#include<iostream>  
#include<iomanip>  
using namespace std;  
int main()  
{  
    cout<<2.226215151515<<endl;  
    cout<<"------------------------"<<endl;  
    double a=1.568922156562;  
    cout<<a<<endl;                //預設有效位數是6   
    cout<<setprecision(0)<<"setprecision(0): "<<a<<endl  
        <<setprecision(1)<<"setprecision(1): "<<a<<endl  
        <<setprecision(2)<<"setprecision(2): "<<a<<endl  
        <<setprecision(3)<<"setprecision(3): "<<a<<endl  
        <<setprecision(4)<<"setprecision(4): "<<a<<endl;  
        cout<<"------------------------"<<endl;  
  
//b的整數有三位，輸出顯示小數精度是兩位，所以系統自動轉化為指數輸出   
    double b=223.0226262;  
    cout<<setprecision(2)<<"輸出指數: "<<b<<endl;  
    cout<<"------------------------"<<endl;  
      
    //設定固定浮點數   
    cout<<setiosflags(ios::fixed);     
    cout<<setprecision(8)<<"setprecision(8): "<<a<<endl;  
    cout<<"------------------------"<<endl;  
      
    cout.unsetf(ostream::floatfield);    //恢復設定             
    cout<<setiosflags(ios::scientific)<<"指數格式: "<<a<<endl;  
    cout<<std::fixed;    //清除使用指數格式   
    cout<<"------------------------"<<endl;  
      
    cout<<setprecision(6);       //沒有清除前面的固定浮點數輸出，之後還是輸出固定浮點數   
    cout<<2.226215151515<<endl;  
    cout<<2.1515<<endl;  
    cout<<"------------------------"<<endl;  
      
    cout.unsetf(ostream::floatfield);    //使用恢復設定   
    cout<<setprecision(6);  
    cout<<2.226215151515<<endl;  
    cout<<2.1515<<endl;  
}

<img src="https://img-blog.csdn.net/20161031195424654?watermark/2/text/aHR0cDovL2Jsb2cuY3Nkbi5uZXQv/font/5a6L5L2T/fontsize/400/fill/I0JBQkFCMA==/dissolve/70/gravity/Center" align="left" alt="" />