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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

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

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