1nil聚合运算符 nil coalescing operator

a ？？ b ==>a!=nil ? a! : b

要求：

1a是一个可选类型

2b必须和a解包后类型一致

var userNickName:String?

//let outputName:String = userNickName != nil ? userNickName! : "Guest"

let outputName:String = userNickName ?? "Guest"

2区间运算符

闭区间 [a,b] a...b

前闭后开 [a,b) a..<b

3for-in循环

for index in 1...10

{

}

//index默认是一个常量值