C++ open multiple ofstreams in a loop

Question

I need to open undefined number of files with ofstream to write in. the file names should have a format of plot1.xpm, plot2.xpm, plot3.xpm,... . The program looks like this: I don't know what should I place in stars.

for(m = 0; m < spf; m++){
    //some calculations on arr[]
    ofstream output(???);
    for(x = 0; x < n; x++){
        for(y = 0; y < n; y++){
            if (arr[x*n + y] == 0)
                output<<0;
            else output<<1;
        }
        output<<'\n';
        output.close();
    }

Any way to make this work would be fine. I haven't worked with strings ever. — Alireza, Jan 25 '17 at 12:50
`std::ofstream output("plot" + std::to_string(m) + ".xpm")` should work there. requires C++11 and above though. — WhiZTiM, Jan 25 '17 at 12:50
If you never have worked with strings, then I recommend you [find a good beginners book](http://stackoverflow.com/questions/388242/the-definitive-c-book-guide-and-list) to read. — Some programmer dude, Jan 25 '17 at 12:51
If you close the output stream in the first loop round, your loop is a bit pointless, non? — Kerrek SB, Jan 25 '17 at 12:53
Kerrek: This piece of code is inside another do_while loop itself with changing m. — Alireza, Jan 25 '17 at 12:55
@AlirezaHashemi: The first iteration in `x` closes the stream. I think @KerrekSB suspects that `output.close()` should be in the `m` loop, not inside the `x` loop. — Pixelchemist, Jan 25 '17 at 12:58
@Pixelchemist , KerrekSB You are right. My mistake. taken care of. Thanks you all. — Alireza, Jan 25 '17 at 13:01
You should break your problem down to its most basic, then perform some research on that basic element. — Lightness Races in Orbit, Jan 25 '17 at 13:26

score 1 · Accepted Answer · answered Jan 25 '17 at 12:56

Use to_string:

std::string filename = "plot" + std::to_string(m) + ".xpm";
std::ofstream output(filename.c_str());

If the pattern is more complex you can also use std::stringstream:

std::stringstream filename_stream;
// use "operator<<" on stream to plug in parts of the file name
filename_stream << "plot" << m << ".xpm";
std::string filename = filename_stream.str();
std::ofstream output(filename.c_str());

C++ open multiple ofstreams in a loop

1 Answers1