在代码业务中，条件判断是必不可少的，控制流程的语句在每种语言中都是差不多的。swift包括了：

if,switch,for-in,for,while,do-while

if 条件语句 (if else)    (if... else if ...  else)  

判断条件为true时执行相关代码。例如：

var a = 0

if a > 0 {

println("a > 0")

}else if a == 0{

println("a = 0")

}else{

println("a < 0")

}

switch switch语句会尝试把某个值与若干个模式（pattern）进行匹配。根据第一个匹配成功的模式，switch语句会执行对应的代码

例如：

let someCharacter: Character = "e"
switch someCharacter {
case "a", "e", "i", "o", "u":
    println("\(someCharacter) is a vowel")
case "b", "c", "d", "f", "g", "h", "j", "k", "l", "m",
"n", "p", "q", "r", "s", "t", "v", "w", "x", "y", "z":
    println("\(someCharacter) is a consonant")
default:
    println("\(someCharacter) is not a vowel or a consonant")
}

switch有几个注意事项：

1、case语句下必须包含至少一行代码

2、case语句下可以不包含break，它不存在隐式的贯穿

3、case可以进行区间匹配

let count = 3_000_000_000_000
let countedThings = "stars in the Milky Way"
var naturalCount: String
switch count {
case 0:
    naturalCount = "no"
case 1...3:
    naturalCount = "a few"
case 4...9:
    naturalCount = "several"
case 10...99:
    naturalCount = "tens of"
case 100...999:
    naturalCount = "hundreds of"
case 1000...999_999:
    naturalCount = "thousands of"
default:
    naturalCount = "millions and millions of"
}
println("There are \(naturalCount) \(countedThings).")

例如：

let somePoint = (1, 1)
switch somePoint {
case (0, 0):
    println("(0, 0) is at the origin")
case (_, 0):
    println("(\(somePoint.0), 0) is on the x-axis")
case (0, _):
    println("(0, \(somePoint.1)) is on the y-axis")
case (-2...2, -2...2):
    println("(\(somePoint.0), \(somePoint.1)) is inside the box")
default:
    println("(\(somePoint.0), \(somePoint.1)) is outside of the box")
}

5、值绑定   case 分支的模式允许将匹配的值绑定到一个临时的常量或变量，这些常量或变量在该 case 分支里就可以被引用了——这种行为被称为值绑定

例如：

let anotherPoint = (2, 0)
switch anotherPoint {
case (let x, 0):
    println("on the x-axis with an x value of \(x)")
case (0, let y):
    println("on the y-axis with a y value of \(y)")
case let (x, y):
    println("somewhere else at (\(x), \(y))")
}
// 输出 "on the x-axis with an x value of 2"

例如

let yetAnotherPoint = (1, -1)
switch yetAnotherPoint {
case let (x, y) where x == y:
    println("(\(x), \(y)) is on the line x == y")
case let (x, y) where x == -y:
    println("(\(x), \(y)) is on the line x == -y")
case let (x, y):
    println("(\(x), \(y)) is just some arbitrary point")
}
// 输出 "(1, -1) is on the line x == -y"

a)continue语句告诉一个循环体立刻停止本次循环迭代，重新开始下次循环迭代。就好像在说“本次循环迭代我已经执行完了”，但是并不会离开整个循环体。

b)break语句会立刻结束整个控制流的执行。当你想要更早的结束一个switch代码块或者一个循环体时，你都可以使用break语句,if for  for-in  while  do-while 这些都能够用break跳出控制，

c)fallthrough 贯穿   这个和第二点一样。

d)return语句会立刻终止当前代码的执行，并返回。用在方法控制中比较多。

8、给while取一个名字  例如

mainLabel: while a > 0{
	if a ==100 {
		break mainLabel
	}
}

for 循环  for-in循环

for循环用来按照指定的次数多次执行一系列语句。Swift 提供两种for循环形式：
a)for-in用来遍历一个区间（range），序列（sequence），集合（collection），系列（progression）里面所有的元素执行一系列语句。
b)for条件递增（for-condition-increment）语句，用来重复执行一系列语句直到达成特定条件达成，一般通过在每次循环完成后增加计数器的值来实现。

遍历区间：

for index in 1...5 {
    println("\(index) times 5 is \(index * 5)")
}

let names = ["Anna", "Alex", "Brian", "Jack"]
for name in names {
    println("Hello, \(name)!")
}

let numberOfLegs = ["spider": 8, "ant": 6, "cat": 4]
for (animalName, legCount) in numberOfLegs {
    println("\(animalName)s have \(legCount) legs")
}

for character in "Hello" {
    println(character)
}

for var index = 0; index < 3; ++index {
    println("index is \(index)")
}

while循环  do-while循环

while循环，每次在循环开始时计算条件是否符合；
do-while循环，每次在循环结束时计算条件是否符合。

while循环：

while condition {
statements
}

while a > 0{
	println("a value \(a)");
}

do {
statements
} while condition

do{
	println("a value \(a)")
}while a > 0

【Swift】学习笔记(五)——控制语句(if,switch,for-in,for,while,do-while)