sort - perl pragma to control sort() behaviour

<!DOCTYPE html> <html lang="en"> <head> <meta charset="utf-8"> <meta http-equiv="X-UA-Compatible" content="IE=edge"> <meta name="viewport" content="width=device-width, initial-scale=1"> <title>sort - perl pragma to control sort() behaviour - Perldoc Browser</title> <link rel="search" href="/opensearch.xml" type="application/opensearchdescription+xml" title="Perldoc Browser"> <link rel="canonical" href="https://perldoc.perl.org/sort"> <link rel="stylesheet" href="https://stackpath.bootstrapcdn.com/bootstrap/4.5.2/css/bootstrap.min.css" integrity="sha384-JcKb8q3iqJ61gNV9KGb8thSsNjpSL0n8PARn9HuZOnIxN0hoP+VmmDGMN5t9UJ0Z" crossorigin="anonymous"> <link rel="stylesheet" href="https://cdnjs.cloudflare.com/ajax/libs/highlight.js/10.5.0/styles/stackoverflow-light.min.css" integrity="sha512-cG1IdFxqipi3gqLmksLtuk13C+hBa57a6zpWxMeoY3Q9O6ooFxq50DayCdm0QrDgZjMUn23z/0PMZlgft7Yp5Q==" crossorigin="anonymous" /> <style> body { background: #f4f4f5; color: #020202; } .navbar-dark { background-image: -webkit-linear-gradient(top, #005f85 0, #002e49 100%); background-image: -o-linear-gradient(top, #005f85 0, #002e49 100%); background-image: linear-gradient(to bottom, #005f85 0, #002e49 100%); filter: progid:DXImageTransform.Microsoft.gradient(startColorstr='#ff005f85', endColorstr='#ff002e49', GradientType=0); background-repeat: repeat-x; } .navbar-dark .navbar-nav .nav-link, .navbar-dark .navbar-nav .nav-link:focus { color: #fff } .navbar-dark .navbar-nav .nav-link:hover { color: #ffef68 } #wrapperlicious { margin: 0 auto; font: 0.9em 'Helvetica Neue', Helvetica, sans-serif; font-weight: normal; line-height: 1.5em; margin: 0; padding: 0; } #wrapperlicious h1 { font-size: 1.5em } #wrapperlicious h2 { font-size: 1.3em } #wrapperlicious h3 { font-size: 1.1em } #wrapperlicious h4 { font-size: 0.9em } #wrapperlicious h1, #wrapperlicious h2, #wrapperlicious h3, #wrapperlicious h4, #wrapperlicious dt { color: #020202; margin-top: 1em; margin-bottom: 1em; position: relative; font-weight: bold; } #wrapperlicious a { color: inherit; text-decoration: underline } #wrapperlicious #toc { text-decoration: none } #wrapperlicious a:hover { color: #2a2a2a } #wrapperlicious a img { border: 0 } #wrapperlicious :not(pre) > code { color: inherit; background-color: rgba(0, 0, 0, 0.04); border-radius: 3px; font: 0.9em Consolas, Menlo, Monaco, monospace; padding: 0.3em; } #wrapperlicious dd { margin: 0; margin-left: 2em; } #wrapperlicious dt { color: #2a2a2a; font-weight: bold; margin-left: 0.9em; } #wrapperlicious p { margin-bottom: 1em; margin-top: 1em; } #wrapperlicious li > p { margin-bottom: 0; margin-top: 0; } #wrapperlicious pre { border: 1px solid #c1c1c1; border-radius: 3px; font: 100% Consolas, Menlo, Monaco, monospace; margin-bottom: 1em; margin-top: 1em; } #wrapperlicious pre > code { display: block; background-color: #f6f6f6; font: 0.9em Consolas, Menlo, Monaco, monospace; line-height: 1.5em; text-align: left; white-space: pre; padding: 1em; } #wrapperlicious dl, #wrapperlicious ol, #wrapperlicious ul { margin-bottom: 1em; margin-top: 1em; } #wrapperlicious ul { list-style-type: square; } #wrapperlicious ul ul { margin-bottom: 0px; margin-top: 0px; } #footer { font-size: 0.8em; padding-top: 0.5em; text-align: center; } #more { display: inline; font-size: 0.8em; } #perldocdiv { background-color: #fff; border: 1px solid #c1c1c1; border-bottom-left-radius: 5px; border-bottom-right-radius: 5px; margin-left: auto; margin-right: auto; padding: 3em; padding-top: 1em; max-width: 960px; } #moduleversion { float: right } #wrapperlicious .leading-notice { font-style: italic; padding-left: 1em; margin-top: 1em; margin-bottom: 1em; } #wrapperlicious .permalink { display: none; left: -0.75em; position: absolute; padding-right: 0.25em; text-decoration: none; } #wrapperlicious h1:hover .permalink, #wrapperlicious h2:hover .permalink, #wrapperlicious h3:hover .permalink, #wrapperlicious h4:hover .permalink, #wrapperlicious dt:hover .permalink { display: block; } </style>  <script async src="https://www.googletagmanager.com/gtag/js?id=G-KVNWBNT5FB"></script> <script> window.dataLayer = window.dataLayer || []; function gtag(){dataLayer.push(arguments);} gtag('js', new Date()); gtag('config', 'G-KVNWBNT5FB'); gtag('config', 'UA-50555-3'); </script> </head> <body> <nav class="navbar navbar-expand-md navbar-dark bg-dark justify-content-between"> <button class="navbar-toggler" type="button" data-toggle="collapse" data-target="#navbarNav" aria-controls="navbarNav" aria-expanded="false" aria-label="Toggle navigation"> <span class="navbar-toggler-icon"></span> </button> <a class="navbar-brand" href="/"><img src="/images/perl_camel_30.png" width="30" height="30" class="d-inline-block align-top" alt="Perl Camel Logo"> Perldoc Browser</a> <div class="collapse navbar-collapse" id="navbarNav"> <ul class="navbar-nav mr-auto"> <li class="nav-item dropdown text-nowrap"> <a class="nav-link dropdown-toggle" href="#" id="dropdownlink-stable" role="button" data-toggle="dropdown" aria-haspopup="true" aria-expanded="false">5.8.0</a> <div class="dropdown-menu" aria-labelledby="dropdownlink-stable"> <a class="dropdown-item" href="/sort">Latest</a> <div class="dropdown-divider"></div> <a class="dropdown-item" href="/5.40.1/sort">5.40.1</a> <a class="dropdown-item" href="/5.40.0/sort">5.40.0</a> <div class="dropdown-divider"></div> <a class="dropdown-item" href="/5.38.3/sort">5.38.3</a> <a class="dropdown-item" href="/5.38.2/sort">5.38.2</a> <a class="dropdown-item" href="/5.38.1/sort">5.38.1</a> <a class="dropdown-item" href="/5.38.0/sort">5.38.0</a> <div class="dropdown-divider"></div> <a class="dropdown-item" href="/5.36.3/sort">5.36.3</a> <a class="dropdown-item" href="/5.36.2/sort">5.36.2</a> <a class="dropdown-item" href="/5.36.1/sort">5.36.1</a> <a class="dropdown-item" href="/5.36.0/sort">5.36.0</a> <div class="dropdown-divider"></div> <a class="dropdown-item" href="/5.34.3/sort">5.34.3</a> <a class="dropdown-item" href="/5.34.2/sort">5.34.2</a> <a class="dropdown-item" href="/5.34.1/sort">5.34.1</a> <a class="dropdown-item" href="/5.34.0/sort">5.34.0</a> <div class="dropdown-divider"></div> <a class="dropdown-item" href="/5.32.1/sort">5.32.1</a> <a class="dropdown-item" href="/5.32.0/sort">5.32.0</a> <div class="dropdown-divider"></div> <a class="dropdown-item" href="/5.30.3/sort">5.30.3</a> <a class="dropdown-item" href="/5.30.2/sort">5.30.2</a> <a class="dropdown-item" href="/5.30.1/sort">5.30.1</a> <a class="dropdown-item" href="/5.30.0/sort">5.30.0</a> <div class="dropdown-divider"></div> <a class="dropdown-item" href="/5.28.3/sort">5.28.3</a> <a class="dropdown-item" href="/5.28.2/sort">5.28.2</a> <a class="dropdown-item" href="/5.28.1/sort">5.28.1</a> <a class="dropdown-item" href="/5.28.0/sort">5.28.0</a> <div class="dropdown-divider"></div> <a class="dropdown-item" href="/5.26.3/sort">5.26.3</a> <a class="dropdown-item" href="/5.26.2/sort">5.26.2</a> <a class="dropdown-item" href="/5.26.1/sort">5.26.1</a> <a class="dropdown-item" href="/5.26.0/sort">5.26.0</a> <div class="dropdown-divider"></div> <a class="dropdown-item" href="/5.24.4/sort">5.24.4</a> <a class="dropdown-item" href="/5.24.3/sort">5.24.3</a> <a class="dropdown-item" href="/5.24.2/sort">5.24.2</a> <a class="dropdown-item" href="/5.24.1/sort">5.24.1</a> <a class="dropdown-item" href="/5.24.0/sort">5.24.0</a> <div class="dropdown-divider"></div> <a class="dropdown-item" href="/5.22.4/sort">5.22.4</a> <a class="dropdown-item" href="/5.22.3/sort">5.22.3</a> <a class="dropdown-item" href="/5.22.2/sort">5.22.2</a> <a class="dropdown-item" href="/5.22.1/sort">5.22.1</a> <a class="dropdown-item" href="/5.22.0/sort">5.22.0</a> <div class="dropdown-divider"></div> <a class="dropdown-item" href="/5.20.3/sort">5.20.3</a> <a class="dropdown-item" href="/5.20.2/sort">5.20.2</a> <a class="dropdown-item" href="/5.20.1/sort">5.20.1</a> <a class="dropdown-item" href="/5.20.0/sort">5.20.0</a> <div class="dropdown-divider"></div> <a class="dropdown-item" href="/5.18.4/sort">5.18.4</a> <a class="dropdown-item" href="/5.18.3/sort">5.18.3</a> <a class="dropdown-item" href="/5.18.2/sort">5.18.2</a> <a class="dropdown-item" href="/5.18.1/sort">5.18.1</a> <a class="dropdown-item" href="/5.18.0/sort">5.18.0</a> <div class="dropdown-divider"></div> <a class="dropdown-item" href="/5.16.3/sort">5.16.3</a> <a class="dropdown-item" href="/5.16.2/sort">5.16.2</a> <a class="dropdown-item" href="/5.16.1/sort">5.16.1</a> <a class="dropdown-item" href="/5.16.0/sort">5.16.0</a> <div class="dropdown-divider"></div> <a class="dropdown-item" href="/5.14.4/sort">5.14.4</a> <a class="dropdown-item" href="/5.14.3/sort">5.14.3</a> <a class="dropdown-item" href="/5.14.2/sort">5.14.2</a> <a class="dropdown-item" href="/5.14.1/sort">5.14.1</a> <a class="dropdown-item" href="/5.14.0/sort">5.14.0</a> <div class="dropdown-divider"></div> <a class="dropdown-item" href="/5.12.5/sort">5.12.5</a> <a class="dropdown-item" href="/5.12.4/sort">5.12.4</a> <a class="dropdown-item" href="/5.12.3/sort">5.12.3</a> <a class="dropdown-item" href="/5.12.2/sort">5.12.2</a> <a class="dropdown-item" href="/5.12.1/sort">5.12.1</a> <a class="dropdown-item" href="/5.12.0/sort">5.12.0</a> <div class="dropdown-divider"></div> <a class="dropdown-item" href="/5.10.1/sort">5.10.1</a> <a class="dropdown-item" href="/5.10.0/sort">5.10.0</a> <div class="dropdown-divider"></div> <a class="dropdown-item" href="/5.8.9/sort">5.8.9</a> <a class="dropdown-item" href="/5.8.8/sort">5.8.8</a> <a class="dropdown-item" href="/5.8.7/sort">5.8.7</a> <a class="dropdown-item" href="/5.8.6/sort">5.8.6</a> <a class="dropdown-item" href="/5.8.5/sort">5.8.5</a> <a class="dropdown-item" href="/5.8.4/sort">5.8.4</a> <a class="dropdown-item" href="/5.8.3/sort">5.8.3</a> <a class="dropdown-item" href="/5.8.2/sort">5.8.2</a> <a class="dropdown-item" href="/5.8.1/sort">5.8.1</a> <a class="dropdown-item active" href="/5.8.0/sort">5.8.0</a> <div class="dropdown-divider"></div> <a class="dropdown-item" href="/5.6.2/sort">5.6.2</a> <a class="dropdown-item" href="/5.6.1/sort">5.6.1</a> <a class="dropdown-item" href="/5.6.0/sort">5.6.0</a> <div class="dropdown-divider"></div> <a class="dropdown-item" href="/5.005_04/sort">5.005_04</a> <a class="dropdown-item" href="/5.005_03/sort">5.005_03</a> <a class="dropdown-item" href="/5.005_02/sort">5.005_02</a> <a class="dropdown-item" href="/5.005_01/sort">5.005_01</a> <a class="dropdown-item" href="/5.005/sort">5.005</a> </div> </li> <li class="nav-item dropdown text-nowrap"> <a class="nav-link dropdown-toggle" href="#" id="dropdownlink-dev" role="button" data-toggle="dropdown" aria-haspopup="true" aria-expanded="false">Dev</a> <div class="dropdown-menu" aria-labelledby="dropdownlink-dev"> <a class="dropdown-item" href="/blead/sort">blead</a> <a class="dropdown-item" href="/5.41.10/sort">5.41.10</a> <a class="dropdown-item" href="/5.41.9/sort">5.41.9</a> <a class="dropdown-item" href="/5.41.8/sort">5.41.8</a> <a class="dropdown-item" href="/5.41.7/sort">5.41.7</a> <a class="dropdown-item" href="/5.41.6/sort">5.41.6</a> <a class="dropdown-item" href="/5.41.5/sort">5.41.5</a> <a class="dropdown-item" href="/5.41.4/sort">5.41.4</a> <a class="dropdown-item" href="/5.41.3/sort">5.41.3</a> <a class="dropdown-item" href="/5.41.2/sort">5.41.2</a> <a class="dropdown-item" href="/5.41.1/sort">5.41.1</a> <div class="dropdown-divider"></div> <a class="dropdown-item" href="/5.40.1-RC1/sort">5.40.1-RC1</a> <a class="dropdown-item" href="/5.40.0-RC2/sort">5.40.0-RC2</a> <a class="dropdown-item" href="/5.40.0-RC1/sort">5.40.0-RC1</a> <div class="dropdown-divider"></div> <a class="dropdown-item" href="/5.39.10/sort">5.39.10</a> <a class="dropdown-item" href="/5.39.9/sort">5.39.9</a> <a class="dropdown-item" href="/5.39.8/sort">5.39.8</a> <a class="dropdown-item" href="/5.39.7/sort">5.39.7</a> <a class="dropdown-item" href="/5.39.6/sort">5.39.6</a> <a class="dropdown-item" href="/5.39.5/sort">5.39.5</a> <a class="dropdown-item" href="/5.39.4/sort">5.39.4</a> <a class="dropdown-item" href="/5.39.3/sort">5.39.3</a> <a class="dropdown-item" href="/5.39.2/sort">5.39.2</a> <a class="dropdown-item" href="/5.39.1/sort">5.39.1</a> <div class="dropdown-divider"></div> <a class="dropdown-item" href="/5.38.3-RC1/sort">5.38.3-RC1</a> </div> </li> <li class="nav-item dropdown text-nowrap"> <a class="nav-link dropdown-toggle" href="#" id="dropdownlink-nav" role="button" data-toggle="dropdown" aria-haspopup="true" aria-expanded="false">Documentation</a> <div class="dropdown-menu" aria-labelledby="dropdownlink-nav"> <a class="dropdown-item" href="/5.8.0/perl">Perl</a> <a class="dropdown-item" href="/5.8.0/perlintro">Intro</a> <a class="dropdown-item" href="/5.8.0/perl#Tutorials">Tutorials</a> <a class="dropdown-item" href="/5.8.0/perlfaq">FAQs</a> <a class="dropdown-item" href="/5.8.0/perl#Reference-Manual">Reference</a> <div class="dropdown-divider"></div> <a class="dropdown-item" href="/5.8.0/perlop">Operators</a> <a class="dropdown-item" href="/5.8.0/functions">Functions</a> <a class="dropdown-item" href="/5.8.0/variables">Variables</a> <a class="dropdown-item" href="/5.8.0/modules">Modules</a> <a class="dropdown-item" href="/5.8.0/perlutil">Utilities</a> <div class="dropdown-divider"></div> <a class="dropdown-item" href="/5.8.0/perldelta">Release Notes</a> <a class="dropdown-item" href="/5.8.0/perlcommunity">Community</a> <a class="dropdown-item" href="/5.8.0/perlhist">History</a> </div> </li> </ul> <ul class="navbar-nav"> <script> function set_expand (expand) { var perldocdiv = document.getElementById('perldocdiv'); var width = window.getComputedStyle(perldocdiv).getPropertyValue('max-width'); var expanded = (width == '' || width == 'none') ? true : false; if (expand === null) { expand = !expanded; } if ((expand && !expanded) || (!expand && expanded)) { perldocdiv.style.setProperty('max-width', expand ? 'none' : '960px'); var button_classlist = document.getElementById('content-expand-button').classList; if (expand) { button_classlist.add('btn-light'); button_classlist.remove('btn-outline-light'); } else { button_classlist.add('btn-outline-light'); button_classlist.remove('btn-light'); } } return expand; } function toggle_expand () { var expand = set_expand(null); document.cookie = 'perldoc_expand=' + (expand ? 1 : 0) + '; path=/; expires=Tue, 19 Jan 2038 03:14:07 UTC'; } function read_expand () { return document.cookie.split(';').some(function (item) { return item.indexOf('perldoc_expand=1') >= 0 }); } if (document.readyState === 'loading') { document.addEventListener('DOMContentLoaded', function () { if (read_expand()) { set_expand(true); } }); } else if (read_expand()) { set_expand(true); } </script> <button id="content-expand-button" type="button" class="btn btn-outline-light d-none d-lg-inline-block mr-4" onclick="toggle_expand()">Expand</button> </ul> <form class="form-inline" method="get" action="/5.8.0/search"> <input class="form-control mr-3" type="search" name="q" placeholder="Search" aria-label="Search" value=""> </form> </div> </nav> <div id="wrapperlicious" class="container-fluid"> <div id="perldocdiv"> <div id="links"> <a href="/5.8.0/sort">sort</a> <div id="more"> (<a href="/5.8.0/sort.txt">source</a>, <a href="https://metacpan.org/pod/sort">CPAN</a>) </div> <div id="moduleversion">version 1.01</div> </div> <div class="leading-notice"> You are viewing the version of this documentation from Perl 5.8.0. <a href="/sort">View the latest version</a> </div> <div class="leading-notice"> <strong>sort</strong> may also refer to the function: <a href="/5.8.0/functions/sort">sort</a> </div> <h1><a id="toc">CONTENTS</a></h1> <ul> <li> <a class="text-decoration-none" href="#NAME">NAME</a> </li> <li> <a class="text-decoration-none" href="#SYNOPSIS">SYNOPSIS</a> </li> <li> <a class="text-decoration-none" href="#DESCRIPTION">DESCRIPTION</a> </li> <li> <a class="text-decoration-none" href="#CAVEATS">CAVEATS</a> </li> </ul> <h1 id="NAME"><a class="permalink" href="#NAME">#</a>NAME</h1> <p>sort - perl pragma to control sort() behaviour</p> <h1 id="SYNOPSIS"><a class="permalink" href="#SYNOPSIS">#</a>SYNOPSIS</h1> <pre><code>use sort 'stable'; # guarantee stability use sort '_quicksort'; # use a quicksort algorithm use sort '_mergesort'; # use a mergesort algorithm use sort '_qsort'; # alias for quicksort my $current = sort::current(); # identify prevailing algorithm</code></pre> <h1 id="DESCRIPTION"><a class="permalink" href="#DESCRIPTION">#</a>DESCRIPTION</h1> <p>With the sort pragma you can control the behaviour of the builtin sort() function.</p> <p>In Perl versions 5.6 and earlier the quicksort algorithm was used to implement sort(), but in Perl 5.8 a mergesort algorithm was also made available, mainly to guarantee worst case O(N log N) behaviour: the worst case of quicksort is O(N**2). In Perl 5.8 and later, quicksort defends against quadratic behaviour by shuffling large arrays before sorting.</p> <p>A stable sort means that for records that compare equal, the original input ordering is preserved. Mergesort is stable, quicksort is not. Stability will matter only if elements that compare equal can be distinguished in some other way. That means that simple numerical and lexical sorts do not profit from stability, since equal elements are indistinguishable. However, with a comparison such as</p> <pre><code>{ substr($a, 0, 3) cmp substr($b, 0, 3) }</code></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 sort to merge several sorted arrays. On the other hand, quicksort is often faster for small arrays, and on platforms with small memory caches that are much faster than main memory. You can force the choice of algorithm with this pragma, but this feels heavy-handed, so the subpragmas beginning with a <code>_</code> may not persist beyond Perl 5.8.</p> <h1 id="CAVEATS"><a class="permalink" href="#CAVEATS">#</a>CAVEATS</h1> <p>This pragma is not lexically scoped : its effect is global to the program it appears in. This may change in future versions.</p> </div> <div id="footer"> <p>Perldoc Browser is maintained by Dan Book (<a href="https://metacpan.org/author/DBOOK">DBOOK</a>). Please contact him via the <a href="https://github.com/Grinnz/perldoc-browser/issues">GitHub issue tracker</a> or <a href="mailto:dbook@cpan.org">email</a> regarding any issues with the site itself, search, or rendering of documentation.</p> <p>The Perl documentation is maintained by the Perl 5 Porters in the development of Perl. Please contact them via the <a href="https://github.com/Perl/perl5/issues">Perl issue tracker</a>, the <a href="https://lists.perl.org/list/perl5-porters.html">mailing list</a>, or <a href="https://kiwiirc.com/client/irc.perl.org/p5p">IRC</a> to report any issues with the contents or format of the documentation.</p> </div> </div> <script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.5.1/jquery.slim.min.js" integrity="sha512-/DXTXr6nQodMUiq+IUJYCt2PPOUjrHJ9wFrqpJ3XkgPNOZVfMok7cRw6CSxyCQxXn6ozlESsSh1/sMCTF1rL/g==" crossorigin="anonymous"></script> <script src="https://cdnjs.cloudflare.com/ajax/libs/popper.js/1.16.1/umd/popper.min.js" integrity="sha512-ubuT8Z88WxezgSqf3RLuNi5lmjstiJcyezx34yIU2gAHonIi27Na7atqzUZCOoY4CExaoFumzOsFQ2Ch+I/HCw==" crossorigin="anonymous"></script> <script src="https://stackpath.bootstrapcdn.com/bootstrap/4.5.2/js/bootstrap.min.js" integrity="sha384-B4gt1jrGC7Jh4AgTPSdUtOBvfO8shuf57BaghqFfPlYxofvL8/KUEfYiJOMMV+rV" crossorigin="anonymous"></script> <script src="/js/highlight.pack.js"></script> <script>hljs.highlightAll();</script> </body> </html>

CINXE.COM

sort - perl pragma to control sort() behaviour - Perldoc Browser