keys - retrieve list of indices from a hash

- NAME
- SYNOPSIS
- DESCRIPTION

# NAME

keys - retrieve list of indices from a hash

# SYNOPSIS

keys HASH

# DESCRIPTION

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

The keys 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 Algorithmic Complexity Attacks in the perlsec manpage).

As a side effect, calling `keys()` resets the HASH's internal iterator (see each in the perlfunc manpage). 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 values in the perlfunc manpage.

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;
        }

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).

See also `each`, `values` and `sort`.

keys - retrieve list of indices from a hash