How to specifiy the number of decimal places using "ostream"

Question

I am getting a bit frustrated by my inability to get a double to print (to stdout) using the desired number of decimal places.

My current code looks like this:

void SomeFunction(vector<string> argv, ostream& oss){
   SomeClass ClassMembers ;
   string directory = argv.at(2) ;
   // specialized code that opens "FileName", defined by "directory"
   ClassMembers.GetValues(FileName) ;
   oss << ClassMembers.SomeNumber << endl ;
}

That is the general idea. The problem is that I cannot get ClassMember.SomeNumber to print out to, say, 5 decimal places by using:

oss << precision(5) ;

Note that "SomeNumber" is a class member that is declared as double.

It always gives me the number of decimal places that the parsed number found in the body of FileName has.

What am I doing wrong?

Thanks!

Answer 1

oss.precision(5); That should do it, according to this page.

So what you would want to do is this:

oss.precision(5);
oss << ClassMembers.SomeNumber << endl ;

Mind you the "precision" implies the maximum number of digits to display (not the minimum, so it will not include leading/trailing zeros or spaces).

If you want the precise number of decimal places to be used, use std::fixed beforehand, like so:

oss.precision(5);
oss << std::fixed << ClassMembers.SomeNumber << endl;

Answer 2

You might try to use std::fixed in combination. There can't be any more unseen digits, than have being parsed from your input.

Answer 3

Set the flag for floatfield, and you should be able to get as many digits after the decimal place as you want.

#include<iomanip>
//...
oss.setf(ios::fixed,ios::floatfield);
oss.precision(5);
oss << ClassMembers.SomeNumber << endl;

If, for example, we were to use this method and print the value 12345.67 instead of ClassMembers.SomeNumber, the output would be:

12345.67000

Answer 4

You may try this..

oss << setiosflags(ios::fixed);
oss << setprecision(5);
oss << ClassMembers.SomeNumber << endl ;

By using setprecision(5) alone, it includes the numbers before decimal place.

E.g: 1234.777 become 1234.8