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 '_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 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
       "_" may not persist beyond Perl 5.8.

CAVEATS

       This pragma is not lexically scoped : its effect is global to the pro-
       gram it appears in.  This may change in future versions.

perl v5.8.0                       2002-06-01                           sort(3)