• 基礎知識

Double的精度為十五個十進制有效數字，
Float的精度為六個十進制有效數字。
兩者都需要擴展后實現保留幾位有效小數的操作，擴展方法需要在classViewController外層增加。

• 代碼：

import UIKit
class ViewController: UIViewController {    
    override func viewDidLoad() {        
        super.viewDidLoad()               
        let n1 : Float = 32.141592654        
        let n2 : Double = 3.141592654        
        print(n1)        
        print(n2)        
        print(n2.format(".3"))        
        // Do any additional setup after loading the view, typically from a nib.    }    
    override func didReceiveMemoryWarning() {       
        super.didReceiveMemoryWarning()        
        // Dispose of any resources that can be recreated.    
    }
}
//擴展
extension Double {    
    func format(f : String) -> String {        
        return NSString.init(format: "%\(f)f", self) as String    
    }
}