cout<<setprecision(4)<<sum; 12.44
cout<<fixed<<setprecision(4)<<sum; 12.4400
http://blog.youkuaiyun.com/acmmsdn/article/details/7191777
cout<<setprecision(4)<<sum; 12.44
cout<<fixed<<setprecision(4)<<sum; 12.4400
http://blog.youkuaiyun.com/acmmsdn/article/details/7191777