hash undef?

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

Hi guys, hope you can help on this one. I have a hash which is filled from lines in a file, after a split of the line a hash is filled defined as $time{$recordNumber}. Later on in the script I want to check if $time{$recordNumber} exists, and if not enter the if statement. I tried using

 if ($time{$recordNumber} == undef) #147   
        {
                $time{$recordNumber} = 10000;
        }
[download]

but got Use of uninitialized value at ././monitor-dataset-age line 147 It's not no, because it's not there...but I'm testing to see if it's not there!? Any help much appreciated, Many thanks, Kim

Comment on hash undef? Select or Download Code

Replies are listed 'Best First'.
Re: hash undef? by moritz (Cardinal) on Nov 15, 2007 at 12:59 UTC
`if (not defined $time{$recordNumber}){ ... }` [download] You can also check with exists in hashes, the semantic is a little bit different. Update: A bit explanation: `==` compares numbers, so in the expression `$foo == undef` the `undef` is promoted to `0` and compared with `$foo` - not what you want.	[reply] [d/l] [select]
Re^2: hash undef? by Anonymous Monk on Nov 15, 2007 at 15:16 UTC
examples: `perl -wMstrict -e "my %hash = (foo => 'x', bar => '', baz => undef); for (qw/foo bar baz quux/) { printf qq($_ exists:%s: $_ defined:%s: $_ true:%s: \n), exists($hash{$_}), defined($hash{$_}), $hash{$_} ? 1 : '' }" foo exists:1: foo defined:1: foo true:1: bar exists:1: bar defined:1: bar true:: baz exists:1: baz defined:: baz true:: quux exists:: quux defined:: quux true::` [download]	[reply] [d/l]
A reply falls below the community's threshold of quality. You may see it by logging in.
Re: hash undef? by zer (Deacon) on Nov 15, 2007 at 14:23 UTC
`if (exists $time{$recordNumber})` This will tell you if it actually exists rather than whether it holds an undef or not. Defined will return false if the value contains an undef or if it just doesnt exist. Update: Thank you Mr. Anony. I had said that Defined returns undef, which isnt accurate and even contradicts the use of the function.	[reply] [d/l]
Re^2: hash undef? by Anonymous Monk on Nov 15, 2007 at 15:11 UTC
`defined` returns a boolean false (an empty string), not undef, if the value it is testing is not defined: `perl -wMstrict -e "print 'defined returns defined' if defined defined undef" defined returns defined` [download]	[reply] [d/l] [select]
Re: hash undef? by dsheroh (Monsignor) on Nov 15, 2007 at 19:12 UTC
As moritz has said, testing `not defined` is better than `== undef`, but `$time{$recordNumber}` isn't the only variable involved on that line... I suspect that `$recordNumber` is the uninitialized value that it's complaining about. (If `$recordNumber` is defined and `$time{$recordNumber}` isn't, I would expect to see the warning "`Use of uninitialized value in numeric eq (==) at...`".)	[reply] [d/l] [select]