MirOS Manual: sort(3p)


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 = 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 sort-
     ing.

     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 indistin-
     guishable.  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 sub-
     sequent 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, mer-
     gesort does fewer comparisons than quicksort, so it may be
     better when complicated comparison routines are used.  Mer-
     gesort 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 sub-
     pragmas beginning with a "_" may not persist beyond Perl

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

     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 program 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.

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

       { 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.

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