if-else

• if 后面的条件只能是Bool类型;

    func test() {
        let num = 3
        if num > 20 { // 正确 ☑️
            
        }
        
        if num { //错误 ✖️
            
        }
    }

• if 后面的条件可以省略小括号;

• 条件后面的大括号不能省略;

  
  

2. while

• while 条件为true才会执行括号里面的代码；
• repeat-while 相当于c语言中的do-while；

    func test() {
        var num = 3
        while num > 0 {
            print(num)
            num -= 1
        }
        
        var num1 = 3
        repeat {
            print(num1)
            num1 -= 1
        } while num1 > 0
    }

3. for

• 闭区间运算符：a...b ，等价于 a<= i <=b；

    func test() {
        let a = 0
        let b = 3
        
        for i in a...b {  // i 默认是let
            print(i)
        }
        
        for var i in a...b { // 可以将 i 改为变量
            i += 1
            print(i)
        }
        
        let range = 0...3
        for i in range {
            print(i)
        }
        
        // 如果用不着i 可以用 _ 省略
        for _ in 0...3 {
            print("hello")
        }
    }

• 半开区间运算符：a..<b，等价于 a<= i <b；

 for i in 0..<3 {  
     print(i)
 } // 0，1，2

• for - 区间运算符

  let names = ["a", "b", "c", "d"]
  for name in names {
      print(name)
  } // a, b, c, d
        
  for name in names[0...3] {
      print(name)
  } // a, b, c, d

• 单侧区间： 让区间的一侧不做限制

  let names = ["a", "b", "c", "d"]
  for name in names[0...] {
      print(name)
  } // a, b, c, d

  for name in names[...3] {
      print(name)
  } // a, b, c, d

  for name in names[..<3] {
      print(name)
  } // a, b, c

  let range = ...4
  print(range.contains(-1))  // true
  print(range.contains(5))   // false

4. 区间类型

• 闭区间类型：ClosedRange

  let range1: ClosedRange<Int> = 0...3
  for i in range1 {
      print(i)
  } // 0 1 2 3

• 半开区间类型：Range

  let range2: Range<Int> = 0..<3
  for i in range2 {
      print(i)
  }

• 单侧区间类型：PartialRangeThrough

   let range3: PartialRangeThrough<Int> = ...3
   // x<=3

• 字符、字符串也可以使用区间运算符，但是不能用在 for-in 中

  let stringRange = "aa"..."dd"
  print(stringRange.contains("bb")) // true
  print(stringRange.contains("ff")) // false

5. 带间隔的区间值

    func test() {
        let from = 0
        let through = 10
        let by = 2
        
        // 从from 开始， 每次累加by，不超过through
        for i in stride(from: from, through: through, by: by) {
            print(i, separator:" ", terminator: " ")
        } // 0 2 4 6 8 10
    }

---

6. switch

• case、default 后面不能写大括号{}；
• 默认可以不写break，并不会贯穿到后面的条件；
• 可以使用 fallthrough，可以实现贯穿效果；

    func test(age: Int) {
        switch age {
        case 1:
            print(age)
        case 2:
            print(age)
            fallthrough
        case 3:
            print(age)
        default:
            print("default")
        }
    }

• switch 必须要保证能够处理所有情况，否则报错；
  
• case 、default后面至少有一条语句（代码），否则报错；
• 如果不想做任何事，写break就可以；
• 如果已经保证能处理全部的情况，就不需要使用default；
• switch 也支持 String、Character类型；

  var name: String = "aa"
        
  switch name {
  case "aa":
      print(name)
  case "bb":
      print(name)
  default:
      print(name)
  }

• 区间匹配、元组匹配

  let num = 10
  switch num {
  case 1:
      print(num)
  case 2...5:
      print(num)
  case 6...10:
      print(num)
  default:
      print(num)
  }

  let point = (1, 1)
  switch point {
  case (0, 1):
      print(point)
  case (_, 0):
      print(point)
  case (0, _):
      print(point)
  case (-1...2, -1...2):
      print("(-1...2, -1...2)")
  default:
      print(point)
  }
  // (-1...2, -1...2)
  // 使用 _ 来忽略某一项

• switch 值绑定

        let point = (1, 1)
        switch point {
        case (let x, 1):
            print(point)
        
        default:
            print("default point \(point)")
        }
        // (1, 1)

---

7. where

        let point = (1, 1)
        switch point {
        case (let x, let y) where x == y:
            print("x = \(x), y = \(y)")
        default:
            print("default point \(point)")
        }
        // x = 1, y = 1

8. 标签语句

        out: for i in 0...4 {
            for j in 0...4 {
                
                if j == 3 {
                    continue out
                }
                if i == 3 {
                    break out
                }
                
                print("i = \(i), j = \(j)")
            }
        }
        /*
         i = 0, j = 0
         i = 0, j = 1
         i = 0, j = 2
         i = 1, j = 0
         i = 1, j = 1
         i = 1, j = 2
         i = 2, j = 0
         i = 2, j = 1
         i = 2, j = 2
         */