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<span id="Working-with-Matrices-and-Arrays-in-Mex_002dFiles"></span><div class="header">
<p>
Next: <a href="Character-Strings-in-Mex_002dFiles.html" accesskey="n" rel="next">Character Strings in Mex-Files</a>, Previous: <a href="Getting-Started-with-Mex_002dFiles.html" accesskey="p" rel="prev">Getting Started 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="Working-with-Matrices-and-Arrays-in-Mex_002dFiles-1"></span><h4 class="subsection">A.2.2 Working with Matrices and Arrays in Mex-Files</h4>
<p>The basic mex type of all variables is <code>mxArray</code>. Any object, such as a
matrix, cell array, or structure, is stored in this basic type. <code>mxArray</code>
serves essentially the same purpose as the <code>octave_value</code> class in
oct-files in that it acts as a container for all the more specialized types.
</p>
<p>The <code>mxArray</code> structure contains at a minimum, the name of the variable it
represents, its dimensions, its type, and whether the variable is real or
complex. It can also contain a number of additional fields depending on the
type of the <code>mxArray</code>. There are a number of functions to create
<code>mxArray</code> structures, including <code>mxCreateDoubleMatrix</code>,
<code>mxCreateCellArray</code>, <code>mxCreateSparse</code>, and the generic
<code>mxCreateNumericArray</code>.
</p>
<p>The basic function to access the data in an array is <code>mxGetPr</code>. Because
the mex interface assumes that real and imaginary parts of a complex array are
stored separately, there is an equivalent function <code>mxGetPi</code> that gets the
imaginary part. Both of these functions are only for use with double precision
matrices. The generic functions <code>mxGetData</code> and <code>mxGetImagData</code>
perform the same operation for all matrix types. For example:
</p>
<div class="example">
<pre class="example">mxArray *m;
mwSize *dims;
UINT32_T *pr;
dims = (mwSize *) mxMalloc (2 * sizeof (mwSize));
dims[0] = 2; dims[1] = 2;
m = mxCreateNumericArray (2, dims, mxUINT32_CLASS, mxREAL);
pr = (UINT32_T *) mxGetData (m);
</pre></div>
<p>There are also the functions <code>mxSetPr</code>, etc., that perform the inverse,
and set the data of an array to use the block of memory pointed to by the
argument of <code>mxSetPr</code>.
</p>
<p>Note the type <code>mwSize</code> used above, and also <code>mwIndex</code>, are defined as
the native precision of the indexing in Octave on the platform on which the
mex-file is built. This allows both 32- and 64-bit platforms to support
mex-files. <code>mwSize</code> is used to define array dimensions and the maximum
number or elements, while <code>mwIndex</code> is used to define indexing into
arrays.
</p>
<p>An example that demonstrates how to work with arbitrary real or complex double
precision arrays is given by the file <samp>mypow2.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[])
{
mwSize n;
mwIndex i;
double *vri, *vro;
if (nrhs != 1 || ! mxIsDouble (prhs[0]))
mexErrMsgTxt ("ARG1 must be a double matrix");
n = mxGetNumberOfElements (prhs[0]);
plhs[0] = mxCreateNumericArray (mxGetNumberOfDimensions (prhs[0]),
mxGetDimensions (prhs[0]),
mxGetClassID (prhs[0]),
mxIsComplex (prhs[0]));
vri = mxGetPr (prhs[0]);
vro = mxGetPr (plhs[0]);
if (mxIsComplex (prhs[0]))
{
double *vii, *vio;
vii = mxGetPi (prhs[0]);
vio = mxGetPi (plhs[0]);
for (i = 0; i < n; i++)
{
vro[i] = vri[i] * vri[i] - vii[i] * vii[i];
vio[i] = 2 * vri[i] * vii[i];
}
}
else
{
for (i = 0; i < n; i++)
vro[i] = vri[i] * vri[i];
}
}
</pre></div>
<p>An example of its use is
</p>
<div class="example">
<pre class="example">b = randn (4,1) + 1i * randn (4,1);
all (b.^2 == mypow2 (b))
⇒ 1
</pre></div>
<p>The example above uses the functions <code>mxGetDimensions</code>,
<code>mxGetNumberOfElements</code>, and <code>mxGetNumberOfDimensions</code> to work with
the dimensions of multi-dimensional arrays. The functions <code>mxGetM</code>, and
<code>mxGetN</code> are also available to find the number of rows and columns in a
2-D matrix (MxN matrix).
</p>
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