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            <h1>sort</h1>


  <!--    -->
<ul><li><a href="#NAME">NAME</a><li><a href="#SYNOPSIS">SYNOPSIS</a><li><a href="#DESCRIPTION">DESCRIPTION</a><li><a href="#CAVEATS">CAVEATS</a></ul><a name="NAME"></a><h1>NAME</h1>
<p>sort - perl pragma to control sort() behaviour</p>
<a name="SYNOPSIS"></a><h1>SYNOPSIS</h1>
<pre class="verbatim"><ol><li>    <a class="l_k" href="functions/use.html">use</a> <a class="l_k" href="functions/sort.html">sort</a> <span class="q">&#39;stable&#39;</span><span class="sc">;</span>		<span class="c"># guarantee stability</span></li><li>    <a class="l_k" href="functions/use.html">use</a> <a class="l_k" href="functions/sort.html">sort</a> <span class="q">&#39;_quicksort&#39;</span><span class="sc">;</span>	<span class="c"># use a quicksort algorithm</span></li><li>    <a class="l_k" href="functions/use.html">use</a> <a class="l_k" href="functions/sort.html">sort</a> <span class="q">&#39;_mergesort&#39;</span><span class="sc">;</span>	<span class="c"># use a mergesort algorithm</span></li><li>    <a class="l_k" href="functions/use.html">use</a> <a class="l_k" href="functions/sort.html">sort</a> <span class="q">&#39;defaults&#39;</span><span class="sc">;</span>	<span class="c"># revert to default behavior</span></li><li>    <a class="l_k" href="functions/no.html">no</a>  <a class="l_k" href="functions/sort.html">sort</a> <span class="q">&#39;stable&#39;</span><span class="sc">;</span>		<span class="c"># stability not important</span></li><li></li><li>    <a class="l_k" href="functions/use.html">use</a> <a class="l_k" href="functions/sort.html">sort</a> <span class="q">&#39;_qsort&#39;</span><span class="sc">;</span>		<span class="c"># alias for quicksort</span></li><li></li><li>    <a class="l_k" href="functions/my.html">my</a> <span class="i">$current</span><span class="sc">;</span></li><li>    BEGIN <span class="s">{</span></li><li>	<span class="i">$current</span> = <span class="i">sort::current</span><span class="s">(</span><span class="s">)</span><span class="sc">;</span>	<span class="c"># identify prevailing algorithm</span></li><li>    <span class="s">}</span></li></ol></pre><a name="DESCRIPTION"></a><h1>DESCRIPTION</h1>
<p>With the <code class="inline"><a class="l_k" href="functions/sort.html">sort</a></code> pragma you can control the behaviour of the builtin
<code class="inline"><a class="l_k" href="functions/sort.html">sort()</a></code> function.</p>
<p>In Perl versions 5.6 and earlier the quicksort algorithm was used to
implement <code class="inline"><a class="l_k" href="functions/sort.html">sort()</a></code>, but in Perl 5.8 a mergesort algorithm was also made
available, mainly to guarantee worst case O(N log N) behaviour:
the worst case of quicksort is O(N**2).  In Perl 5.8 and later,
quicksort defends against quadratic behaviour by shuffling large
arrays before sorting.</p>
<p>A stable sort means that for records that compare equal, the original
input ordering is preserved.  Mergesort is stable, quicksort is not.
Stability will matter only if elements that compare equal can be
distinguished in some other way.  That means that simple numerical
and lexical sorts do not profit from stability, since equal elements
are indistinguishable.  However, with a comparison such as</p>
<pre class="verbatim"><ol><li>   <span class="s">{</span> <a class="l_k" href="functions/substr.html">substr</a><span class="s">(</span><span class="i">$a</span><span class="cm">,</span> <span class="n">0</span><span class="cm">,</span> <span class="n">3</span><span class="s">)</span> cmp <a class="l_k" href="functions/substr.html">substr</a><span class="s">(</span><span class="i">$b</span><span class="cm">,</span> <span class="n">0</span><span class="cm">,</span> <span class="n">3</span><span class="s">)</span> <span class="s">}</span></li></ol></pre><p>stability might matter because elements that compare equal on the
first 3 characters may be distinguished based on subsequent characters.
In Perl 5.8 and later, quicksort can be stabilized, but doing so will
add overhead, so it should only be done if it matters.</p>
<p>The best algorithm depends on many things.  On average, mergesort
does fewer comparisons than quicksort, so it may be better when
complicated comparison routines are used.  Mergesort also takes
advantage of pre-existing order, so it would be favored for using
<code class="inline"><a class="l_k" href="functions/sort.html">sort()</a></code> to merge several sorted arrays.  On the other hand, quicksort
is often faster for small arrays, and on arrays of a few distinct
values, repeated many times.  You can force the
choice of algorithm with this pragma, but this feels heavy-handed,
so the subpragmas beginning with a <code class="inline"><span class="w">_</span></code>
 may not persist beyond Perl 5.8.
The default algorithm is mergesort, which will be stable even if
you do not explicitly demand it.
But the stability of the default sort is a side-effect that could
change in later versions.  If stability is important, be sure to
say so with a</p>
<pre class="verbatim"><ol><li>  <a class="l_k" href="functions/use.html">use</a> <a class="l_k" href="functions/sort.html">sort</a> <span class="q">&#39;stable&#39;</span><span class="sc">;</span></li></ol></pre><p>The <code class="inline"><a class="l_k" href="functions/no.html">no</a> <a class="l_k" href="functions/sort.html">sort</a></code>
 pragma doesn't
<i>forbid</i> what follows, it just leaves the choice open.  Thus, after</p>
<pre class="verbatim"><ol><li>  <a class="l_k" href="functions/no.html">no</a> <a class="l_k" href="functions/sort.html">sort</a> <span class="q">qw(_mergesort stable)</span><span class="sc">;</span></li></ol></pre><p>a mergesort, which happens to be stable, will be employed anyway.
Note that</p>
<pre class="verbatim"><ol><li>  <a class="l_k" href="functions/no.html">no</a> <a class="l_k" href="functions/sort.html">sort</a> <span class="q">&quot;_quicksort&quot;</span><span class="sc">;</span></li><li>  <a class="l_k" href="functions/no.html">no</a> <a class="l_k" href="functions/sort.html">sort</a> <span class="q">&quot;_mergesort&quot;</span><span class="sc">;</span></li></ol></pre><p>have exactly the same effect, leaving the choice of sort algorithm open.</p>
<a name="CAVEATS"></a><h1>CAVEATS</h1>
<p>As of Perl 5.10, this pragma is lexically scoped and takes effect
at compile time. In earlier versions its effect was global and took
effect at run-time; the documentation suggested using <code class="inline"><a class="l_k" href="functions/eval.html">eval()</a></code> to
change the behaviour:</p>
<pre class="verbatim"><ol><li>  <span class="s">{</span> <a class="l_k" href="functions/eval.html">eval</a> <span class="q">'use sort qw(defaults _quicksort)'</span><span class="sc">;</span> <span class="c"># force quicksort</span></li><li>    <a class="l_k" href="functions/eval.html">eval</a> <span class="q">'no sort &quot;stable&quot;'</span><span class="sc">;</span>      <span class="c"># stability not wanted</span></li><li>    <a class="l_k" href="functions/print.html">print</a> <span class="i">sort::current</span> . <span class="q">&quot;\n&quot;</span><span class="sc">;</span></li><li>    <span class="i">@a</span> = <a class="l_k" href="functions/sort.html">sort</a> <span class="i">@b</span><span class="sc">;</span></li><li>    <a class="l_k" href="functions/eval.html">eval</a> <span class="q">'use sort &quot;defaults&quot;'</span><span class="sc">;</span>   <span class="c"># clean up, for others</span></li><li>  <span class="s">}</span></li><li>  <span class="s">{</span> <a class="l_k" href="functions/eval.html">eval</a> <span class="q">'use sort qw(defaults stable)'</span><span class="sc">;</span>     <span class="c"># force stability</span></li><li>    <a class="l_k" href="functions/print.html">print</a> <span class="i">sort::current</span> . <span class="q">&quot;\n&quot;</span><span class="sc">;</span></li><li>    <span class="i">@c</span> = <a class="l_k" href="functions/sort.html">sort</a> <span class="i">@d</span><span class="sc">;</span></li><li>    <a class="l_k" href="functions/eval.html">eval</a> <span class="q">'use sort &quot;defaults&quot;'</span><span class="sc">;</span>   <span class="c"># clean up, for others</span></li><li>  <span class="s">}</span></li></ol></pre><p>Such code no longer has the desired effect, for two reasons.
Firstly, the use of <code class="inline"><a class="l_k" href="functions/eval.html">eval()</a></code> means that the sorting algorithm
is not changed until runtime, by which time it's too late to
have any effect. Secondly, <code class="inline"><a class="l_k" href="functions/sort.html">sort::current</a></code> is also called at
run-time, when in fact the compile-time value of <code class="inline"><a class="l_k" href="functions/sort.html">sort::current</a></code>
is the one that matters.</p>
<p>So now this code would be written:</p>
<pre class="verbatim"><ol><li>  <span class="s">{</span> <a class="l_k" href="functions/use.html">use</a> <a class="l_k" href="functions/sort.html">sort</a> <span class="q">qw(defaults _quicksort)</span><span class="sc">;</span> <span class="c"># force quicksort</span></li><li>    <a class="l_k" href="functions/no.html">no</a> <a class="l_k" href="functions/sort.html">sort</a> <span class="q">&quot;stable&quot;</span><span class="sc">;</span>      <span class="c"># stability not wanted</span></li><li>    <a class="l_k" href="functions/my.html">my</a> <span class="i">$current</span><span class="sc">;</span></li><li>    <a class="l_k" href="functions/BEGIN.html">BEGIN</a> <span class="s">{</span> <span class="i">$current</span> = <span class="w">sort::current</span><span class="sc">;</span> <span class="s">}</span></li><li>    <a class="l_k" href="functions/print.html">print</a> <span class="q">&quot;$current\n&quot;</span><span class="sc">;</span></li><li>    <span class="i">@a</span> = <a class="l_k" href="functions/sort.html">sort</a> <span class="i">@b</span><span class="sc">;</span></li><li>    <span class="c"># Pragmas go out of scope at the end of the block</span></li><li>  <span class="s">}</span></li><li>  <span class="s">{</span> <a class="l_k" href="functions/use.html">use</a> <a class="l_k" href="functions/sort.html">sort</a> <span class="q">qw(defaults stable)</span><span class="sc">;</span>     <span class="c"># force stability</span></li><li>    <a class="l_k" href="functions/my.html">my</a> <span class="i">$current</span><span class="sc">;</span></li><li>    <a class="l_k" href="functions/BEGIN.html">BEGIN</a> <span class="s">{</span> <span class="i">$current</span> = <span class="w">sort::current</span><span class="sc">;</span> <span class="s">}</span></li><li>    <a class="l_k" href="functions/print.html">print</a> <span class="q">&quot;$current\n&quot;</span><span class="sc">;</span></li><li>    <span class="i">@c</span> = <a class="l_k" href="functions/sort.html">sort</a> <span class="i">@d</span><span class="sc">;</span></li><li>  <span class="s">}</span></li></ol></pre>



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