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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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keys HASH

keys ARRAY

Returns a list consisting of all the keys of the named hash, or the indices of an array. (In scalar context, returns the number of keys or indices.)

The keys of a hash are returned in an apparently random order. The actual random order is subject to change in future versions of Perl, but it is guaranteed to be the same order as either the values or each function produces (given that the hash has not been modified). Since Perl 5.8.1 the ordering is different even between different runs of Perl for security reasons (see 22.2.9).

As a side effect, calling keys() resets the HASH or ARRAY's internal iterator (see ). In particular, calling keys() in void context resets the iterator with no other overhead.

Here is yet another way to print your environment:

@keys = keys %ENV;
@values = values %ENV;
while (@keys) {
    print pop(@keys), '=', pop(@values), "\n";
}

or how about sorted by key:

foreach $key (sort(keys %ENV)) {
    print $key, '=', $ENV{$key}, "\n";
}

The returned values are copies of the original keys in the hash, so modifying them will not affect the original hash. Compare .

To sort a hash by value, you'll need to use a sort function. Here's a descending numeric sort of a hash by its values:

foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
    printf "%4d %s\n", $hash{$key}, $key;
}

Used as an lvalue, keys allows you to increase the number of hash buckets allocated for the given hash. This can gain you a measure of efficiency if you know the hash is going to get big. (This is similar to pre-extending an array by assigning a larger number to $#array.) If you say

keys %hash = 200;

then %hash will have at least 200 buckets allocated for it--256 of them, in fact, since it rounds up to the next power of two. These buckets will be retained even if you do %hash = (), use undef %hash if you want to free the storage while %hash is still in scope. You can't shrink the number of buckets allocated for the hash using keys in this way (but you needn't worry about doing this by accident, as trying has no effect). keys @array in an lvalue context is a syntax error.

See also each, values and sort.

ISBN 9781906966027Perl Language Reference ManualSee the print edition