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A.2.3 Character Strings in Mex-Files

As mex-files do not make the distinction between single and double quoted strings within Octave, there is perhaps less complexity in the use of strings and character matrices in mex-files. An example of their use, that parallels the demo in stringdemo.cc, is given in the file mystring.c, as seen below.

     /*
     
     Copyright (C) 2007 John W. Eaton
     
     This file is part of Octave.
     
     Octave is free software; you can redistribute it and/or modify it
     under the terms of the GNU General Public License as published by the
     Free Software Foundation; either version 3 of the License, or (at your
     option) any later version.
     
     Octave is distributed in the hope that it will be useful, but WITHOUT
     ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
     FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
     for more details.
     
     You should have received a copy of the GNU General Public License
     along with Octave; see the file COPYING.  If not, see
     <http://www.gnu.org/licenses/>.
     
     */
     
     #include <string.h>
     #include "mex.h"
     
     void
     mexFunction (int nlhs, mxArray *plhs[], int nrhs, const mxArray *prhs[])
     {
       mwIndex i, j;
       mwSize m, n;
       mxChar *pi, *po;
     
       if (nrhs != 1 || ! mxIsChar (prhs[0]) || 
           mxGetNumberOfDimensions (prhs[0]) > 2)
         mexErrMsgTxt ("expecting char matrix");
     
       m = mxGetM (prhs[0]);
       n = mxGetN (prhs[0]);
       pi = mxGetChars (prhs[0]);
       plhs[0] = mxCreateNumericMatrix (m, n, mxCHAR_CLASS, mxREAL);
       po = mxGetChars (plhs[0]);
     
       for (j = 0; j < n; j++)
         for (i = 0; i < m; i++)
           po [j*m + m - 1 - i] = pi [j*m + i];
     }

An example of its expected output is

     mystring(["First String"; "Second String"])
      s1 = Second String
             First String

There are a couple of additional functions available in mex-files of interest in the treatment of strings. These are mxCreateString, mxArrayToString and mxCreateCharMatrixFromStrings. A string in a mex-file is considered to be a vector rather than a matrix. This is perhaps an arbitrary distinction as the data in the mxArray for the matrix is consecutive in any case.