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<title>Computing the rank - GNU Scientific Library -- Reference Manual</title>
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<h3 class="section">11.4 Computing the rank</h3>
<p>The <dfn>rank</dfn> of an element is its order in the sorted data. The rank
is the inverse of the index permutation, <var>p</var>. It can be computed
using the following algorithm,
<pre class="example"> for (i = 0; i < p->size; i++)
{
size_t pi = p->data[i];
rank->data[pi] = i;
}
</pre>
<p class="noindent">This can be computed directly from the function
<code>gsl_permutation_inverse(rank,p)</code>.
<p>The following function will print the rank of each element of the vector
<var>v</var>,
<pre class="example"> void
print_rank (gsl_vector * v)
{
size_t i;
size_t n = v->size;
gsl_permutation * perm = gsl_permutation_alloc(n);
gsl_permutation * rank = gsl_permutation_alloc(n);
gsl_sort_vector_index (perm, v);
gsl_permutation_inverse (rank, perm);
for (i = 0; i < n; i++)
{
double vi = gsl_vector_get(v, i);
printf ("element = %d, value = %g, rank = %d\n",
i, vi, rank->data[i]);
}
gsl_permutation_free (perm);
gsl_permutation_free (rank);
}
</pre>
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