Learning Perl, 5th Edition

Hash Functions

Naturally, there are some useful functions that can work on an entire hash at once.

The keys and values Functions

The keys function yields a list of all the keys in a hash, while the values function gives the corresponding values. If there are no elements to the hash, then either function returns an empty list:

my %hash = ("a" => 1, "b" => 2, "c" => 3);
my @k = keys %hash;
my @v = values %hash;

So, @k will contain "a", "b", and "c", and @v will contain 1, 2, and 3—in some order. Remember, Perl doesn’t maintain the order of elements in a hash. But, whatever order the keys are in, the values will be in the corresponding order: if "b" is last in the keys, 2 will be last in the values; if "c" is the first key, 3 will be the first value. That’s true as long as you don’t modify the hash between the request for the keys and the one for the values. If you add elements to the hash, Perl reserves the right to rearrange it as needed, to keep the access quick.^[*] In a scalar context, these functions give the number of elements (key-value pairs) in the hash. They do this quite efficiently, without having to visit each element of the hash:

my $count = keys %hash;  # gets 3, meaning three key-value pairs

Once in a long while, you’ll see that someone has used a hash as a Boolean (true/false) expression, something like this:

if (%hash) {
  print "That was a true value!\n";
}

That will be true if (and only if) the hash has at least one key-value pair.^[†] So, it’s just saying, “If the hash is ...