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sort(3)		       Perl Programmers	Reference Guide		       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;
	   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 { $current = sort::current; }
	   print "$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 { $current = sort::current; }
	   print "$current\n";
	   @c =	sort @d;
	 }

perl v5.26.0			  2017-02-28			       sort(3)

NAME | SYNOPSIS | DESCRIPTION | CAVEATS

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