undef(%hash) not doing what I expect for HoH

doowah2004 has asked for the wisdom of the Perl Monks concerning the following question:

I have had a job change, and had to step away from the perl arena for a while. I have start a little script and I am a bit rusty. That being said, I have something that confuses me.

I have a HoH: %hash
%hash has 5 keys that are also hashes (%key1, %key2, %key3, %key4, %key5)
each of these hashes have 5 keys/value pairs.

When I undef(%hash) and I do a key count on it
$count = keys %hash;
$count = 0 and I would expect this. But when I do a key count on the hashes one level in:
$count = keys %{$hash->{$key1}};
I get $count = 5. This I did not expect!

I know that I can loop through the hash and undef everything, but why is this happening? What am I missing?

I did a super search and did not see this discussed.

Thanks in advance!

Comment on undef(%hash) not doing what I expect for HoH

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Re: undef(%hash) not doing what I expect for HoH by monarch (Priest) on Oct 12, 2006 at 14:56 UTC
I'm confused. `%hash` is your hash of hashes. This implies that `%hash` is a hash, not a reference to a hash. So when you attempt to recover `keys %{$hash->{$key1}}` you're attempting to treat `%hash` as a reference. Don't you mean to try `keys %{$hash{$key1}}`? Update: actually I tried a little script of my own and when I added `keys %{$hash->{$key1}}` I got a compiler error saying that I hadn't defined `$hash`. Try adding `use strict` to your script and see if the compiler helps you solve the problem..	[reply] [d/l] [select]
Re^2: undef(%hash) not doing what I expect for HoH by doowah2004 (Monk) on Oct 12, 2006 at 15:15 UTC
Wow, I am rusty. You are exactly right! And of course an undef on a reference does not affect the hash and an undef on a hash that I was not even using affects it even less... Thank you!	[reply]
Re: undef(%hash) not doing what I expect for HoH by Fletch (Bishop) on Oct 12, 2006 at 15:12 UTC
`%hash` is a hash; `$hash` is a scalar with an unfortunate name. `$hash{foo}` is the item in `%hash` at key `"foo"`. `$hash->{foo}` is the item in the hashref stored in `$hash` at key `"foo"`. `undef %hash` clears everything from `%hash`; `$hash` is untouched. `undef %{ $hash }` clears everything from the hash referenced by `$hash`; `%hash` is untouched. `undef $hash` replaces the reference to the hash which was in `$hash` with `undef`; again, `%hash` is untouched. A judicious `use strict` would have caught this.	[reply] [d/l] [select]
Re: undef(%hash) not doing what I expect for HoH by blazar (Canon) on Oct 12, 2006 at 15:26 UTC
That doesn't seem to be the case for me: `#!/usr/bin/perl -l use strict; use warnings; my %hoh=( h => {foo=>1}, g => {bar=>2} ); print scalar keys %hoh; print scalar keys %{ $hoh{h} }; undef %hoh; print scalar keys %hoh; print scalar keys %{ $hoh{h} }; __END__` [download] I get: `2 1 0 0` [download] But I wouldn't use undef on aggregates anyway.	[reply] [d/l] [select]