#### 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