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<h4 class="subsection">A.2.5 Structures with Mex-Files</h4>
<p>The basic function to create a structure in a mex-file is
<code>mxCreateStructMatrix</code>, which creates a structure array with a two
dimensional matrix, or <code>mxCreateStructArray</code>.
<pre class="example"> mxArray *mxCreateStructArray (int ndims, int *dims,
int num_keys,
const char **keys);
mxArray *mxCreateStructMatrix (int rows, int cols,
int num_keys,
const char **keys);
</pre>
<p>Accessing the fields of the structure can then be performed with the
<code>mxGetField</code> and <code>mxSetField</code> or alternatively with the
<code>mxGetFieldByNumber</code> and <code>mxSetFieldByNumber</code> functions.
<pre class="example"> 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);
</pre>
<p>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 <code>index</code> over the elements of the
arrays of elements per <code>field</code>. Whereas the oct-file structure
includes a Cell Array per field of the structure.
<p>An example that demonstrates the use of structures in mex-file can be
found in the file <samp><span class="file">mystruct.c</span></samp>, as seen below
<pre class="example"><pre class="verbatim"> #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;
}
</pre>
</pre>
<p>An example of the behavior of this function within Octave is then
<pre class="example"> 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
,)
}
</pre>
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