sort - sort a list of values

- NAME
- SYNOPSIS
- DESCRIPTION

# NAME

sort - sort a list of values

# SYNOPSIS

sort SUBNAME LIST

sort BLOCK LIST

sort LIST

# DESCRIPTION

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, `sort`s 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 the perlsyn manpage or with `goto`.

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

`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 manpage.

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

        @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
        @nums = @caps = ();
        for (@old) {
            push @nums, /=(\d+)/;
            push @caps, uc($_);
        }

        @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;

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

        @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` 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.

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

sort - sort a list of values