How can I add padding zeros to a number written to an ofstream?

Question

I'm trying to write numeric values into a text file aligned to columns. My code looks like this:

ofstream file;
file.open("try.txt", ios::app);
file << num << "\t" << max << "\t" << mean << "\t << a << "\n";

It works, except if the values don't have the same number of digits, they don't align. What I would like is the following:

1.234567  ->  1.234
1.234     ->  1.234
1.2       ->  1.200

possible duplicate of [C++ stream output with 3 digits after the decimal point. How?](http://stackoverflow.com/questions/8554441/c-stream-output-with-3-digits-after-the-decimal-point-how) — Paul R, Jan 24 '12 at 11:09

score 6 · Accepted Answer · answered Jan 24 '12 at 11:15

It depends on what format you want. For a fixed decimal place, something like:

class FFmt
{
    int myWidth;
    int myPrecision;
public:
    FFmt( int width, int precision )
        : myWidth( width )
        , myPrecision( precision )
    {
    }
    friend std::ostream& operator<<(
        std::ostream& dest,
        FFmt const& fmt )
    {
        dest.setf( std::ios::fixed, std::ios::floatfield );
        dest.precision( myPrecision );
        dest.width( myWidth );
    }
};

should do the trick, so you can write:

file << nume << '\t' << FFmt( 8, 2 ) << max ...

(or whatever width and precision you want).

If you're doing any floating point work at all, you should probably have such a manipulator in your took kit (although in many cases, it will be more appropriate to use a logical manipulator, named after the logical meaning of the data it formats, e.g. degree, distances, etc.).

IMHO, it's also worth extending the manipulators so that they save the formatting state, and restore it at the end of the full expression. (All of my manipulators derive from a base class which handles this.)

NPE · Answer 2 · 2012-01-24T11:18:01.207

4

Take a look at std::fixed, std::setw() and std::setprecision().

edited Jan 24 '12 at 11:18

answered Jan 24 '12 at 11:07

NPE

486,780
108
951
1,012

score 2 · Answer 3 · answered Jan 24 '12 at 11:06

2

You'll need to alter the precision first.

There's a good example here.

answered Jan 24 '12 at 11:06

Linus Kleen

33,871
11
91
99

answer is your for beeing the first one – andrea Jan 24 '12 at 11:12
You need to specify that the output is fixed format, then specify the precision, and specify the width for each output. – James Kanze Jan 24 '12 at 11:16
Thanks, andrea, but I suggest accepting [James's answer](http://stackoverflow.com/a/8985852/255756). It's way more detailed. – Linus Kleen Jan 24 '12 at 11:19

score 2 · Answer 4 · edited May 23 '17 at 12:03

2

The method is the same as when using cout. See this answer.

edited May 23 '17 at 12:03

Community

1
1

answered Jan 24 '12 at 11:08

Samuel Harmer

4,264
5
33
67

How can I add padding zeros to a number written to an ofstream?

4 Answers4