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<a name="Sparse-Matrices-with-Mex_002dFiles"></a>
<div class="header">
<p>
Next: <a href="Calling-Other-Functions-in-Mex_002dFiles.html#Calling-Other-Functions-in-Mex_002dFiles" accesskey="n" rel="next">Calling Other Functions in Mex-Files</a>, Previous: <a href="Structures-with-Mex_002dFiles.html#Structures-with-Mex_002dFiles" accesskey="p" rel="prev">Structures with Mex-Files</a>, Up: <a href="Mex_002dFiles.html#Mex_002dFiles" accesskey="u" rel="up">Mex-Files</a> &nbsp; [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html#Concept-Index" title="Index" rel="index">Index</a>]</p>
</div>
<hr>
<a name="Sparse-Matrices-with-Mex_002dFiles-1"></a>
<h4 class="subsection">A.2.6 Sparse Matrices with Mex-Files</h4>

<p>The Octave format for sparse matrices is identical to the mex format in
that it is a compressed column sparse format.  Also in both, sparse
matrices are required to be two-dimensional.  The only difference is that
the real and imaginary parts of the matrix are stored separately.
</p>
<p>The mex-file interface, in addition to using <code>mxGetM</code>, <code>mxGetN</code>,
<code>mxSetM</code>, <code>mxSetN</code>, <code>mxGetPr</code>, <code>mxGetPi</code>,
<code>mxSetPr</code>, and <code>mxSetPi</code>, also supplies the following functions.
</p>
<div class="example">
<pre class="example">mwIndex *mxGetIr (const mxArray *ptr);
mwIndex *mxGetJc (const mxArray *ptr);
mwSize mxGetNzmax (const mxArray *ptr);

void mxSetIr (mxArray *ptr, mwIndex *ir);
void mxSetJc (mxArray *ptr, mwIndex *jc);
void mxSetNzmax (mxArray *ptr, mwSize nzmax);
</pre></div>

<p><code>mxGetNzmax</code> gets the maximum number of elements that can be stored
in the sparse matrix.  This is not necessarily the number of nonzero
elements in the sparse matrix.  <code>mxGetJc</code> returns an array with one
additional value than the number of columns in the sparse matrix.  The
difference between consecutive values of the array returned by
<code>mxGetJc</code> define the number of nonzero elements in each column of
the sparse matrix.  Therefore,
</p>
<div class="example">
<pre class="example">mwSize nz, n;
mwIndex *Jc;
mxArray *m;
&hellip;
n = mxGetN (m);
Jc = mxGetJc (m);
nz = Jc[n];
</pre></div>

<p>returns the actual number of nonzero elements stored in the matrix in
<code>nz</code>.  As the arrays returned by <code>mxGetPr</code> and <code>mxGetPi</code>
only contain the nonzero values of the matrix, we also need a pointer
to the rows of the nonzero elements, and this is given by
<code>mxGetIr</code>.  A complete example of the use of sparse matrices in
mex-files is given by the file <samp>mysparse.c</samp> shown below.
</p>
<div class="example">
<pre class="verbatim">#include &quot;mex.h&quot;

void
mexFunction (int nlhs, mxArray *plhs[],
             int nrhs, const mxArray *prhs[])
{
  mwSize m, n, nz;
  mxArray *v;
  mwIndex i;
  double *pr, *pi;
  double *pr2, *pi2;
  mwIndex *ir, *jc;
  mwIndex *ir2, *jc2;

  if (nrhs != 1 || ! mxIsSparse (prhs[0]))
    mexErrMsgTxt (&quot;ARG1 must be a sparse matrix&quot;);

  m = mxGetM (prhs[0]);
  n = mxGetN (prhs[0]);
  nz = mxGetNzmax (prhs[0]);

  if (mxIsComplex (prhs[0]))
    {
      mexPrintf (&quot;Matrix is %d-by-%d complex sparse matrix&quot;, m, n);
      mexPrintf (&quot; with %d elements\n&quot;, nz);

      pr = mxGetPr (prhs[0]);
      pi = mxGetPi (prhs[0]);
      ir = mxGetIr (prhs[0]);
      jc = mxGetJc (prhs[0]);

      i = n;
      while (jc[i] == jc[i-1] &amp;&amp; i != 0) i--;

      mexPrintf (&quot;last nonzero element (%d, %d) = (%g, %g)\n&quot;,
                 ir[nz-1]+ 1, i, pr[nz-1], pi[nz-1]);

      v = mxCreateSparse (m, n, nz, mxCOMPLEX);
      pr2 = mxGetPr (v);
      pi2 = mxGetPi (v);
      ir2 = mxGetIr (v);
      jc2 = mxGetJc (v);

      for (i = 0; i &lt; nz; i++)
        {
          pr2[i] = 2 * pr[i];
          pi2[i] = 2 * pi[i];
          ir2[i] = ir[i];
        }
      for (i = 0; i &lt; n + 1; i++)
        jc2[i] = jc[i];

      if (nlhs &gt; 0)
        plhs[0] = v;
    }
  else if (mxIsLogical (prhs[0]))
    {
      mxLogical *pbr, *pbr2;
      mexPrintf (&quot;Matrix is %d-by-%d logical sparse matrix&quot;, m, n);
      mexPrintf (&quot; with %d elements\n&quot;, nz);

      pbr = mxGetLogicals (prhs[0]);
      ir = mxGetIr (prhs[0]);
      jc = mxGetJc (prhs[0]);

      i = n;
      while (jc[i] == jc[i-1] &amp;&amp; i != 0) i--;
      mexPrintf (&quot;last nonzero element (%d, %d) = %d\n&quot;,
                 ir[nz-1]+ 1, i, pbr[nz-1]);

      v = mxCreateSparseLogicalMatrix (m, n, nz);
      pbr2 = mxGetLogicals (v);
      ir2 = mxGetIr (v);
      jc2 = mxGetJc (v);

      for (i = 0; i &lt; nz; i++)
        {
          pbr2[i] = pbr[i];
          ir2[i] = ir[i];
        }
      for (i = 0; i &lt; n + 1; i++)
        jc2[i] = jc[i];

      if (nlhs &gt; 0)
        plhs[0] = v;
    }
  else
    {
      mexPrintf (&quot;Matrix is %d-by-%d real sparse matrix&quot;, m, n);
      mexPrintf (&quot; with %d elements\n&quot;, nz);

      pr = mxGetPr (prhs[0]);
      ir = mxGetIr (prhs[0]);
      jc = mxGetJc (prhs[0]);

      i = n;
      while (jc[i] == jc[i-1] &amp;&amp; i != 0) i--;
      mexPrintf (&quot;last nonzero element (%d, %d) = %g\n&quot;,
                 ir[nz-1]+ 1, i, pr[nz-1]);

      v = mxCreateSparse (m, n, nz, mxREAL);
      pr2 = mxGetPr (v);
      ir2 = mxGetIr (v);
      jc2 = mxGetJc (v);

      for (i = 0; i &lt; nz; i++)
        {
          pr2[i] = 2 * pr[i];
          ir2[i] = ir[i];
        }
      for (i = 0; i &lt; n + 1; i++)
        jc2[i] = jc[i];

      if (nlhs &gt; 0)
        plhs[0] = v;
    }
}
</pre><pre class="example">

</pre></div>

<p>A sample usage of <code>mysparse</code> is
</p>
<div class="example">
<pre class="example">sm = sparse ([1, 0; 0, pi]);
mysparse (sm)
&rArr;
Matrix is 2-by-2 real sparse matrix with 2 elements
last nonzero element (2, 2) = 3.14159
</pre></div>

<hr>
<div class="header">
<p>
Next: <a href="Calling-Other-Functions-in-Mex_002dFiles.html#Calling-Other-Functions-in-Mex_002dFiles" accesskey="n" rel="next">Calling Other Functions in Mex-Files</a>, Previous: <a href="Structures-with-Mex_002dFiles.html#Structures-with-Mex_002dFiles" accesskey="p" rel="prev">Structures with Mex-Files</a>, Up: <a href="Mex_002dFiles.html#Mex_002dFiles" accesskey="u" rel="up">Mex-Files</a> &nbsp; [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html#Concept-Index" title="Index" rel="index">Index</a>]</p>
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