how to check if a hash reference is empty in perl

Question

I need to test, whether my hashref contains 0 elements. I used this code:

$self = { fld => 1 };
%h = ( "a" => "b" );
$self->{href} = { %h };
print STDERR $self->{href}{ "a" };
print STDERR "\n";
print "size of hash:  " . keys( %h ) . ".\n";
print "size of hashref:  " . keys( $self->{href} ) . ".\n";

It works well with perl 5.16, but fails with perl 5.10:

Type of arg 1 to keys must be hash (not hash element) at - line 7, near "} ) "
Execution of - aborted due to compilation errors.

Answer 1

If you'd use

%hash

for a hash, you'd use

%{ $hash }

for a reference, so it's

keys %{ $self->{href} }

Note: In some versions of Perl, keys accepts a reference. However, this was an experimental feature that was abandoned. One shouldn't use it.

Answer 2

To find out if a hash has elements, you just use it in scalar context:

scalar %h

or

%h ? "yup" : "nope"

scalar keys %h accomplishes the same purpose by counting the keys in %h, but it's better to ask for what you actually want to know.

Either way, however, %h is a hash and not a hashref. (Though some versions of Perl do tolerate a hashref as an argument to keys.) Given an expression EXPR that evaluates to a hashref, you get at the corresponding hash by saying %{ EXPR }. Putting this together with your sample code, we get

print "size of hashref:  " . keys( %{ $self->{href} } ) . ".\n";
print "hash " . (%{ $self->{href} } ? "does" : "does not") . " contain elements\n";