条件判断(whilec循环＆for＆switch循环＆if-else判断)

if-else

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")
}
/* 1.if后面的条件可以省略小括号 2.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

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

1.repeat-while相当于C语言中的do-while
2.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 "Alex", "Brian", "Jack"]

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

// 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

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

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~囊括了所有可能要用到的ASCII字符
let characterRange: ClosedRange<Character> = "\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

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
//case、 default后面不能写大括号{}

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
//默认可以不写break ，并不会贯穿到后面的条件

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必须要保证能处理所有情况

    
        /* switch注意点: 1.switch必须保证能处理所有情况 2.case ,default后面至少一条语句; 3.如果不想做任何事,加个break即可; 4.如果能保证已处理所有的情况,也可以不必使用default */
        // 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")
}

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

值绑定

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
//必要时let也可以改为var

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)")
}
}