Question

How could I find out if this hash has an odd number of elements?

my %hash = ( 1, 2, 3, 4, 5 );

Ok, I should have written more information.

sub routine {
    my ( $first, $hash_ref ) = @_;
    if ( $hash_ref refers to a hash with odd numbers of elements ) {
        "Second argument refers to a hash with odd numbers of elements.\nFalling back to default values";
        $hash_ref = { option1 => 'office', option2 => 34, option3 => 'fast'  };
    }
    ...
    ...
}


routine( [ 'one', 'two', 'three' ], { option1 =>, option2 => undef, option3 => 'fast' );

Answer 1

The simple answer is: You get a warning about it:

Odd number of elements in hash assignment at...

Assuming you have not been foolish and turned warnings off.

The hard answer is, once assignment to the hash has been done (and warning issued), it is not odd anymore. So you can’t.

my %hash = (1,2,3,4,5);
use Data::Dumper;
print Dumper \%hash;

$VAR1 = {
      '1' => 2,
      '3' => 4,
      '5' => undef
    };

As you can see, undef has been inserted in the empty spot. Now, you can check for undefined values and pretend that any existing undefined values constitutes an odd number of elements in the hash. However, should an undefined value be a valid value in your hash, you’re in trouble.

perl -lwe '
    sub isodd { my $count = @_ = grep defined, @_; return ($count % 2) }; 
    %a=(a=>1,2); 
    print isodd(%a);'
Odd number of elements in hash assignment at -e line 1.
1

In this one-liner, the function isodd counts the defined arguments and returns whether the amount of arguments is odd or not. But as you can see, it still gives the warning.