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<h4 class="subsection" id="Sparse-Matrices-with-Mex_002dFiles-1"><span>A.2.6 Sparse Matrices with Mex-Files<a class="copiable-link" href="#Sparse-Matrices-with-Mex_002dFiles-1"> ¶</a></span></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 implementations sparse
matrices are required to be two-dimensional. The only difference of importance
to the programmer is that the real and imaginary parts of the matrix are stored
separately.
</p>
<p>The mex-file interface, in addition to using <code class="code">mxGetM</code>, <code class="code">mxGetN</code>,
<code class="code">mxSetM</code>, <code class="code">mxSetN</code>, <code class="code">mxGetPr</code>, <code class="code">mxGetPi</code>, <code class="code">mxSetPr</code>,
and <code class="code">mxSetPi</code>, also supplies the following functions.
</p>
<div class="example">
<div class="group"><pre class="example-preformatted">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></div>
<p><code class="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 class="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 class="code">mxGetJc</code> define the number of nonzero
elements in each column of the sparse matrix. Therefore,
</p>
<div class="example">
<div class="group"><pre class="example-preformatted">mwSize nz, n;
mwIndex *Jc;
mxArray *m;
...
n = mxGetN (m);
Jc = mxGetJc (m);
nz = Jc[n];
</pre></div></div>
<p>returns the actual number of nonzero elements stored in the matrix in
<code class="code">nz</code>. As the arrays returned by <code class="code">mxGetPr</code> and <code class="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 class="code">mxGetIr</code>. A complete example
of the use of sparse matrices in mex-files is given by the file
<samp class="file">mysparse.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 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 ("ARG1 must be a sparse matrix");
m = mxGetM (prhs[0]);
n = mxGetN (prhs[0]);
nz = mxGetNzmax (prhs[0]);
if (mxIsComplex (prhs[0]))
{
mexPrintf ("Matrix is %d-by-%d complex sparse matrix", m, n);
mexPrintf (" with %d elements\n", 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] && i != 0) i--;
mexPrintf ("last nonzero element (%d, %d) = (%g, %g)\n",
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 < nz; i++)
{
pr2[i] = 2 * pr[i];
pi2[i] = 2 * pi[i];
ir2[i] = ir[i];
}
for (i = 0; i < n + 1; i++)
jc2[i] = jc[i];
if (nlhs > 0)
plhs[0] = v;
}
else if (mxIsLogical (prhs[0]))
{
mxLogical *pbr, *pbr2;
mexPrintf ("Matrix is %d-by-%d logical sparse matrix", m, n);
mexPrintf (" with %d elements\n", nz);
pbr = mxGetLogicals (prhs[0]);
ir = mxGetIr (prhs[0]);
jc = mxGetJc (prhs[0]);
i = n;
while (jc[i] == jc[i-1] && i != 0) i--;
mexPrintf ("last nonzero element (%d, %d) = %d\n",
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 < nz; i++)
{
pbr2[i] = pbr[i];
ir2[i] = ir[i];
}
for (i = 0; i < n + 1; i++)
jc2[i] = jc[i];
if (nlhs > 0)
plhs[0] = v;
}
else
{
mexPrintf ("Matrix is %d-by-%d real sparse matrix", m, n);
mexPrintf (" with %d elements\n", nz);
pr = mxGetPr (prhs[0]);
ir = mxGetIr (prhs[0]);
jc = mxGetJc (prhs[0]);
i = n;
while (jc[i] == jc[i-1] && i != 0) i--;
mexPrintf ("last nonzero element (%d, %d) = %g\n",
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 < nz; i++)
{
pr2[i] = 2 * pr[i];
ir2[i] = ir[i];
}
for (i = 0; i < n + 1; i++)
jc2[i] = jc[i];
if (nlhs > 0)
plhs[0] = v;
}
}
</pre></div>
<p>A sample usage of <code class="code">mysparse</code> is
</p>
<div class="example">
<div class="group"><pre class="example-preformatted">sm = sparse ([1, 0; 0, pi]);
mysparse (sm)
⇒
Matrix is 2-by-2 real sparse matrix with 2 elements
last nonzero element (2, 2) = 3.14159
</pre></div></div>
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