代码如下，输入要保留的小数个数
extension Double {
/// Rounds the double to decimal places value
func roundTo(places:Int) -> Double {
let divisor = pow(10.0, Double(places))
return (self * divisor).rounded() / divisor
}
}
原文：http://www.cnblogs.com/qiutangfengmian/p/6898532.html