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sort(3pm)              Perl Programmers Reference Guide              sort(3pm)

NAME
       sort - perl pragma to control sort() behaviour
SYNOPSIS
           use sort 'stable';          # guarantee stability
           use sort '_quicksort';      # use a quicksort algorithm
           use sort '_mergesort';      # use a mergesort algorithm
           use sort 'defaults';        # revert to default behavior
           no  sort 'stable';          # stability not important
           use sort '_qsort';          # alias for quicksort
           my $current;
           BEGIN {
               $current = sort::current();     # identify prevailing algorithm
           }
DESCRIPTION
       With the "sort" pragma you can control the behaviour of the builtin
       "sort()" function.
       In Perl versions 5.6 and earlier the quicksort algorithm was used to
       implement "sort()", but in Perl 5.8 a mergesort algorithm was also made
       available, mainly to guarantee worst case O(N log N) behaviour: the
       worst case of quicksort is O(N**2).  In Perl 5.8 and later, quicksort
       defends against quadratic behaviour by shuffling large arrays before
       sorting.
       A stable sort means that for records that compare equal, the original
       input ordering is preserved.  Mergesort is stable, quicksort is not.
       Stability will matter only if elements that compare equal can be
       distinguished in some other way.  That means that simple numerical and
       lexical sorts do not profit from stability, since equal elements are
       indistinguishable.  However, with a comparison such as
          { substr($a, 0, 3) cmp substr($b, 0, 3) }
       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.
       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 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 "_" 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
         use sort 'stable';
       The "no sort" pragma doesn't forbid what follows, it just leaves the
       choice open.  Thus, after
         no sort qw(_mergesort stable);
       a mergesort, which happens to be stable, will be employed anyway.  Note
       that
         no sort "_quicksort";
         no sort "_mergesort";
       have exactly the same effect, leaving the choice of sort algorithm
       open.
CAVEATS
       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 "eval()" to change the
       behaviour:
         { eval 'use sort qw(defaults _quicksort)'; # force quicksort
           eval 'no sort "stable"';      # stability not wanted
           print sort::current . "\n";
           @a = sort @b;
           eval 'use sort "defaults"';   # clean up, for others
         }
         { eval 'use sort qw(defaults stable)';     # force stability
           print sort::current . "\n";
           @c = sort @d;
           eval 'use sort "defaults"';   # clean up, for others
         }
       Such code no longer has the desired effect, for two reasons.  Firstly,
       the use of "eval()" means that the sorting algorithm is not changed
       until runtime, by which time it's too late to have any effect.
       Secondly, "sort::current" is also called at run-time, when in fact the
       compile-time value of "sort::current" is the one that matters.
       So now this code would be written:
         { use sort qw(defaults _quicksort); # force quicksort
           no sort "stable";      # stability not wanted
           my $current;
           BEGIN { $current = print sort::current; }
           print "$current\n";
           @a = sort @b;
           # Pragmas go out of scope at the end of the block
         }
         { use sort qw(defaults stable);     # force stability
           my $current;
           BEGIN { $current = print sort::current; }
           print "$current\n";
           @c = sort @d;
         }

perl v5.16.3                      2013-03-04                         sort(3pm)