1.循环结构

//for
for index in 1...10{
    index//常量
}
for index in 1..<10{
    index
}

for i in -99...99{
    i*i
}
for var i = -99 ; i <= 99; i++ {
    i*i
}
var i = -99
for  ; i <= 99; {
    i*i
    i++
}

//while不知道会执行多少次
//repeat  while至少执行一次
var awin = 0
var bwin = 0
var game = 0
while awin < 3 && bwin < 3 {
    game++
    let a = arc4random_uniform(6)+1
    let b = arc4random_uniform(6)+1
    //print("a is \(a), b is \(b)." , termimator:" ")
    if a > b {
        print("A win")
        bwin = 0
        awin++
    }
    else if a < b {
        print("B win")
        awin = 0
        bwin++
    }
    else{
        print("draw")
        awin = 0
        bwin = 0
    }
    
}
let winner = awin == 3 ? "A" : "B"
print("after \(game) games, \(winner) win")

//break   continue控制循环的转移

2.选择结构

var rating = "a"
if rating == "A" {
    print("A")
}else if rating == "B" {
    print("B")
}else {
    print("Error")
}

//switch更加灵活，rating的类型比较多，整形，布尔型
switch rating{
case "a", "A":
    print("A")
case "b", "B":
    print("B")
default:
    print("Error")//不要error显示时，可以用break或者空括号（）
}

let point = (0,0)//point:(x:Int,y:Int)=(1,1)
switch point{
case (0,0):
    print("origin")
    fallthrough//可以继续进入下一个case
case (_,0):
    print("x-axis")
    fallthrough
case (0,_):
    print("y-axis")
case (-2...2,-2...2):
    print("near point")
default:
    print("")
}
switch point{
case (0,0):
    print("origin")
case (let x,0):
    print("x-axis")
case (0,let y):
    print("y-axis")
case (let x, let y):
    print("near point")//不需要default
}

//x^4 - y^2 = 15*x*y  只要一组结果，为循环取一个名字
getAnswer:for m in 1...300{
    for n in 1...300 {
        if m*m*m*m - n*n == 15*m*n {
            print(m,n)
            break getAnswer
        }
    }
}

3.where关键字

let point = (3,3)
switch point{
case let(x,y) where x == y:
    print("It's on the line x==y")
case let(x,y) where x == -y:
    print("It's on the line x==-y")
case let(x,y):
    print("The point is (\(x),\(y))")
}

let age = 19
if case 10...19 = age{
    print("teenager")
}

if case 10...19 = age where age >= 18{
    print("teenager and in a college")
}

let vector = (4,0)
if case (let x,0) = vector where x > 2 && x < 5 {
    print("vector")
}

for i in 1...100 {
    if i % 3 == 0 {
        print(i)
    }
}
//等价于
for case let i in 1...100 where i % 3 == 0 {
    print(i)
}

4.guard关键字

func buy(money:Int , price:Int , capacity:Int , volume:Int){
    if money >= price {
        if capacity >= volume {
            print("I can buy it")
            print("\(money - price) left")
        }else{
            print("Not enough capacity")
        }
    }else{
        print("Not enough money")
    }
}
//等价于
func buy2(money:Int , price:Int , capacity:Int , volume:Int){
    guard money >= price else{//guard确保一个的条件，不满足就直接return
        print("Not enough money")
        return
    }
    guard capacity >= volume else {//guard确保一个的条件，不满足就直接return
        print("Not enough capacity")
        return
    }
    //剩下的就是我们需要的了
    print("I can buy it")
    print("\(money - price) left")
}