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Mastering Modern Perl: Writing Efficient and Maintainable Code

Hash Idioms

Get to know about hash idioms and locking hashes in Perl.

We'll cover the following...

Finding unique values with undef

Each key exists only once in a hash, so assigning multiple values to the same key stores only the most recent value. This behavior has advantages! For example, we can find unique elements of a list:

Perl
my @items = ('Pencil', 'Sharpner', 'Pencil', 'Eraser', 'Paper', 'Eraser');
my %uniq;
undef @uniq{ @items };
my @uniques = keys %uniq;
say "@uniques";

Using undef with a hash slice sets the hash values to undef. This idiom is the cheapest way to perform set operations with a hash.

Using undef with hash slice
Using undef with hash slice

Counting occurrences

Hashes are useful for counting elements, such as IP addresses in a log file:

Perl
sub analyze_line { split /\s/, shift; }
open(my $logfile, '<', 'log');
my %ip_addresses;
while (my $line = <$logfile>) {
    chomp $line;
    my ($ip, $resource) = analyze_line( $line );
    $ip_addresses{$ip}++;
}
close($logfile);
#printing the hash
for (keys %ip_addresses) {
    say "$_ => $ip_addresses{$_}";
}

The initial value of a hash value is undef. The post-increment operator ++ treats that as zero. This in-place modification ...