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Perl Language Reference Manual
by Larry Wall and others
Paperback (6"x9"), 724 pages
ISBN 9781906966027
RRP £29.95 ($39.95)

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sort SUBNAME LIST

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.

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 4 or with goto.

When use locale is in effect, sort LIST sorts LIST according to the current collation locale. See "Perl locale handling (internationalization and localization)" (perllocale) in the Perl Unicode and Locales Manual.

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 the sort pragma ("Perl pragma to control sort() behaviour" (sort) in the Perl Library Reference Manual (Volume 1)).

Examples:

# sort lexically
@articles = sort @files;
# same thing, but with explicit sort routine
@articles = sort {$a cmp $b} @files;
# now case-insensitively
@articles = sort {uc($a) cmp uc($b)} @files;
# same thing in reversed order
@articles = sort {$b cmp $a} @files;
# sort numerically ascending
@articles = sort {$a <=> $b} @files;
# sort numerically descending
@articles = sort {$b <=> $a} @files;
# this sorts the %age hash by value instead of key
# using an in-line function
@eldest = sort { $age{$b} <=> $age{$a} } keys %age;
# sort using explicit subroutine name
sub byage {
$age{$a} <=> $age{$b};  # presuming numeric
}
@sortedclass = sort byage @class;
sub backwards { $b cmp $a }
@harry  = qw(dog cat x Cain Abel);
@george = qw(gone chased yz Punished Axed);
print sort @harry;
    # prints AbelCaincatdogx
print sort backwards @harry;
    # prints xdogcatCainAbel
print sort @george, 'to', @harry;
    # prints AbelAxedCainPunishedcatchaseddoggonetoxyz
# inefficiently sort by descending numeric compare using
# the first integer after the first = sign, or the
# whole record case-insensitively otherwise
my @new = sort {
    ($b =~ /=(\d+)/)[0] <=> ($a =~ /=(\d+)/)[0]
        ||
    uc($a)  cmp  uc($b)
} @old;
# same thing, but much more efficiently;
# we'll build auxiliary indices instead
# for speed
my @nums = @caps = ();
for (@old) {
    push @nums, ( /=(\d+)/ ? $1 : undef );
    push @caps, uc($_);
}
my @new = @old[ sort {
    $nums[$b] <=> $nums[$a]
        ||
    $caps[$a] cmp $caps[$b]
    } 0..$#old
];
# same thing, but without any temps
@new = map { $_->[0] }
       sort { $b->[1] <=> $a->[1]
           ||
       $a->[2] cmp $b->[2]
} map { [$_, /=(\d+)/, uc($_)] } @old;
# using a prototype allows you to use any comparison subroutine
# as a sort subroutine (including other package's subroutines)
package other;
sub backwards ($$) { $_[1] cmp $_[0]; }  # $a and $b are not set here
package main;
@new = sort other::backwards @old;
# guarantee stability, regardless of algorithm
use sort 'stable';
@new = sort { substr($a, 3, 5) cmp substr($b, 3, 5) } @old;
# force use of mergesort (not portable outside Perl 5.8)
use sort '_mergesort';  # note discouraging _
@new = sort { substr($a, 3, 5) cmp substr($b, 3, 5) } @old;

Warning: syntactical care is required when sorting the list returned from a function. If you want to sort the list returned by the function call find_records(@key), you can use:

@contact = sort { $a cmp $b } find_records @key;
@contact = sort +find_records(@key);
@contact = sort &find_records(@key);
@contact = sort(find_records(@key));

If instead you want to sort the array @key with the comparison routine find_records() then you can use:

@contact = sort { find_records() } @key;
@contact = sort find_records(@key);
@contact = sort(find_records @key);
@contact = sort(find_records (@key));

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

@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

@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 raises an exception 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 that NaN != NaN to eliminate any NaNs from the input list.

@result = sort { $a <=> $b } grep { $_ == $_ } @input;
ISBN 9781906966027Perl Language Reference ManualSee the print edition