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<h3 class="section">19.38 Shuffling and Sampling</h3>
<p>The following functions allow the shuffling and sampling of a set of
objects. The algorithms rely on a random number generator as a source of
randomness and a poor quality generator can lead to correlations in the
output. In particular it is important to avoid generators with a short
period. For more information see Knuth, v2, 3rd ed, Section 3.4.2,
“Random Sampling and Shuffling”.
<div class="defun">
— Function: void <b>gsl_ran_shuffle</b> (<var>const gsl_rng * r, void * base, size_t n, size_t size</var>)<var><a name="index-gsl_005fran_005fshuffle-1805"></a></var><br>
<blockquote>
<p>This function randomly shuffles the order of <var>n</var> objects, each of
size <var>size</var>, stored in the array <var>base</var>[0..<var>n</var>-1]. The
output of the random number generator <var>r</var> is used to produce the
permutation. The algorithm generates all possible n!
permutations with equal probability, assuming a perfect source of random
numbers.
<p>The following code shows how to shuffle the numbers from 0 to 51,
<pre class="example"> int a[52];
for (i = 0; i < 52; i++)
{
a[i] = i;
}
gsl_ran_shuffle (r, a, 52, sizeof (int));
</pre>
</blockquote></div>
<div class="defun">
— Function: int <b>gsl_ran_choose</b> (<var>const gsl_rng * r, void * dest, size_t k, void * src, size_t n, size_t size</var>)<var><a name="index-gsl_005fran_005fchoose-1806"></a></var><br>
<blockquote><p>This function fills the array <var>dest</var>[k] with <var>k</var> objects taken
randomly from the <var>n</var> elements of the array
<var>src</var>[0..<var>n</var>-1]. The objects are each of size <var>size</var>. The
output of the random number generator <var>r</var> is used to make the
selection. The algorithm ensures all possible samples are equally
likely, assuming a perfect source of randomness.
<p>The objects are sampled <em>without</em> replacement, thus each object can
only appear once in <var>dest</var>[k]. It is required that <var>k</var> be less
than or equal to <code>n</code>. The objects in <var>dest</var> will be in the
same relative order as those in <var>src</var>. You will need to call
<code>gsl_ran_shuffle(r, dest, n, size)</code> if you want to randomize the
order.
<p>The following code shows how to select a random sample of three unique
numbers from the set 0 to 99,
<pre class="example"> double a[3], b[100];
for (i = 0; i < 100; i++)
{
b[i] = (double) i;
}
gsl_ran_choose (r, a, 3, b, 100, sizeof (double));
</pre>
</blockquote></div>
<div class="defun">
— Function: void <b>gsl_ran_sample</b> (<var>const gsl_rng * r, void * dest, size_t k, void * src, size_t n, size_t size</var>)<var><a name="index-gsl_005fran_005fsample-1807"></a></var><br>
<blockquote><p>This function is like <code>gsl_ran_choose</code> but samples <var>k</var> items
from the original array of <var>n</var> items <var>src</var> with replacement, so
the same object can appear more than once in the output sequence
<var>dest</var>. There is no requirement that <var>k</var> be less than <var>n</var>
in this case.
</p></blockquote></div>
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