How a Program Can Handle Multiple Output Files

FILES_MULTIPLE is a C++ program which demonstrates how a program can open multiple output files at one time, and write data to any one specific file it chooses.

To write data to a file, a FORTRAN90 program needs a filename, an open() statement that assigns a unique unit number to that filename, and write() statements that send data to the file identified by unit number.

To handle two or three files is easy, but to handle an arbitrary number of files requires a little planning.

Supposing we need "n" files, we can create a "template" filename with some zeros in it, like "file00.txt", and call a function like "filename_inc()" which, each time, will return a filename with the numeric text incremented by 1:

The filenames can be stored in a character array of dimension n, which might be called "filename".

In order to store the unique unit number of each file, we can simply try to set the first file to unit 1, the second to unit 2, and so on. (For some versions of FORTRAN, this might cause problems, in which case, a function called get_unit() can be used instead.) The unit numbers can be stored in an integer vector of dimension n, which might be called "fileunit".

Now we use a loop from 1 to n, to open the files:

        do i = 1, n
          open ( unit = fileunit(i), file = filename(i), status = 'replace' )
        end do

At this point, to write data to the 17th file, we would use a statement like:

        write ( unit(17), '(2g14.6)' ) x, y

When we are all done, we need to close the files:

        do i = 1, n
          close ( unit = fileunit(i) )
        end do


The computer code and data files described and made available on this web page are distributed under the GNU LGPL license.


FILES_MULTIPLE is available in a C version and a C++ version and a FORTRAN90 version and a MATLAB version.

Related Data and Programs:

FILE_NAME_SEQUENCE, a C++ program which demonstrates ways to generate a sequence of filenames that can be used when generating images for an animation.

FILUM, a C++ library which can work with information in text files.

TABLE_IO, a C++ library which reads and writes table files.


  1. Jeanne Adams, Walter Brainerd, Jeanne Martin, Brian Smith, Jerrold Wagener,
    Fortran90 Handbook,
    Complete ANSI/ISO Reference,
    McGraw Hill, 1992,
    ISBN: 0-07-000406-4,
    LC: QA76.73.F28.F67.

Source Code:

test01() counts from 0 to 100. Each divisor of 2 is written to "divisor1.txt", each divisor of 3 is written to "divisor2.txt" and so on.

test02() watches a vector X, of length 100, as it changes 20 times.

List of Routines:

You can go up one level to the C++ source codes.

Last revised on 03 December 2012.