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<h1>sort</h1>
<div class="mod_az_list">
<a href="../index-functions.html">Perl functions A-Z</a> |
<a href="../index-functions-by-cat.html">Perl functions by category</a> |
<a href="../perlfunc.html">The 'perlfunc' manpage</a>
</div>
<!-- -->
<ul>
<li><a name="sort-SUBNAME-LIST"></a><b>sort SUBNAME LIST</b>
<p> </p>
</li>
</ul>
<ul>
<li><a name="sort-BLOCK-LIST"></a><b>sort BLOCK LIST</b>
</li>
<li><a name="sort-LIST"></a><b>sort LIST</b>
<p>In list context, this sorts the LIST and returns the sorted list value.
In scalar context, the behaviour of <code class="inline"><a class="l_k" href="../functions/sort.html">sort()</a></code> is undefined.</p>
<p>If SUBNAME or BLOCK is omitted, <code class="inline"><a class="l_k" href="../functions/sort.html">sort</a></code>s in standard string comparison
order. If SUBNAME is specified, it gives the name of a subroutine
that returns an integer less than, equal to, or greater than <code class="inline"><span class="n">0</span></code>
,
depending on how the elements of the list are to be ordered. (The <code class="inline"><=></code>
and <code class="inline">cmp</code>
operators are extremely useful in such routines.)
SUBNAME may be a scalar variable name (unsubscripted), in which case
the value provides the name of (or a reference to) the actual
subroutine to use. In place of a SUBNAME, you can provide a BLOCK as
an anonymous, in-line sort subroutine.</p>
<p>If the subroutine's prototype is <code class="inline"><span class="s">(</span><span class="i">$$</span><span class="s">)</span></code>
, the elements to be compared
are passed by reference in <code class="inline"><span class="i">@_</span></code>
, as for a normal subroutine. This is
slower than unprototyped subroutines, where the elements to be
compared are passed into the subroutine
as the package global variables $a and $b (see example below). Note that
in the latter case, it is usually counter-productive to declare $a and
$b as lexicals.</p>
<p>The values to be compared are always passed by reference and should not
be modified.</p>
<p>You also cannot exit out of the sort block or subroutine using any of the
loop control operators described in <a href="../perlsyn.html">perlsyn</a> or with <code class="inline"><a class="l_k" href="../functions/goto.html">goto</a></code>.</p>
<p>When <code class="inline"><a class="l_k" href="../functions/use.html">use</a> <span class="w">locale</span></code>
is in effect, <code class="inline"><a class="l_k" href="../functions/sort.html">sort</a> <span class="i">LIST</span></code>
sorts LIST according to the
current collation locale. See <a href="../perllocale.html">perllocale</a>.</p>
<p>sort() returns aliases into the original list, much as a for loop's index
variable aliases the list elements. That is, modifying an element of a
list returned by sort() (for example, in a <code class="inline">foreach</code>
, <code class="inline"><a class="l_k" href="../functions/map.html">map</a></code> or <code class="inline"><a class="l_k" href="../functions/grep.html">grep</a></code>)
actually modifies the element in the original list. This is usually
something to be avoided when writing clear code.</p>
<p>Perl 5.6 and earlier used a quicksort algorithm to implement sort.
That algorithm was not stable, and <i>could</i> go quadratic. (A <i>stable</i> sort
preserves the input order of elements that compare equal. Although
quicksort's run time is O(NlogN) when averaged over all arrays of
length N, the time can be O(N**2), <i>quadratic</i> behavior, for some
inputs.) In 5.7, the quicksort implementation was replaced with
a stable mergesort algorithm whose worst-case behavior is O(NlogN).
But benchmarks indicated that for some inputs, on some platforms,
the original quicksort was faster. 5.8 has a sort pragma for
limited control of the sort. Its rather blunt control of the
underlying algorithm may not persist into future Perls, but the
ability to characterize the input or output in implementation
independent ways quite probably will. See <a href="../sort.html">the sort pragma</a>.</p>
<p>Examples:</p>
<pre class="verbatim"><ol><li> <span class="c"># sort lexically</span></li><li> <span class="i">@articles</span> = <a class="l_k" href="../functions/sort.html">sort</a> <span class="i">@files</span><span class="sc">;</span></li><li></li><li> <span class="c"># same thing, but with explicit sort routine</span></li><li> <span class="i">@articles</span> = <a class="l_k" href="../functions/sort.html">sort</a> <span class="s">{</span><span class="i">$a</span> cmp <span class="i">$b</span><span class="s">}</span> <span class="i">@files</span><span class="sc">;</span></li><li></li><li> <span class="c"># now case-insensitively</span></li><li> <span class="i">@articles</span> = <a class="l_k" href="../functions/sort.html">sort</a> <span class="s">{</span><a class="l_k" href="../functions/uc.html">uc</a><span class="s">(</span><span class="i">$a</span><span class="s">)</span> cmp <a class="l_k" href="../functions/uc.html">uc</a><span class="s">(</span><span class="i">$b</span><span class="s">)</span><span class="s">}</span> <span class="i">@files</span><span class="sc">;</span></li><li></li><li> <span class="c"># same thing in reversed order</span></li><li> <span class="i">@articles</span> = <a class="l_k" href="../functions/sort.html">sort</a> <span class="s">{</span><span class="i">$b</span> cmp <span class="i">$a</span><span class="s">}</span> <span class="i">@files</span><span class="sc">;</span></li><li></li><li> <span class="c"># sort numerically ascending</span></li><li> <span class="i">@articles</span> = <a class="l_k" href="../functions/sort.html">sort</a> <span class="s">{</span><span class="i">$a</span> <=> <span class="i">$b</span><span class="s">}</span> <span class="i">@files</span><span class="sc">;</span></li><li></li><li> <span class="c"># sort numerically descending</span></li><li> <span class="i">@articles</span> = <a class="l_k" href="../functions/sort.html">sort</a> <span class="s">{</span><span class="i">$b</span> <=> <span class="i">$a</span><span class="s">}</span> <span class="i">@files</span><span class="sc">;</span></li><li></li><li> <span class="c"># this sorts the %age hash by value instead of key</span></li><li> <span class="c"># using an in-line function</span></li><li> <span class="i">@eldest</span> = <a class="l_k" href="../functions/sort.html">sort</a> <span class="s">{</span> <span class="i">$age</span>{<span class="i">$b</span>} <=> <span class="i">$age</span>{<span class="i">$a</span>} <span class="s">}</span> <a class="l_k" href="../functions/keys.html">keys</a> <span class="i">%age</span><span class="sc">;</span></li><li></li><li> <span class="c"># sort using explicit subroutine name</span></li><li><a name="byage"></a> sub <span class="m">byage</span> <span class="s">{</span></li><li> <span class="i">$age</span>{<span class="i">$a</span>} <=> <span class="i">$age</span>{<span class="i">$b</span>}<span class="sc">;</span> <span class="c"># presuming numeric</span></li><li> <span class="s">}</span></li><li> <span class="i">@sortedclass</span> = <a class="l_k" href="../functions/sort.html">sort</a> <span class="i">byage</span> <span class="i">@class</span><span class="sc">;</span></li><li></li><li><a name="backwards"></a> sub <span class="m">backwards</span> <span class="s">{</span> <span class="i">$b</span> cmp <span class="i">$a</span> <span class="s">}</span></li><li> <span class="i">@harry</span> = <span class="q">qw(dog cat x Cain Abel)</span><span class="sc">;</span></li><li> <span class="i">@george</span> = <span class="q">qw(gone chased yz Punished Axed)</span><span class="sc">;</span></li><li> <a class="l_k" href="../functions/print.html">print</a> <a class="l_k" href="../functions/sort.html">sort</a> <span class="i">@harry</span><span class="sc">;</span></li><li> <span class="c"># prints AbelCaincatdogx</span></li><li> <a class="l_k" href="../functions/print.html">print</a> <a class="l_k" href="../functions/sort.html">sort</a> <span class="i">backwards</span> <span class="i">@harry</span><span class="sc">;</span></li><li> <span class="c"># prints xdogcatCainAbel</span></li><li> <a class="l_k" href="../functions/print.html">print</a> <a class="l_k" href="../functions/sort.html">sort</a> <span class="i">@george</span><span class="cm">,</span> <span class="q">'to'</span><span class="cm">,</span> <span class="i">@harry</span><span class="sc">;</span></li><li> <span class="c"># prints AbelAxedCainPunishedcatchaseddoggonetoxyz</span></li><li></li><li> <span class="c"># inefficiently sort by descending numeric compare using</span></li><li> <span class="c"># the first integer after the first = sign, or the</span></li><li> <span class="c"># whole record case-insensitively otherwise</span></li><li></li><li> <span class="i">@new</span> = <a class="l_k" href="../functions/sort.html">sort</a> <span class="s">{</span></li><li> <span class="s">(</span><span class="i">$b</span> =~ <span class="q">/=(\d+)/</span><span class="s">)</span>[<span class="n">0</span>] <=> <span class="s">(</span><span class="i">$a</span> =~ <span class="q">/=(\d+)/</span><span class="s">)</span>[<span class="n">0</span>]</li><li> ||</li><li> <a class="l_k" href="../functions/uc.html">uc</a><span class="s">(</span><span class="i">$a</span><span class="s">)</span> cmp <a class="l_k" href="../functions/uc.html">uc</a><span class="s">(</span><span class="i">$b</span><span class="s">)</span></li><li> <span class="s">}</span> <span class="i">@old</span><span class="sc">;</span></li><li></li><li> <span class="c"># same thing, but much more efficiently;</span></li><li> <span class="c"># we'll build auxiliary indices instead</span></li><li> <span class="c"># for speed</span></li><li> <span class="i">@nums</span> = <span class="i">@caps</span> = <span class="s">(</span><span class="s">)</span><span class="sc">;</span></li><li> for <span class="s">(</span><span class="i">@old</span><span class="s">)</span> <span class="s">{</span></li><li> <a class="l_k" href="../functions/push.html">push</a> <span class="i">@nums</span><span class="cm">,</span> <span class="q">/=(\d+)/</span><span class="sc">;</span></li><li> <a class="l_k" href="../functions/push.html">push</a> <span class="i">@caps</span><span class="cm">,</span> <a class="l_k" href="../functions/uc.html">uc</a><span class="s">(</span><span class="i">$_</span><span class="s">)</span><span class="sc">;</span></li><li> <span class="s">}</span></li><li></li><li> <span class="i">@new</span> = <span class="i">@old</span>[ <a class="l_k" href="../functions/sort.html">sort</a> <span class="s">{</span></li><li> <span class="i">$nums</span>[<span class="i">$b</span>] <=> <span class="i">$nums</span>[<span class="i">$a</span>]</li><li> ||</li><li> <span class="i">$caps</span>[<span class="i">$a</span>] cmp <span class="i">$caps</span>[<span class="i">$b</span>]</li><li> <span class="s">}</span> <span class="n">0</span>..<span class="i">$#old</span></li><li> ]<span class="sc">;</span></li><li></li><li> <span class="c"># same thing, but without any temps</span></li><li> <span class="i">@new</span> = <a class="l_k" href="../functions/map.html">map</a> <span class="s">{</span> <span class="i">$_</span>->[<span class="n">0</span>] <span class="s">}</span></li><li> <a class="l_k" href="../functions/sort.html">sort</a> <span class="s">{</span> <span class="i">$b</span>->[<span class="n">1</span>] <=> <span class="i">$a</span>->[<span class="n">1</span>]</li><li> ||</li><li> <span class="i">$a</span>->[<span class="n">2</span>] cmp <span class="i">$b</span>->[<span class="n">2</span>]</li><li> <span class="s">}</span> <a class="l_k" href="../functions/map.html">map</a> <span class="s">{</span> <span class="s">[</span><span class="i">$_</span><span class="cm">,</span> <span class="q">/=(\d+)/</span><span class="cm">,</span> <a class="l_k" href="../functions/uc.html">uc</a><span class="s">(</span><span class="i">$_</span><span class="s">)</span><span class="s">]</span> <span class="s">}</span> <span class="i">@old</span><span class="sc">;</span></li><li></li><li> <span class="c"># using a prototype allows you to use any comparison subroutine</span></li><li> <span class="c"># as a sort subroutine (including other package's subroutines)</span></li><li><a name="package-other"></a> package <span class="i">other</span><span class="sc">;</span></li><li><a name="backwards-1"></a> sub <span class="m">backwards ($$)</span> <span class="s">{</span> <span class="i">$_</span>[<span class="n">1</span>] cmp <span class="i">$_</span>[<span class="n">0</span>]<span class="sc">;</span> <span class="s">}</span> <span class="c"># $a and $b are not set here</span></li><li></li><li><a name="package-main"></a> package <span class="i">main</span><span class="sc">;</span></li><li> <span class="i">@new</span> = <a class="l_k" href="../functions/sort.html">sort</a> <span class="i">other::backwards</span> <span class="i">@old</span><span class="sc">;</span></li><li></li><li> <span class="c"># guarantee stability, regardless of algorithm</span></li><li> <a class="l_k" href="../functions/use.html">use</a> <a class="l_k" href="../functions/sort.html">sort</a> <span class="q">'stable'</span><span class="sc">;</span></li><li> <span class="i">@new</span> = <a class="l_k" href="../functions/sort.html">sort</a> <span class="s">{</span> <a class="l_k" href="../functions/substr.html">substr</a><span class="s">(</span><span class="i">$a</span><span class="cm">,</span> <span class="n">3</span><span class="cm">,</span> <span class="n">5</span><span class="s">)</span> cmp <a class="l_k" href="../functions/substr.html">substr</a><span class="s">(</span><span class="i">$b</span><span class="cm">,</span> <span class="n">3</span><span class="cm">,</span> <span class="n">5</span><span class="s">)</span> <span class="s">}</span> <span class="i">@old</span><span class="sc">;</span></li><li></li><li> <span class="c"># force use of mergesort (not portable outside Perl 5.8)</span></li><li> <a class="l_k" href="../functions/use.html">use</a> <a class="l_k" href="../functions/sort.html">sort</a> <span class="q">'_mergesort'</span><span class="sc">;</span> <span class="c"># note discouraging _</span></li><li> <span class="i">@new</span> = <a class="l_k" href="../functions/sort.html">sort</a> <span class="s">{</span> <a class="l_k" href="../functions/substr.html">substr</a><span class="s">(</span><span class="i">$a</span><span class="cm">,</span> <span class="n">3</span><span class="cm">,</span> <span class="n">5</span><span class="s">)</span> cmp <a class="l_k" href="../functions/substr.html">substr</a><span class="s">(</span><span class="i">$b</span><span class="cm">,</span> <span class="n">3</span><span class="cm">,</span> <span class="n">5</span><span class="s">)</span> <span class="s">}</span> <span class="i">@old</span><span class="sc">;</span></li></ol></pre><p>Warning: syntactical care is required when sorting the list returned from
a function. If you want to sort the list returned by the function call
<code class="inline"><span class="i">find_records</span><span class="s">(</span><span class="i">@key</span><span class="s">)</span></code>
, you can use:</p>
<pre class="verbatim"><ol><li> <span class="i">@contact</span> = <a class="l_k" href="../functions/sort.html">sort</a> <span class="s">{</span> <span class="i">$a</span> cmp <span class="i">$b</span> <span class="s">}</span> <span class="w">find_records</span> <span class="i">@key</span><span class="sc">;</span></li><li> <span class="i">@contact</span> = <a class="l_k" href="../functions/sort.html">sort</a> +<span class="i">find_records</span><span class="s">(</span><span class="i">@key</span><span class="s">)</span><span class="sc">;</span></li><li> <span class="i">@contact</span> = <a class="l_k" href="../functions/sort.html">sort</a> <span class="i">&find_records</span><span class="s">(</span><span class="i">@key</span><span class="s">)</span><span class="sc">;</span></li><li> <span class="i">@contact</span> = <a class="l_k" href="../functions/sort.html">sort</a><span class="s">(</span><span class="i">find_records</span><span class="s">(</span><span class="i">@key</span><span class="s">)</span><span class="s">)</span><span class="sc">;</span></li></ol></pre><p>If instead you want to sort the array @key with the comparison routine
<code class="inline"><span class="i">find_records</span><span class="s">(</span><span class="s">)</span></code>
then you can use:</p>
<pre class="verbatim"><ol><li> <span class="i">@contact</span> = <a class="l_k" href="../functions/sort.html">sort</a> <span class="s">{</span> <span class="i">find_records</span><span class="s">(</span><span class="s">)</span> <span class="s">}</span> <span class="i">@key</span><span class="sc">;</span></li><li> <span class="i">@contact</span> = <a class="l_k" href="../functions/sort.html">sort</a> <span class="i">find_records</span><span class="s">(</span><span class="i">@key</span><span class="s">)</span><span class="sc">;</span></li><li> <span class="i">@contact</span> = <a class="l_k" href="../functions/sort.html">sort</a><span class="s">(</span><span class="i">find_records</span> <span class="i">@key</span><span class="s">)</span><span class="sc">;</span></li><li> <span class="i">@contact</span> = <a class="l_k" href="../functions/sort.html">sort</a><span class="s">(</span><span class="i">find_records</span> <span class="s">(</span><span class="i">@key</span><span class="s">)</span><span class="s">)</span><span class="sc">;</span></li></ol></pre><p>If you're using strict, you <i>must not</i> declare $a
and $b as lexicals. They are package globals. That means
that if you're in the <code class="inline"><span class="w">main</span></code>
package and type</p>
<pre class="verbatim"><ol><li> <span class="i">@articles</span> = <a class="l_k" href="../functions/sort.html">sort</a> <span class="s">{</span><span class="i">$b</span> <=> <span class="i">$a</span><span class="s">}</span> <span class="i">@files</span><span class="sc">;</span></li></ol></pre><p>then <code class="inline"><span class="i">$a</span></code>
and <code class="inline"><span class="i">$b</span></code>
are <code class="inline"><span class="i">$main::a</span></code>
and <code class="inline"><span class="i">$main::b</span></code>
(or <code class="inline"><span class="i">$::a</span></code>
and <code class="inline"><span class="i">$::b</span></code>
),
but if you're in the <code class="inline"><span class="w">FooPack</span></code>
package, it's the same as typing</p>
<pre class="verbatim"><ol><li> <span class="i">@articles</span> = <a class="l_k" href="../functions/sort.html">sort</a> <span class="s">{</span><span class="i">$FooPack::b</span> <=> <span class="i">$FooPack::a</span><span class="s">}</span> <span class="i">@files</span><span class="sc">;</span></li></ol></pre><p>The comparison function is required to behave. If it returns
inconsistent results (sometimes saying <code class="inline"><span class="i">$x</span>[<span class="n">1</span>]</code>
is less than <code class="inline"><span class="i">$x</span>[<span class="n">2</span>]</code>
and
sometimes saying the opposite, for example) the results are not
well-defined.</p>
<p>Because <code class="inline"><=></code>
returns <code class="inline"><a class="l_k" href="../functions/undef.html">undef</a></code> when either operand is <code class="inline"><span class="w">NaN</span></code>
(not-a-number), and because <code class="inline"><a class="l_k" href="../functions/sort.html">sort</a></code> will trigger a fatal error unless the
result of a comparison is defined, when sorting with a comparison function
like <code class="inline"><span class="i">$a</span> <=> <span class="i">$b</span></code>
, be careful about lists that might contain a <code class="inline"><span class="w">NaN</span></code>
.
The following example takes advantage of the fact that <code class="inline"><span class="w">NaN</span> != <span class="w">NaN</span></code>
to
eliminate any <code class="inline"><span class="w">NaN</span></code>
s from the input.</p>
<pre class="verbatim"><ol><li> <span class="i">@result</span> = <a class="l_k" href="../functions/sort.html">sort</a> <span class="s">{</span> <span class="i">$a</span> <=> <span class="i">$b</span> <span class="s">}</span> <a class="l_k" href="../functions/grep.html">grep</a> <span class="s">{</span> <span class="i">$_</span> == <span class="i">$_</span> <span class="s">}</span> <span class="i">@input</span><span class="sc">;</span></li></ol></pre></li>
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