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Mastering Modern Perl: Writing Efficient and Maintainable Code

Hash Keys and Values

Explore how to work effectively with Perl hashes by learning to check key existence, access keys and values, and understand hash ordering and iterators to write reliable data structures.

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Checking hash key existence

The exists operator returns a boolean value to indicate whether a hash contains the given key:

Perl
my %addresses = (
    Leonardo => '1123 Fib Place',
    Utako => 'Cantor Hotel, Room 1'
);
say "Have Leonardo's address" if exists $addresses{Leonardo};
say "Have Warnie's address" if exists $addresses{Warnie};

Using exists instead of accessing the hash directly avoids two problems. First, it doesn’t check the boolean nature of the hash value; a hash key may exist with a value even if that value evaluates to a boolean false (including undef):

Perl
use Test::More;
my %false_key_value = ( 0 => '' );
ok %false_key_value,
   'hash containing false key & value should evaluate to a true value';
done_testing();

Second, exists avoids autovivificationAuto-creation of nested data structures on demand. ...