sort(3)

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 = 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 dis-
       tinguished 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 sev-
       eral 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 sta-
       bility 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

       This pragma is not lexically scoped: its effect is global to the pro-
       gram it appears in.  That means the following will probably not do what
       you expect, because both pragmas take effect at compile time, before
       either "sort()" happens.

         { use sort "_quicksort";
           print sort::current . "\n";
           @a = sort @b;
         }
         { use sort "stable";
           print sort::current . "\n";
           @c = sort @d;
         }
         # prints:
         # quicksort stable
         # quicksort stable

       You can achieve the effect you probably wanted by using "eval()" to
       defer the pragmas until run time.  Use the quoted argument form of
       "eval()", not the BLOCK form, as in

         eval { use sort "_quicksort" }; # WRONG

       or the effect will still be at compile time.  Reset to default options
       before selecting other subpragmas (in case somebody carelessly left
       them on) and after sorting, as a courtesy to others.

         { 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
         }
         # prints:
         # quicksort
         # stable

       Scoping for this pragma may change in future versions.

perl v5.8.8                       2006-06-14                           sort(3)