Swift - 数字格式化转成字符串（保留两位小数，前面补0等）

http://www.hangge.com/blog/cache/detail_699.html

1，下面是一个浮点类型的数字转成String字符串的例子

var  f = 123.32342342
var  s: String  =  "\(f)"  //123.32342342

2，如果要保留两位小数

var  f = 123.32342342
var  s =  String (format:  "%.2f" , f)  //123.32

3，转成十六进制格式字符串

let  i = 255
let  s: String  =  String (format:  "%x" , i)  //ff

4，不足六位前面补0

let  i = 255
let  s: String  =  String (format:  "%06x" , i)  //0000ff