Odd number of elements in hash assignment warning. Use of uninitialized value warning

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

Replies are listed 'Best First'.
Re: Odd number of elements in hash assignment warning. Use of uninitialized value warning by Fastolfe (Vicar) on Sep 21, 2000 at 20:21 UTC
Your statement: `my %hash = undef;` [download] is incorrect. In this context, since `%hash` is more like an array than a scalar, 'undef' is actually a value you're trying to set to `%hash`. This "list-ish" behavior lets you do stuff like this: `my %hash = ( key => "value", key2 => "value2" ); # OK my %hash = ( "key", "value", "key2", "value2" ); # OK (same) my %hash = ( "key", "value", "key2" ); # WRONG - gives the + error you describe my %hash = ( undef ); # WRONG - equivalen +t to what you were doing!` [download] If you're wanting to "un-set" `%hash`, try either: `undef %hash; %hash = (); # empty list = empty hash` [download] But in your code above, this is unnecessary. Simply declaring `my %hash;` is sufficient, since in Perl this would initialize your hash to be empty.	[reply] [d/l] [select]
Re: Odd number of elements in hash assignment warning. Use of uninitialized value warning by Adam (Vicar) on Sep 21, 2000 at 20:16 UTC
Don't assign your hash `undef`, instead assign it an empty array () Also, use exists when testing for hash elements.	[reply]
Re: Odd number of elements in hash assignment warning. Use of uninitialized value warning by merlyn (Sage) on Sep 21, 2000 at 20:17 UTC
Yes. You've committed the same flaw as: `@array = undef;` [download] which is that it makes `@array` have one element of `undef`, not zero elements (empty). Do not use `undef` for anything other than the scalar undefined value. Your program could use: `my %href = ();` [download] But I'd probably flag that on code review as "potential signs of cluelessness - look very carefully at the remaining code", since that's also the default, and people who override the default with the default tend to make other mistakes. {grin} -- Randal L. Schwartz, Perl hacker	[reply] [d/l] [select]
RE: Re: Odd number of elements in hash assignment warning. Use of uninitialized value warning by swiftone (Curate) on Sep 21, 2000 at 20:20 UTC
"potential signs of cluelessness - look very carefully at the remaining code", since that's also the default I think what he means (in less sarcastic terms :) ) is that there is no reason not to simply have: `my %hash;` [download] I'm sure some people would defend variable initialization, even to the default, but in general I think it's more readable to simply accept the default (undef) that Perl assigns. Trust Perl. Perl is your friend.	[reply] [d/l]
"cluelessness" = best practice by blogical (Pilgrim) on May 30, 2007 at 16:05 UTC
Initializing variables with `undef` or `()` upon creation is a best practice for those who might ever have their code run under mod_perl.	[reply] [d/l] [select]
undef has a noun form and verb form. I should have used the verb form by princepawn (Parson) on Sep 21, 2000 at 20:22 UTC
You are right. And by the way, Net::FTP was causing problems for us along these lines... it is best to get an array ref from Net::FTP instead of an array because you will get an array with one element (undef) if there are no files resulting from an $ftp->ls call. But anyway, undef %h or undef @x will work fine. But the noun form is the one that I errantly used. What a dummy am I. oops, there I go again. That should've been What a dummy I am... misplaced noun and verb again. heh.	[reply]