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<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>. All variables,
such as matrices, cell arrays or structures are all stored in this basic
type, and this type serves basically the same purpose as the
octave_value class in oct-files. That is it acts as a container for the
more specialized types.
<p>The <code>mxArray</code> structure contains at a minimum, the variable it
represents name, its dimensions, its type and whether the variable is
real or complex. It can however 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>mxCreateCellArray</code>, <code>mxCreateSparse</code> and the generic
<code>mxCreateNumericArray</code>.
<p>The basic functions to access the data contained in an array is
<code>mxGetPr</code>. As the mex interface assumes that the real and imaginary
parts of a complex array are stored separately, there is an equivalent
function <code>mxGetPi</code> that get the imaginary part. Both of these
functions are for use only with double precision matrices. There also
exists the generic function <code>mxGetData</code> and <code>mxGetImagData</code>
that perform the same operation on all matrix types. For 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>
<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>Note the type <code>mwSize</code> used above, and <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 dimension
and maximum number or elements, while <code>mwIndex</code> is used to define
indexing into arrays.
<p>An example that demonstration how to work with arbitrary real or complex
double precision arrays is given by the file <samp><span class="file">mypow2.c</span></samp> as given
below.
<pre class="example"><pre class="verbatim"> #include "mex.h"
void
mexFunction (int nlhs, mxArray* plhs[], int nrhs,
const mxArray* prhs[])
{
mwIndex i;
mwSize n;
double *vri, *vro;
if (nrhs != 1 || ! mxIsNumeric (prhs[0]))
mexErrMsgTxt ("expects matrix");
n = mxGetNumberOfElements (prhs[0]);
plhs[0] = (mxArray *) 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></pre>
<p class="noindent">with an example of its use
<pre class="example"> b = randn(4,1) + 1i * randn(4,1);
all(b.^2 == mypow2(b))
1
</pre>
<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 matrix.
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