In Perl, how do I process an entire hash?

Question

I would like to process all elements of a hash table in Perl. How can I do that?

---

This is a question from the official perlfaq. We're importing the perlfaq to Stack Overflow.

Answer 1

(This is the official perlfaq answer, minus any subsequent edits)

There are a couple of ways that you can process an entire hash. You can get a list of keys, then go through each key, or grab a one key-value pair at a time.

To go through all of the keys, use the keys function. This extracts all of the keys of the hash and gives them back to you as a list. You can then get the value through the particular key you're processing:

foreach my $key ( keys %hash ) {
    my $value = $hash{$key}
    ...
    }

Once you have the list of keys, you can process that list before you process the hash elements. For instance, you can sort the keys so you can process them in lexical order:

foreach my $key ( sort keys %hash ) {
    my $value = $hash{$key}
    ...
    }

Or, you might want to only process some of the items. If you only want to deal with the keys that start with text:, you can select just those using grep:

foreach my $key ( grep /^text:/, keys %hash ) {
    my $value = $hash{$key}
    ...
    }

If the hash is very large, you might not want to create a long list of keys. To save some memory, you can grab one key-value pair at a time using each(), which returns a pair you haven't seen yet:

while( my( $key, $value ) = each( %hash ) ) {
    ...
    }

The each operator returns the pairs in apparently random order, so if ordering matters to you, you'll have to stick with the keys method.

The each() operator can be a bit tricky though. You can't add or delete keys of the hash while you're using it without possibly skipping or re-processing some pairs after Perl internally rehashes all of the elements. Additionally, a hash has only one iterator, so if you use keys, values, or each on the same hash, you can reset the iterator and mess up your processing. See the each entry in perlfunc for more details.