FAQ 4.71 How can I check if a key exists in a multilevel hash?

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This is an excerpt from the latest version perlfaq4.pod, which
comes with the standard Perl distribution. These postings aim to
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to review and update the answers. The latest version of the complete
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4.71: How can I check if a key exists in a multilevel hash?

(contributed by brian d foy)

The trick to this problem is avoiding accidental autovivification. If
you want to check three keys deep, you might naïvely try this:

my %hash;
if( exists $hash{key1}{key2}{key3} ) {
...;
}

Even though you started with a completely empty hash, after that call to
"exists" you've created the structure you needed to check for "key3":

%hash = (
'key1' => {
'key2' => {}
}
);

That's autovivification. You can get around this in a few ways. The
easiest way is to just turn it off. The lexical "autovivification"
pragma is available on CPAN. Now you don't add to the hash:

{
no autovivification;
my %hash;
if( exists $hash{key1}{key2}{key3} ) {
...;
}
}

The "Data::Diver" module on CPAN can do it for you too. Its "Dive"
subroutine can tell you not only if the keys exist but also get the
value:

use Data::Diver qw(Dive);

my @exists = Dive( \%hash, qw(key1 key2 key3) );
if( ! @exists ) {
...; # keys do not exist
}
elsif( ! defined $exists[0] ) {
...; # keys exist but value is undef
}

You can easily do this yourself too by checking each level of the hash
before you move onto the next level. This is essentially what
"Data::Diver" does for you:

if( check_hash( \%hash, qw(key1 key2 key3) ) ) {
...;
}

sub check_hash {
my( $hash, @keys ) = @_;

return unless @keys;

foreach my $key ( @keys ) {
return unless eval { exists $hash->{$key} };
$hash = $hash->{$key};
}

return 1;
}



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