- keys ARRAY
- keys EXPR
Called in list context, returns a list consisting of all the keys of the named hash, or in Perl 5.12 or later only, the indices of an array. Perl releases prior to 5.12 will produce a syntax error if you try to use an array argument. In scalar context, returns the number of keys or indices.
Hash entries are returned in an apparently random order. The actual random order is specific to a given hash; the exact same series of operations on two hashes may result in a different order for each hash. Any insertion into the hash may change the order, as will any deletion, with the exception that the most recent key returned by
keysmay be deleted without changing the order. So long as a given hash is unmodified you may rely on
eachto repeatedly return the same order as each other. See Algorithmic Complexity Attacks in perlsec for details on why hash order is randomized. Aside from the guarantees provided here the exact details of Perl's hash algorithm and the hash traversal order are subject to change in any release of Perl.
As a side effect, calling keys() resets the internal iterator of the HASH or ARRAY (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:
or how about sorted by key:
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
sortfunction. Here's a descending numeric sort of a hash by its values:
Used as an lvalue,
keysallows 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;
%hashwill 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 %hashif you want to free the storage while
%hashis still in scope. You can't shrink the number of buckets allocated for the hash using
keysin this way (but you needn't worry about doing this by accident, as trying has no effect).
keys @arrayin an lvalue context is a syntax error.
Starting with Perl 5.14,
keyscan take a scalar EXPR, which must contain a reference to an unblessed hash or array. The argument will be dereferenced automatically. This aspect of
keysis considered highly experimental. The exact behaviour may change in a future version of Perl.
To avoid confusing would-be users of your code who are running earlier versions of Perl with mysterious syntax errors, put this sort of thing at the top of your file to signal that your code will work only on Perls of a recent vintage: