1.var let 变量和常量，常量不能再修改值。
2.特性就是用var定义不同类型。
var s:String----也可以制定类型。
var swiftString = "swift is fun";
var hours = 24;
var PI = 3.14;
var me = ("mar",11,"string");

3.整数 但是默认用 Int

//Int8。有符号
//Int16
//Int32
//Int64

//UInt8 无符号
//UInt16
//UInt32
//UInt64
======================
Int.min
Int.max

Int64.min
Int64.max
=========进制=============
//0x 十六进制
let fiften = 15
let fiftenHex = 0xF
//0o 八进制
let eight = 8
let eightHex = 0o17
//二进制
let two = 2
let twoHex = 0b111
//十进制 可以使用拆分
let shihex = 1_000_000
=========整数／浮点数=============
var oneFloat:Float = 1/3
var twoDouble:Double = 1/3
var three = 3
type(of:three)

var zero = 3.14e-2
type(of: zero)

//three + zero
var year = Double(three) + zero
var yearnew = 3 + 0.3
========unicode=============
utf-8
utf-16