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.

     #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

Other functions in the mex interface for handling character strings are mxCreateString, mxArrayToString, and mxCreateCharMatrixFromStrings. In a mex-file, a character string 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.