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From: PerlFAQ Server on 13 Aug 2010 00:00
This is an excerpt from the latest version perlfaq4.pod, which
comes with the standard Perl distribution. These postings aim to
reduce the number of repeated questions as well as allow the community
to review and update the answers. The latest version of the complete
perlfaq is at http://faq.perl.org .

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4.56: How do I merge two hashes?

(contributed by brian d foy)

Before you decide to merge two hashes, you have to decide what to do if
both hashes contain keys that are the same and if you want to leave the
original hashes as they were.

If you want to preserve the original hashes, copy one hash (%hash1) to a
new hash (%new_hash), then add the keys from the other hash (%hash2 to
the new hash. Checking that the key already exists in %new_hash gives
you a chance to decide what to do with the duplicates:

my %new_hash = %hash1; # make a copy; leave %hash1 alone

foreach my $key2 ( keys %hash2 )
{
if( exists $new_hash{$key2} )
{
warn "Key [$key2] is in both hashes!";
# handle the duplicate (perhaps only warning)
...
next;
}
else
{
$new_hash{$key2} = $hash2{$key2};
}
}

If you don't want to create a new hash, you can still use this looping
technique; just change the %new_hash to %hash1.

foreach my $key2 ( keys %hash2 )
{
if( exists $hash1{$key2} )
{
warn "Key [$key2] is in both hashes!";
# handle the duplicate (perhaps only warning)
...
next;
}
else
{
$hash1{$key2} = $hash2{$key2};
}
}

If you don't care that one hash overwrites keys and values from the
other, you could just use a hash slice to add one hash to another. In
this case, values from %hash2 replace values from %hash1 when they have
keys in common:

@hash1{ keys %hash2 } = values %hash2;



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are not necessarily experts in every domain where Perl might show up,
so please include as much information as possible and relevant in any
corrections. The perlfaq-workers also don't have access to every
operating system or platform, so please include relevant details for
corrections to examples that do not work on particular platforms.
Working code is greatly appreciated.

If you'd like to help maintain the perlfaq, see the details in
perlfaq.pod.
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