<html lang="en"> <head> <title>Structures with Mex-Files - Untitled</title> <meta http-equiv="Content-Type" content="text/html"> <meta name="description" content="Untitled"> <meta name="generator" content="makeinfo 4.13"> <link title="Top" rel="start" href="index.html#Top"> <link rel="up" href="Mex_002dFiles.html#Mex_002dFiles" title="Mex-Files"> <link rel="prev" href="Cell-Arrays-with-Mex_002dFiles.html#Cell-Arrays-with-Mex_002dFiles" title="Cell Arrays with Mex-Files"> <link rel="next" href="Sparse-Matrices-with-Mex_002dFiles.html#Sparse-Matrices-with-Mex_002dFiles" title="Sparse Matrices with Mex-Files"> <link href="http://www.gnu.org/software/texinfo/" rel="generator-home" title="Texinfo Homepage"> <meta http-equiv="Content-Style-Type" content="text/css"> <style type="text/css"><!-- pre.display { font-family:inherit } pre.format { font-family:inherit } pre.smalldisplay { font-family:inherit; font-size:smaller } pre.smallformat { font-family:inherit; font-size:smaller } pre.smallexample { font-size:smaller } pre.smalllisp { font-size:smaller } span.sc { font-variant:small-caps } span.roman { font-family:serif; font-weight:normal; } span.sansserif { font-family:sans-serif; font-weight:normal; } --></style> </head> <body> <div class="node"> <a name="Structures-with-Mex-Files"></a> <a name="Structures-with-Mex_002dFiles"></a> <p> Next: <a rel="next" accesskey="n" href="Sparse-Matrices-with-Mex_002dFiles.html#Sparse-Matrices-with-Mex_002dFiles">Sparse Matrices with Mex-Files</a>, Previous: <a rel="previous" accesskey="p" href="Cell-Arrays-with-Mex_002dFiles.html#Cell-Arrays-with-Mex_002dFiles">Cell Arrays with Mex-Files</a>, Up: <a rel="up" accesskey="u" href="Mex_002dFiles.html#Mex_002dFiles">Mex-Files</a> <hr> </div> <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> </body></html>