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      FILES_MULTIPLE - How a Program Can Handle Multiple Output Files
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      FILES_MULTIPLE <br> How a Program Can Handle Multiple Output Files
    </h1>

    <hr>

    <p>
      <b>FILES_MULTIPLE</b>
      is a FORTRAN77 program which
      demonstrates how a program can open multiple output files at one time,
      and write data to any one specific file it chooses.
    </p>

    <p>
      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.
    </p>

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

    <p>
      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:
      <pre>
        file01.txt
        file02.txt
        file03.txt
        ...
      </pre>
      The filenames can be stored in a character array of dimension n,
      which might be called "filename".
    </p>

    <p>
      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".
    </p>

    <p>
      Now we use a loop from 1 to n, to open the files:
      <pre>
        do i = 1, n
          open ( unit = fileunit(i), file = filename(i), status = 'replace' )
        end do
      </pre>
    </p>

    <p>
      At this point, to write data to the 17th file, we would use a statement like:
      <pre>
        write ( unit(17), '(2g14.6)' ) x, y
      </pre>
    </p>

    <p>
      When we are all done, we need to close the files:
      <pre>
        do i = 1, n
          close ( unit = fileunit(i) )
        end do
      </pre>
    </p>

    <h3 align = "center">
      Licensing:
    </h3>

    <p>
      The computer code and data files described and made available on this
      web page are distributed under
      <a href = "../../txt/gnu_lgpl.txt">the GNU LGPL license.</a>
    </p>

    <h3 align = "center">
      Languages:
    </h3>

    <p>
      <b>FILES_MULTIPLE</b> is available in
      <a href = "../../f77_src/files_multiple/files_multiple.html">a FORTRAN77 version</a> and
      <a href = "../../f_src/files_multiple/files_multiple.html">a FORTRAN90 version</a> and
      <a href = "../../m_src/files_multiple/files_multiple.html">a MATLAB version</a>.
    </p>

    <h3 align = "center">
      Related Data and Programs:
    </h3>

    <p>
      <a href = "../../f77_src/file_name_sequence/file_name_sequence.html">
      FILE_NAME_SEQUENCE</a>,
      a FORTRAN77 program which
      demonstrates ways to generate a sequence of filenames
      that can be used when generating images for an animation.
    </p>

    <p>
      <a href = "../../f77_src/filum/filum.html">
      FILUM</a>,
      a FORTRAN77 library which
      can work with information in text files.
    </p>

    <p>
      <a href = "../../f77_src/table_io/table_io.html">
      TABLE_IO</a>,
      a FORTRAN77 library which
      reads and writes table files.
    </p>

    <h3 align = "center">
      Source Code:
    </h3>

    <p>
      <ul>
        <li>
          <a href = "files_multiple.f">files_multiple.f</a>, the source code.
        </li>
        <li>
          <a href = "files_multiple.sh">files_multiple.sh</a>,
          BASH commands to compile and run the source code.
        </li>
        <li>
          <a href = "files_multiple_output.txt">files_multiple_output.txt</a>, 
          the output file.
        </li>
      </ul>
    </p>

    <p>
      <b>test01()</b> 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.
      <ul>
        <li>
          <a href = "divisor1.txt">divisor1.txt</a>,
          multiples of 2.
        </li>
        <li>
          <a href = "divisor2.txt">divisor2.txt</a>,
          multiples of 3.
        </li>
        <li>
          <a href = "divisor3.txt">divisor3.txt</a>,
          multiples of 5.
        </li>
        <li>
          <a href = "divisor4.txt">divisor4.txt</a>,
          multiples of 7.
        </li>
      </ul>
    </p>

    <p>
      <b>test02()</b> watches a vector X, of length 100, as it changes 20 times.
      <ul>
        <li>
          <a href = "x1.txt">x1.txt</a>,
          the values of X(10).
        </li>
        <li>
          <a href = "x2.txt">x2.txt</a>,
          the values of X(25).
        </li>
        <li>
          <a href = "x3.txt">x3.txt</a>,
          the values of X(64).
        </li>
        <li>
          <a href = "x4.txt">x4.txt</a>,
          the values of X(81).
        </li>
      </ul>
    </p>

    <h3 align = "center">
      List of Routines:
    </h3>

    <p>
      <ul>
        <li>
          <b>FILES_MULTIPLE</b> demonstrates how to work with multiple files.
        </li>
        <li>
          <b>TEST01</b> writes data to four files which are open simultaneously.
        </li>
        <li>
          <b>TEST02</b> writes selected data to four files which are open simultaneously.
        </li>
        <li>
          <b>FILENAME_INC</b> increments a partially numeric filename.
        </li>
        <li>
          <b>GET_UNIT</b> returns a free FORTRAN unit number.
        </li>
        <li>
          <b>R8VEC_UNIFORM_01</b> returns a unit pseudorandom R8VEC.
        </li>
        <li>
          <b>TIMESTAMP</b> prints the current YMDHMS date as a time stamp.
        </li>
      </ul>
    </p>

    <p>
      You can go up one level to <a href = "../f77_src.html">
      the FORTRAN77 source codes</a>.
    </p>

    <hr>

    <i>
      Last revised on 01 December 2012.
    </i>

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