Question

Sorry, this seems like such a basic question but I still don’t understand. If I have a hash, for example:

my %md_hash = ();
$md_hash{'top'}{'primary'}{'secondary'} = 0;

How come this is true?

if ($md_hash{'top'}{'foobar'}{'secondary'} == 0) {
    print "I'm true even though I'm not in that hash\n";
}

There is no “foobar” level in the hash so shouldn’t that result in false?

TIA

Answer 1

Try a search on “Perl autovivification”.

The hash values “spring into existence” when you first access them. In this case, the value is undef, which when interpreted as a number is zero.

To test for existence of a hash value without auto-vivifying it, use the exists operator:

if (exists $md_hash{'top'}{'foobar'}{'secondary'}
    && $md_hash{'top'}{'foobar'}{'secondary'} == 0) {
    print "I exist and I am zero\n";
}

Note that this will still auto-vivify $md_hash{'top'} and
$md_hash{'top'}{'foobar'} (i.e. the sub-hashes).

[edit]

As tchrist points out in a comment, it is poor style to compare undef against anything. So a better way to write this code would be:

if (defined $md_hash{'top'}{'foobar'}{'secondary'}
    && $md_hash{'top'}{'foobar'}{'secondary'} == 0) {
    print "I exist and I am zero\n";
}

(Although this will now auto-vivify all three levels of the nested hash, setting the lowest level to undef‘.)