1sort(3pm) Perl Programmers Reference Guide sort(3pm)
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6 sort - perl pragma to control sort() behaviour
7
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
14 use sort '_qsort'; # alias for quicksort
16 my $current = sort::current(); # identify prevailing algorithm
18
20 With the "sort" pragma you can control the behaviour of the builtin
21 "sort()" function.
22 In Perl versions 5.6 and earlier the quicksort algorithm was used to
24 implement "sort()", but in Perl 5.8 a mergesort algorithm was also made
25 available, mainly to guarantee worst case O(N log N) behaviour: the
26 worst case of quicksort is O(N**2). In Perl 5.8 and later, quicksort
27 defends against quadratic behaviour by shuffling large arrays before
28 sorting.
29 A stable sort means that for records that compare equal, the original
31 input ordering is preserved. Mergesort is stable, quicksort is not.
32 Stability will matter only if elements that compare equal can be dis‐
33 tinguished in some other way. That means that simple numerical and
34 lexical sorts do not profit from stability, since equal elements are
35 indistinguishable. However, with a comparison such as
36 { substr($a, 0, 3) cmp substr($b, 0, 3) }
38 stability might matter because elements that compare equal on the first
40 3 characters may be distinguished based on subsequent characters. In
41 Perl 5.8 and later, quicksort can be stabilized, but doing so will add
42 overhead, so it should only be done if it matters.
43 The best algorithm depends on many things. On average, mergesort does
45 fewer comparisons than quicksort, so it may be better when complicated
46 comparison routines are used. Mergesort also takes advantage of pre-
47 existing order, so it would be favored for using "sort()" to merge sev‐
48 eral sorted arrays. On the other hand, quicksort is often faster for
49 small arrays, and on arrays of a few distinct values, repeated many
50 times. You can force the choice of algorithm with this pragma, but
51 this feels heavy-handed, so the subpragmas beginning with a "_" may not
52 persist beyond Perl 5.8. The default algorithm is mergesort, which
53 will be stable even if you do not explicitly demand it. But the sta‐
54 bility of the default sort is a side-effect that could change in later
55 versions. If stability is important, be sure to say so with a
56 use sort 'stable';
58 The "no sort" pragma doesn't forbid what follows, it just leaves the
60 choice open. Thus, after
61 no sort qw(_mergesort stable);
63 a mergesort, which happens to be stable, will be employed anyway. Note
65 that
66 no sort "_quicksort";
68 no sort "_mergesort";
69 have exactly the same effect, leaving the choice of sort algorithm
71 open.
72
74 This pragma is not lexically scoped: its effect is global to the pro‐
75 gram it appears in. That means the following will probably not do what
76 you expect, because both pragmas take effect at compile time, before
77 either "sort()" happens.
78 { use sort "_quicksort";
80 print sort::current . "\n";
81 @a = sort @b;
82 }
83 { use sort "stable";
84 print sort::current . "\n";
85 @c = sort @d;
86 }
87 # prints:
88 # quicksort stable
89 # quicksort stable
90 You can achieve the effect you probably wanted by using "eval()" to
92 defer the pragmas until run time. Use the quoted argument form of
93 "eval()", not the BLOCK form, as in
94 eval { use sort "_quicksort" }; # WRONG
96 or the effect will still be at compile time. Reset to default options
98 before selecting other subpragmas (in case somebody carelessly left
99 them on) and after sorting, as a courtesy to others.
100 { eval 'use sort qw(defaults _quicksort)'; # force quicksort
102 eval 'no sort "stable"'; # stability not wanted
103 print sort::current . "\n";
104 @a = sort @b;
105 eval 'use sort "defaults"'; # clean up, for others
106 }
107 { eval 'use sort qw(defaults stable)'; # force stability
108 print sort::current . "\n";
109 @c = sort @d;
110 eval 'use sort "defaults"'; # clean up, for others
111 }
112 # prints:
113 # quicksort
114 # stable
115 Scoping for this pragma may change in future versions.
117perl v5.8.8 2001-09-21 sort(3pm)