流程控制

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

  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后面的条件只能是Bool类型

while

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

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

• 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..<b， a <= 取值 < b

  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

• 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 range1: ClosedRange<Int> = 1...3
  let range2: Range<Int> = 1..<3
  let range3: PartialRangeThrough<Int> = ...5
    
  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~囊括了所有可能要用到的ASCII字符
  let characterRange: ClosedRange<Character> = "\0"..."~"
  characterRange.contains("G") // true
  

带区间的间隔符

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

• 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
  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

• 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，请听下回分解

  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
  

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)")
    }
}