Set precision with fstream to output file - format

问题:

I cannot find an answer for how I might use an already opened fstream to output formatted doubles to a text file. Currently my code is doing this:

int writeToFile (fstream& f, product_record& p)
{
   if (!f){
      cout << "out file failed!!error" << endl;
      return -1;
   } else {

      f << p.idnumber << endl;
      f << p.name << endl;
      f << p.price << endl;
      f << p.number << endl;
      f << p.tax << endl;
      f << p.sANDh << endl;
      f << p.total << endl;
      f << intArrToString( p.stations ) << endl;

   }
}

Where p is struct called product_record and price, tax, sANDh, and total are all doubles I have tried doing f << setprecision(4) << p.price << endl; but this does not work. How can i format this double to have a precision of two decimal places. Like this "#.00". How can I achieve this using specifically fstream?

Also, fyi, the overall task is to simply read a txt file, store data in a struct, add structs to a vector and then read from the structs to print to an output file. The input file already has the doubles formatted like 10.00, 2.00 and such(2-decimal places)

回答1:

Try using

f << fixed << setprecision(2) << endl;

set precision sets the number of significant digits, not the number of decimals places. For example

cout << setprecision(3) << 12.3456 << endl;

would output 12.3
Sending in fixed first makes it so you are setting the precision from a fixed position (the decimal place) rather than from the first digit of the floating point value.