How do I undefine the value for a hash key in Perl?

Question

How can I undefine the value for a hash key in Perl? How can my code be corrected?

#!/usr/bin/perl

use strict;
use warnings;
my %hash;

undef($hash{"a"});
undef($hash{"b"});
print scalar values %hash; # Here I need 0
print scalar keys %hash; # And 2 here

Answer 1

undef($hash{"a"});

is equivalent to

$hash{"a"} = undef;

So you add key 'a' with value undef. To delete a value from a hash, use "delete".

delete $hash{"a"};

It is not possible to have different sizes of 'keys' and 'values' for the same hash. You can use grep to filter unwanted elements.

Answer 2

Alexandr Ciornii's point about definedness vs. existence is a very good one, but if you actually do want to know the number of defined values, you can always do:

print scalar grep { defined $_ } values %hash