


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


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,



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


       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:







perl v5.10.1                2008-09-29                          2





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


         { 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 v5.10.1                2008-09-29                          3


