FAQ 4.55 How do I process an entire hash?


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This is an excerpt from the latest version perlfaq4.pod, which
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4.55: How do I process an entire hash?

(contributed by brian d foy)

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.



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