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<title>GNU Scientific Library – Reference Manual: Accessing matrix elements</title>
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<a name="Accessing-matrix-elements"></a>
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Next: <a href="Initializing-matrix-elements.html#Initializing-matrix-elements" accesskey="n" rel="next">Initializing matrix elements</a>, Previous: <a href="Matrix-allocation.html#Matrix-allocation" accesskey="p" rel="previous">Matrix allocation</a>, Up: <a href="Matrices.html#Matrices" accesskey="u" rel="up">Matrices</a> [<a href="Function-Index.html#Function-Index" title="Index" rel="index">Index</a>]</p>
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<a name="Accessing-matrix-elements-1"></a>
<h4 class="subsection">8.4.2 Accessing matrix elements</h4>
<a name="index-matrices_002c-range_002dchecking"></a>
<a name="index-range_002dchecking-for-matrices"></a>
<p>The functions for accessing the elements of a matrix use the same range
checking system as vectors. You can turn off range checking by recompiling
your program with the preprocessor definition
<code>GSL_RANGE_CHECK_OFF</code>.
</p>
<p>The elements of the matrix are stored in “C-order”, where the second
index moves continuously through memory. More precisely, the element
accessed by the function <code>gsl_matrix_get(m,i,j)</code> and
<code>gsl_matrix_set(m,i,j,x)</code> is
</p>
<div class="example">
<pre class="example">m->data[i * m->tda + j]
</pre></div>
<p>where <var>tda</var> is the physical row-length of the matrix.
</p>
<dl>
<dt><a name="index-gsl_005fmatrix_005fget"></a>Function: <em>double</em> <strong>gsl_matrix_get</strong> <em>(const gsl_matrix * <var>m</var>, size_t <var>i</var>, size_t <var>j</var>)</em></dt>
<dd><p>This function returns the <em>(i,j)</em>-th element of a matrix
<var>m</var>. If <var>i</var> or <var>j</var> lie outside the allowed range of 0 to
<em><var>n1</var>-1</em> and 0 to <em><var>n2</var>-1</em> then the error handler is invoked and 0
is returned. An inline version of this function is used when <code>HAVE_INLINE</code> is defined.
</p></dd></dl>
<dl>
<dt><a name="index-gsl_005fmatrix_005fset"></a>Function: <em>void</em> <strong>gsl_matrix_set</strong> <em>(gsl_matrix * <var>m</var>, size_t <var>i</var>, size_t <var>j</var>, double <var>x</var>)</em></dt>
<dd><p>This function sets the value of the <em>(i,j)</em>-th element of a
matrix <var>m</var> to <var>x</var>. If <var>i</var> or <var>j</var> lies outside the
allowed range of 0 to <em><var>n1</var>-1</em> and 0 to <em><var>n2</var>-1</em> then the error
handler is invoked. An inline version of this function is used when <code>HAVE_INLINE</code> is defined.
</p></dd></dl>
<dl>
<dt><a name="index-gsl_005fmatrix_005fptr"></a>Function: <em>double *</em> <strong>gsl_matrix_ptr</strong> <em>(gsl_matrix * <var>m</var>, size_t <var>i</var>, size_t <var>j</var>)</em></dt>
<dt><a name="index-gsl_005fmatrix_005fconst_005fptr"></a>Function: <em>const double *</em> <strong>gsl_matrix_const_ptr</strong> <em>(const gsl_matrix * <var>m</var>, size_t <var>i</var>, size_t <var>j</var>)</em></dt>
<dd><p>These functions return a pointer to the <em>(i,j)</em>-th element of a
matrix <var>m</var>. If <var>i</var> or <var>j</var> lie outside the allowed range of
0 to <em><var>n1</var>-1</em> and 0 to <em><var>n2</var>-1</em> then the error handler is invoked
and a null pointer is returned. Inline versions of these functions are used when <code>HAVE_INLINE</code> is defined.
</p></dd></dl>
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