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#### 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 the `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 mex-file can be found in the file `mystruct.c`, as seen 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 ("expects 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, 0, 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 =
             {
               this =

               (,
                 [1] = this1
                 [2] = this2
                 [3] = this3
                 [4] = this4
               ,)

               that =

               (,
                 [1] = that1
                 [2] = that2
                 [3] = that3
                 [4] = that4
               ,)

             }