Mastering Modern Perl: Writing Efficient and Maintainable Code/

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Hash Idioms

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
Files
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 ...