Perl 5 version 8.8 documentation



  • sort BLOCK LIST
  • sort LIST

    In list context, this sorts the LIST and returns the sorted list value. In scalar context, the behaviour of sort() is undefined.

    If SUBNAME or BLOCK is omitted, sorts in standard string comparison order. If SUBNAME is specified, it gives the name of a subroutine that returns an integer less than, equal to, or greater than 0 , depending on how the elements of the list are to be ordered. (The <=> and cmp operators are extremely useful in such routines.) SUBNAME may be a scalar variable name (unsubscripted), in which case the value provides the name of (or a reference to) the actual subroutine to use. In place of a SUBNAME, you can provide a BLOCK as an anonymous, in-line sort subroutine.

    If the subroutine's prototype is ($$) , the elements to be compared are passed by reference in @_ , as for a normal subroutine. This is slower than unprototyped subroutines, where the elements to be compared are passed into the subroutine as the package global variables $a and $b (see example below). Note that in the latter case, it is usually counter-productive to declare $a and $b as lexicals.

    In either case, the subroutine may not be recursive. The values to be compared are always passed by reference and should not be modified.

    You also cannot exit out of the sort block or subroutine using any of the loop control operators described in perlsyn or with goto.

    When use locale is in effect, sort LIST sorts LIST according to the current collation locale. See perllocale.

    sort() returns aliases into the original list, much as a for loop's index variable aliases the list elements. That is, modifying an element of a list returned by sort() (for example, in a foreach , map or grep) actually modifies the element in the original list. This is usually something to be avoided when writing clear code.

    Perl 5.6 and earlier used a quicksort algorithm to implement sort. That algorithm was not stable, and could go quadratic. (A stable sort preserves the input order of elements that compare equal. Although quicksort's run time is O(NlogN) when averaged over all arrays of length N, the time can be O(N**2), quadratic behavior, for some inputs.) In 5.7, the quicksort implementation was replaced with a stable mergesort algorithm whose worst-case behavior is O(NlogN). But benchmarks indicated that for some inputs, on some platforms, the original quicksort was faster. 5.8 has a sort pragma for limited control of the sort. Its rather blunt control of the underlying algorithm may not persist into future Perls, but the ability to characterize the input or output in implementation independent ways quite probably will. See sort.


    1. # sort lexically
    2. @articles = sort @files;
    3. # same thing, but with explicit sort routine
    4. @articles = sort {$a cmp $b} @files;
    5. # now case-insensitively
    6. @articles = sort {uc($a) cmp uc($b)} @files;
    7. # same thing in reversed order
    8. @articles = sort {$b cmp $a} @files;
    9. # sort numerically ascending
    10. @articles = sort {$a <=> $b} @files;
    11. # sort numerically descending
    12. @articles = sort {$b <=> $a} @files;
    13. # this sorts the %age hash by value instead of key
    14. # using an in-line function
    15. @eldest = sort { $age{$b} <=> $age{$a} } keys %age;
    16. # sort using explicit subroutine name
    17. sub byage {
    18. $age{$a} <=> $age{$b}; # presuming numeric
    19. }
    20. @sortedclass = sort byage @class;
    21. sub backwards { $b cmp $a }
    22. @harry = qw(dog cat x Cain Abel);
    23. @george = qw(gone chased yz Punished Axed);
    24. print sort @harry;
    25. # prints AbelCaincatdogx
    26. print sort backwards @harry;
    27. # prints xdogcatCainAbel
    28. print sort @george, 'to', @harry;
    29. # prints AbelAxedCainPunishedcatchaseddoggonetoxyz
    30. # inefficiently sort by descending numeric compare using
    31. # the first integer after the first = sign, or the
    32. # whole record case-insensitively otherwise
    33. @new = sort {
    34. ($b =~ /=(\d+)/)[0] <=> ($a =~ /=(\d+)/)[0]
    35. ||
    36. uc($a) cmp uc($b)
    37. } @old;
    38. # same thing, but much more efficiently;
    39. # we'll build auxiliary indices instead
    40. # for speed
    41. @nums = @caps = ();
    42. for (@old) {
    43. push @nums, /=(\d+)/;
    44. push @caps, uc($_);
    45. }
    46. @new = @old[ sort {
    47. $nums[$b] <=> $nums[$a]
    48. ||
    49. $caps[$a] cmp $caps[$b]
    50. } 0..$#old
    51. ];
    52. # same thing, but without any temps
    53. @new = map { $_->[0] }
    54. sort { $b->[1] <=> $a->[1]
    55. ||
    56. $a->[2] cmp $b->[2]
    57. } map { [$_, /=(\d+)/, uc($_)] } @old;
    58. # using a prototype allows you to use any comparison subroutine
    59. # as a sort subroutine (including other package's subroutines)
    60. package other;
    61. sub backwards ($$) { $_[1] cmp $_[0]; } # $a and $b are not set here
    62. package main;
    63. @new = sort other::backwards @old;
    64. # guarantee stability, regardless of algorithm
    65. use sort 'stable';
    66. @new = sort { substr($a, 3, 5) cmp substr($b, 3, 5) } @old;
    67. # force use of mergesort (not portable outside Perl 5.8)
    68. use sort '_mergesort'; # note discouraging _
    69. @new = sort { substr($a, 3, 5) cmp substr($b, 3, 5) } @old;

    If you're using strict, you must not declare $a and $b as lexicals. They are package globals. That means if you're in the main package and type

    1. @articles = sort {$b <=> $a} @files;

    then $a and $b are $main::a and $main::b (or $::a and $::b ), but if you're in the FooPack package, it's the same as typing

    1. @articles = sort {$FooPack::b <=> $FooPack::a} @files;

    The comparison function is required to behave. If it returns inconsistent results (sometimes saying $x[1] is less than $x[2] and sometimes saying the opposite, for example) the results are not well-defined.

    Because <=> returns undef when either operand is NaN (not-a-number), and because sort will trigger a fatal error unless the result of a comparison is defined, when sorting with a comparison function like $a <=> $b , be careful about lists that might contain a NaN . The following example takes advantage of the fact that NaN != NaN to eliminate any NaN s from the input.

    1. @result = sort { $a <=> $b } grep { $_ == $_ } @input;

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