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<h4 class="subsection">8.4.2 Accessing matrix elements</h4>
<p><a name="index-matrices_002c-range_002dchecking-914"></a><a name="index-range_002dchecking-for-matrices-915"></a>
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>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
<pre class="example"> m->data[i * m->tda + j]
</pre>
<p class="noindent">where <var>tda</var> is the physical row-length of the matrix.
<div class="defun">
— Function: double <b>gsl_matrix_get</b> (<var>const gsl_matrix * m, size_t i, size_t j</var>)<var><a name="index-gsl_005fmatrix_005fget-916"></a></var><br>
<blockquote><p>This function returns the (i,j)-th element of a matrix
<var>m</var>. If <var>i</var> or <var>j</var> lie outside the allowed range of 0 to
<var>n1</var>-1 and 0 to <var>n2</var>-1 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></blockquote></div>
<div class="defun">
— Function: void <b>gsl_matrix_set</b> (<var>gsl_matrix * m, size_t i, size_t j, double x</var>)<var><a name="index-gsl_005fmatrix_005fset-917"></a></var><br>
<blockquote><p>This function sets the value of the (i,j)-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 <var>n1</var>-1 and 0 to <var>n2</var>-1 then the error
handler is invoked. An inline version of this function is used when <code>HAVE_INLINE</code> is defined.
</p></blockquote></div>
<div class="defun">
— Function: double * <b>gsl_matrix_ptr</b> (<var>gsl_matrix * m, size_t i, size_t j</var>)<var><a name="index-gsl_005fmatrix_005fptr-918"></a></var><br>
— Function: const double * <b>gsl_matrix_const_ptr</b> (<var>const gsl_matrix * m, size_t i, size_t j</var>)<var><a name="index-gsl_005fmatrix_005fconst_005fptr-919"></a></var><br>
<blockquote><p>These functions return a pointer to the (i,j)-th element of a
matrix <var>m</var>. If <var>i</var> or <var>j</var> lie outside the allowed range of
0 to <var>n1</var>-1 and 0 to <var>n2</var>-1 then the error handler is invoked
and a null pointer is returned. Inline versions of these function are used when <code>HAVE_INLINE</code> is defined.
</p></blockquote></div>
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