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