- keys HASH
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
oreach
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 perlsec).As a side effect, calling keys() resets the HASH's internal iterator, see each. (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.
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 saykeys %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 = ()
, useundef %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 usingkeys
in this way (but you needn't worry about doing this by accident, as trying has no effect).