1sort(3pm)              Perl Programmers Reference Guide              sort(3pm)
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NAME

6       sort - perl pragma to control sort() behaviour
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SYNOPSIS

9           use sort 'stable';          # guarantee stability
10           use sort '_quicksort';      # use a quicksort algorithm
11           use sort '_mergesort';      # use a mergesort algorithm
12           use sort 'defaults';        # revert to default behavior
13           no  sort 'stable';          # stability not important
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15           use sort '_qsort';          # alias for quicksort
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17           my $current;
18           BEGIN {
19               $current = sort::current();     # identify prevailing algorithm
20           }
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DESCRIPTION

23       With the "sort" pragma you can control the behaviour of the builtin
24       "sort()" function.
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26       In Perl versions 5.6 and earlier the quicksort algorithm was used to
27       implement "sort()", but in Perl 5.8 a mergesort algorithm was also made
28       available, mainly to guarantee worst case O(N log N) behaviour: the
29       worst case of quicksort is O(N**2).  In Perl 5.8 and later, quicksort
30       defends against quadratic behaviour by shuffling large arrays before
31       sorting.
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33       A stable sort means that for records that compare equal, the original
34       input ordering is preserved.  Mergesort is stable, quicksort is not.
35       Stability will matter only if elements that compare equal can be
36       distinguished in some other way.  That means that simple numerical and
37       lexical sorts do not profit from stability, since equal elements are
38       indistinguishable.  However, with a comparison such as
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40          { substr($a, 0, 3) cmp substr($b, 0, 3) }
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42       stability might matter because elements that compare equal on the first
43       3 characters may be distinguished based on subsequent characters.  In
44       Perl 5.8 and later, quicksort can be stabilized, but doing so will add
45       overhead, so it should only be done if it matters.
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47       The best algorithm depends on many things.  On average, mergesort does
48       fewer comparisons than quicksort, so it may be better when complicated
49       comparison routines are used.  Mergesort also takes advantage of pre-
50       existing order, so it would be favored for using "sort()" to merge
51       several sorted arrays.  On the other hand, quicksort is often faster
52       for small arrays, and on arrays of a few distinct values, repeated many
53       times.  You can force the choice of algorithm with this pragma, but
54       this feels heavy-handed, so the subpragmas beginning with a "_" may not
55       persist beyond Perl 5.8.  The default algorithm is mergesort, which
56       will be stable even if you do not explicitly demand it.  But the
57       stability of the default sort is a side-effect that could change in
58       later versions.  If stability is important, be sure to say so with a
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60         use sort 'stable';
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62       The "no sort" pragma doesn't forbid what follows, it just leaves the
63       choice open.  Thus, after
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65         no sort qw(_mergesort stable);
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67       a mergesort, which happens to be stable, will be employed anyway.  Note
68       that
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70         no sort "_quicksort";
71         no sort "_mergesort";
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73       have exactly the same effect, leaving the choice of sort algorithm
74       open.
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CAVEATS

77       As of Perl 5.10, this pragma is lexically scoped and takes effect at
78       compile time. In earlier versions its effect was global and took effect
79       at run-time; the documentation suggested using "eval()" to change the
80       behaviour:
81
82         { eval 'use sort qw(defaults _quicksort)'; # force quicksort
83           eval 'no sort "stable"';      # stability not wanted
84           print sort::current . "\n";
85           @a = sort @b;
86           eval 'use sort "defaults"';   # clean up, for others
87         }
88         { eval 'use sort qw(defaults stable)';     # force stability
89           print sort::current . "\n";
90           @c = sort @d;
91           eval 'use sort "defaults"';   # clean up, for others
92         }
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94       Such code no longer has the desired effect, for two reasons.  Firstly,
95       the use of "eval()" means that the sorting algorithm is not changed
96       until runtime, by which time it's too late to have any effect.
97       Secondly, "sort::current" is also called at run-time, when in fact the
98       compile-time value of "sort::current" is the one that matters.
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100       So now this code would be written:
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102         { use sort qw(defaults _quicksort); # force quicksort
103           no sort "stable";      # stability not wanted
104           my $current;
105           BEGIN { $current = print sort::current; }
106           print "$current\n";
107           @a = sort @b;
108           # Pragmas go out of scope at the end of the block
109         }
110         { use sort qw(defaults stable);     # force stability
111           my $current;
112           BEGIN { $current = print sort::current; }
113           print "$current\n";
114           @c = sort @d;
115         }
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119perl v5.16.3                      2013-03-04                         sort(3pm)
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