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<a name="Structures-with-Mex_002dFiles"></a>
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<p>
Next: <a href="Sparse-Matrices-with-Mex_002dFiles.html#Sparse-Matrices-with-Mex_002dFiles" accesskey="n" rel="next">Sparse Matrices with Mex-Files</a>, Previous: <a href="Cell-Arrays-with-Mex_002dFiles.html#Cell-Arrays-with-Mex_002dFiles" accesskey="p" rel="prev">Cell Arrays with Mex-Files</a>, Up: <a href="Mex_002dFiles.html#Mex_002dFiles" accesskey="u" rel="up">Mex-Files</a> &nbsp; [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html#Concept-Index" title="Index" rel="index">Index</a>]</p>
</div>
<hr>
<a name="Structures-with-Mex_002dFiles-1"></a>
<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>.
</p>
<div class="example">
<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></div>

<p>Accessing the fields of the structure can then be performed with
<code>mxGetField</code> and <code>mxSetField</code> or alternatively with the
<code>mxGetFieldByNumber</code> and <code>mxSetFieldByNumber</code> functions.
</p>
<div class="example">
<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></div>

<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>
<p>An example that demonstrates the use of structures in a mex-file can be found
in the file <samp>mystruct.c</samp> shown below.
</p>
<div class="example">
<pre class="verbatim">#include &quot;mex.h&quot;

void
mexFunction (int nlhs, mxArray *plhs[],
             int nrhs, const mxArray *prhs[])
{
  int i;
  mwIndex j;
  mxArray *v;
  const char *keys[] = { &quot;this&quot;, &quot;that&quot; };

  if (nrhs != 1 || ! mxIsStruct (prhs[0]))
    mexErrMsgTxt (&quot;ARG1 must be a struct&quot;);

  for (i = 0; i &lt; mxGetNumberOfFields (prhs[0]); i++)
    for (j = 0; j &lt; mxGetNumberOfElements (prhs[0]); j++)
      {
        mexPrintf (&quot;field %s(%d) = &quot;, mxGetFieldNameByNumber (prhs[0], i), j);
        v = mxGetFieldByNumber (prhs[0], j, i);
        mexCallMATLAB (0, NULL, 1, &amp;v, &quot;disp&quot;);
      }

  v = mxCreateStructMatrix (2, 2, 2, keys);

  mxSetFieldByNumber (v, 0, 0, mxCreateString (&quot;this1&quot;));
  mxSetFieldByNumber (v, 0, 1, mxCreateString (&quot;that1&quot;));
  mxSetFieldByNumber (v, 1, 0, mxCreateString (&quot;this2&quot;));
  mxSetFieldByNumber (v, 1, 1, mxCreateString (&quot;that2&quot;));
  mxSetFieldByNumber (v, 2, 0, mxCreateString (&quot;this3&quot;));
  mxSetFieldByNumber (v, 2, 1, mxCreateString (&quot;that3&quot;));
  mxSetFieldByNumber (v, 3, 0, mxCreateString (&quot;this4&quot;));
  mxSetFieldByNumber (v, 3, 1, mxCreateString (&quot;that4&quot;));

  if (nlhs)
    plhs[0] = v;
}
</pre></div>

<p>An example of the behavior of this function within Octave is then
</p>
<div class="example">
<pre class="example">a(1).f1 = &quot;f11&quot;; a(1).f2 = &quot;f12&quot;;
a(2).f1 = &quot;f21&quot;; a(2).f2 = &quot;f22&quot;;
b = mystruct (a);
&rArr;  field f1(0) = f11
    field f1(1) = f21
    field f2(0) = f12
    field f2(1) = f22
b
&rArr; 2x2 struct array containing the fields:

     this
     that

b(3)
&rArr; scalar structure containing the fields:

     this = this3
     that = that3
</pre></div>

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<p>
Next: <a href="Sparse-Matrices-with-Mex_002dFiles.html#Sparse-Matrices-with-Mex_002dFiles" accesskey="n" rel="next">Sparse Matrices with Mex-Files</a>, Previous: <a href="Cell-Arrays-with-Mex_002dFiles.html#Cell-Arrays-with-Mex_002dFiles" accesskey="p" rel="prev">Cell Arrays with Mex-Files</a>, Up: <a href="Mex_002dFiles.html#Mex_002dFiles" accesskey="u" rel="up">Mex-Files</a> &nbsp; [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html#Concept-Index" title="Index" rel="index">Index</a>]</p>
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