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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 &lt; n; i++)
     	{
     	  vro [i] = vri [i] * vri [i] - vii [i] * vii [i];
     	  vio [i] = 2 * vri [i] * vii [i];
     	}
         }
       else
         {
           for (i = 0; i &lt; 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))
     &rArr; 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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