Info Catalog octave: Cell Arrays with Mex-Files octave: Mex-Files octave: Sparse Matrices with Mex-Files

octave: Structures with Mex-Files

 
 A.2.5 Structures with Mex-Files
 -------------------------------
 
 The basic function to create a structure in a mex-file is
 ‘mxCreateStructMatrix’ which creates a structure array with a two
 dimensional matrix, or ‘mxCreateStructArray’.
 
      mxArray *mxCreateStructArray (int ndims, int *dims,
                                    int num_keys,
                                    const char **keys);
      mxArray *mxCreateStructMatrix (int rows, int cols,
                                     int num_keys,
                                     const char **keys);
 
    Accessing the fields of the structure can then be performed with
 ‘mxGetField’ and ‘mxSetField’ or alternatively with the
 ‘mxGetFieldByNumber’ and ‘mxSetFieldByNumber’ functions.
 
      mxArray *mxGetField (const mxArray *ptr, mwIndex index,
                           const char *key);
      mxArray *mxGetFieldByNumber (const mxArray *ptr,
                                   mwIndex index, int key_num);
      void mxSetField (mxArray *ptr, mwIndex index,
                       const char *key, mxArray *val);
      void mxSetFieldByNumber (mxArray *ptr, mwIndex index,
                               int key_num, mxArray *val);
 
    A difference between the oct-file interface to structures and the
 mex-file version is that the functions to operate on structures in
 mex-files directly include an ‘index’ over the elements of the arrays of
 elements per ‘field’; Whereas, the oct-file structure includes a Cell
 Array per field of the structure.
 
    An example that demonstrates the use of structures in a mex-file can
 be found in the file ‘mystruct.c’ shown below.
 
      #include "mex.h"
      
      void
      mexFunction (int nlhs, mxArray* plhs[],
                   int nrhs, const mxArray* prhs[])
      {
        int i;
        mwIndex j;
        mxArray *v;
        const char *keys[] = { "this", "that" };
      
        if (nrhs != 1 || ! mxIsStruct (prhs[0]))
          mexErrMsgTxt ("ARG1 must be a struct");
      
        for (i = 0; i < mxGetNumberOfFields (prhs[0]); i++)
          for (j = 0; j < mxGetNumberOfElements (prhs[0]); j++)
            {
              mexPrintf ("field %s(%d) = ", mxGetFieldNameByNumber (prhs[0], i), j);
              v = mxGetFieldByNumber (prhs[0], j, i);
              mexCallMATLAB (0, NULL, 1, &v, "disp");
            }
      
        v = mxCreateStructMatrix (2, 2, 2, keys);
      
        mxSetFieldByNumber (v, 0, 0, mxCreateString ("this1"));
        mxSetFieldByNumber (v, 0, 1, mxCreateString ("that1"));
        mxSetFieldByNumber (v, 1, 0, mxCreateString ("this2"));
        mxSetFieldByNumber (v, 1, 1, mxCreateString ("that2"));
        mxSetFieldByNumber (v, 2, 0, mxCreateString ("this3"));
        mxSetFieldByNumber (v, 2, 1, mxCreateString ("that3"));
        mxSetFieldByNumber (v, 3, 0, mxCreateString ("this4"));
        mxSetFieldByNumber (v, 3, 1, mxCreateString ("that4"));
      
        if (nlhs)
          plhs[0] = v;
      }
 
    An example of the behavior of this function within Octave is then
 
      a(1).f1 = "f11"; a(1).f2 = "f12";
      a(2).f1 = "f21"; a(2).f2 = "f22";
      b = mystruct (a);
      ⇒  field f1(0) = f11
          field f1(1) = f21
          field f2(0) = f12
          field f2(1) = f22
      b
      ⇒ 2x2 struct array containing the fields:
 
           this
           that
 
      b(3)
      ⇒ scalar structure containing the fields:
 
           this = this3
           that = that3
 

Info Catalog octave: Cell Arrays with Mex-Files octave: Mex-Files octave: Sparse Matrices with Mex-Files