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<title>GNU Scientific Library – Reference Manual: Accessing vector elements</title>
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<a name="Accessing-vector-elements"></a>
<div class="header">
<p>
Next: <a href="Initializing-vector-elements.html#Initializing-vector-elements" accesskey="n" rel="next">Initializing vector elements</a>, Previous: <a href="Vector-allocation.html#Vector-allocation" accesskey="p" rel="previous">Vector allocation</a>, Up: <a href="Vectors.html#Vectors" accesskey="u" rel="up">Vectors</a> [<a href="Function-Index.html#Function-Index" title="Index" rel="index">Index</a>]</p>
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<hr>
<a name="Accessing-vector-elements-1"></a>
<h4 class="subsection">8.3.2 Accessing vector elements</h4>
<a name="index-vectors_002c-range_002dchecking"></a>
<a name="index-range_002dchecking-for-vectors"></a>
<a name="index-bounds-checking_002c-extension-to-GCC"></a>
<a name="index-gcc-extensions_002c-range_002dchecking"></a>
<a name="index-Fortran-range-checking_002c-equivalent-in-gcc"></a>
<p>Unlike <small>FORTRAN</small> compilers, C compilers do not usually provide
support for range checking of vectors and matrices.<a name="DOCF8" href="#FOOT8"><sup>8</sup></a> The functions <code>gsl_vector_get</code> and
<code>gsl_vector_set</code> can perform portable range checking for you and
report an error if you attempt to access elements outside the allowed
range.
</p>
<p>The functions for accessing the elements of a vector or matrix are
defined in <samp>gsl_vector.h</samp> and declared <code>extern inline</code> to
eliminate function-call overhead. You must compile your program with
the preprocessor macro <code>HAVE_INLINE</code> defined to use these
functions.
</p>
<a name="index-GSL_005fRANGE_005fCHECK_005fOFF"></a>
<p>If necessary you can turn off range checking completely without
modifying any source files by recompiling your program with the
preprocessor definition <code>GSL_RANGE_CHECK_OFF</code>. Provided your
compiler supports inline functions the effect of turning off range
checking is to replace calls to <code>gsl_vector_get(v,i)</code> by
<code>v->data[i*v->stride]</code> and calls to <code>gsl_vector_set(v,i,x)</code> by
<code>v->data[i*v->stride]=x</code>. Thus there should be no performance
penalty for using the range checking functions when range checking is
turned off.
</p>
<a name="index-GSL_005fC99_005fINLINE-2"></a>
<p>If you use a C99 compiler which requires inline functions in header
files to be declared <code>inline</code> instead of <code>extern inline</code>,
define the macro <code>GSL_C99_INLINE</code> (see <a href="Inline-functions.html#Inline-functions">Inline functions</a>).
With GCC this is selected automatically when compiling in C99 mode
(<code>-std=c99</code>).
</p>
<a name="index-gsl_005fcheck_005frange"></a>
<p>If inline functions are not used, calls to the functions
<code>gsl_vector_get</code> and <code>gsl_vector_set</code> will link to the
compiled versions of these functions in the library itself. The range
checking in these functions is controlled by the global integer
variable <code>gsl_check_range</code>. It is enabled by default—to
disable it, set <code>gsl_check_range</code> to zero. Due to function-call
overhead, there is less benefit in disabling range checking here than
for inline functions.
</p>
<dl>
<dt><a name="index-gsl_005fvector_005fget"></a>Function: <em>double</em> <strong>gsl_vector_get</strong> <em>(const gsl_vector * <var>v</var>, const size_t <var>i</var>)</em></dt>
<dd><p>This function returns the <var>i</var>-th element of a vector <var>v</var>. If
<var>i</var> lies outside the allowed range of 0 to <em><var>n</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_005fvector_005fset"></a>Function: <em>void</em> <strong>gsl_vector_set</strong> <em>(gsl_vector * <var>v</var>, const size_t <var>i</var>, double <var>x</var>)</em></dt>
<dd><p>This function sets the value of the <var>i</var>-th element of a vector
<var>v</var> to <var>x</var>. If <var>i</var> lies outside the allowed range of 0 to
<em><var>n</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_005fvector_005fptr"></a>Function: <em>double *</em> <strong>gsl_vector_ptr</strong> <em>(gsl_vector * <var>v</var>, size_t <var>i</var>)</em></dt>
<dt><a name="index-gsl_005fvector_005fconst_005fptr"></a>Function: <em>const double *</em> <strong>gsl_vector_const_ptr</strong> <em>(const gsl_vector * <var>v</var>, size_t <var>i</var>)</em></dt>
<dd><p>These functions return a pointer to the <var>i</var>-th element of a vector
<var>v</var>. If <var>i</var> lies outside the allowed range of 0 to <em><var>n</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>
<div class="footnote">
<hr>
<h4 class="footnotes-heading">Footnotes</h4>
<h3><a name="FOOT8" href="#DOCF8">(8)</a></h3>
<p>Range
checking is available in the GNU C Compiler bounds-checking extension,
but it is not part of the default installation of GCC. Memory accesses
can also be checked with Valgrind or the <code>gcc -fmudflap</code>
memory protection option.</p>
</div>
<hr>
<div class="header">
<p>
Next: <a href="Initializing-vector-elements.html#Initializing-vector-elements" accesskey="n" rel="next">Initializing vector elements</a>, Previous: <a href="Vector-allocation.html#Vector-allocation" accesskey="p" rel="previous">Vector allocation</a>, Up: <a href="Vectors.html#Vectors" accesskey="u" rel="up">Vectors</a> [<a href="Function-Index.html#Function-Index" title="Index" rel="index">Index</a>]</p>
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