When called on a hash in list context, returns a 2-element list consisting of the key and value for the next element of a hash. In Perl 5.12 and later only, it will also return the index and value for the next element of an array so that you can iterate over it; older Perls consider this a syntax error. When called in scalar context, returns only the key (not the value) in a hash, or the index in an array.
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 each or keys may be deleted without changing the order. So long as a given hash is unmodified you may rely on keys, values and each to 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.
After each has returned all entries from the hash or array, the next call to each returns the empty list in list context and undef in scalar context; the next call following that one restarts iteration. Each hash or array has its own internal iterator, accessed by each, keys, and values. The iterator is implicitly reset when each has reached the end as just described; it can be explicitly reset by calling keys or values on the hash or array, or by referencing the hash (but not array) in list context. If you add or delete a hash's elements while iterating over it, the effect on the iterator is unspecified; for example, entries may be skipped or duplicated--so don't do that. Exception: It is always safe to delete the item most recently returned by each, so the following code works properly:
while (my ($key, $value) = each %hash) {
print $key, "\n";
delete $hash{$key}; # This is safe
}Tied hashes may have a different ordering behaviour to perl's hash implementation.
The iterator used by each is attached to the hash or array, and is shared between all iteration operations applied to the same hash or array. Thus all uses of each on a single hash or array advance the same iterator location. All uses of each are also subject to having the iterator reset by any use of keys or values on the same hash or array, or by the hash (but not array) being referenced in list context. This makes each-based loops quite fragile: it is easy to arrive at such a loop with the iterator already part way through the object, or to accidentally clobber the iterator state during execution of the loop body. It's easy enough to explicitly reset the iterator before starting a loop, but there is no way to insulate the iterator state used by a loop from the iterator state used by anything else that might execute during the loop body. To avoid these problems, use a foreach loop rather than while-each.
This extends to using each on the result of an anonymous hash or array constructor. A new underlying array or hash is created each time so each will always start iterating from scratch, eg:
# loops forever
while (my ($key, $value) = each @{ +{ a => 1 } }) {
print "$key=$value\n";
}This prints out your environment like the printenv(1) program, but in a different order:
while (my ($key,$value) = each %ENV) {
print "$key=$value\n";
}Starting with Perl 5.14, an experimental feature allowed each to take a scalar expression. This experiment has been deemed unsuccessful, and was removed as of Perl 5.24.
As of Perl 5.18 you can use a bare each in a while loop, which will set $_ on every iteration. If either an each expression or an explicit assignment of an each expression to a scalar is used as a while/for condition, then the condition actually tests for definedness of the expression's value, not for its regular truth value.
while (each %ENV) {
print "$_=$ENV{$_}\n";
}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:
use v5.12; # so keys/values/each work on arrays
use v5.18; # so each assigns to $_ in a lone while test