Using OO hash

monk2b has asked for the wisdom of the Perl Monks concerning the following question:

Hi Monks

I am creating a script using what I think are object oriented coding techniques. I am not able to get hash variables working properly. Could one of you let me know what I am missing here that would make my second print statement work properly.

my %BldEnv = (
  "TTT2" => {
    VersionOne => "one",
    VersionTwo => "two",
    VersionThree => "three",
    VersionFour => "four",
    }
);

# commented line works
#print "$BldEnv{TTT2}{VersionOne}\n";

# uncommented line doesn't work 
print "$BldEnv->{TTT2}->{VersionTwo}\n";
[download]

I got 99 problems, but a @%$()_ ain't one.

Comment on Using OO hash Download Code

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Re: Using OO hash by moritz (Cardinal) on Nov 27, 2007 at 21:11 UTC
There is no OO involved in your example. `%BldEnv` is a plain hash, and the values can be accessed with the `$hash{$key}` syntax. The arrow `->` is for references to hashes, blessed or not.	[reply] [d/l] [select]
Re^2: Using OO hash by monk2b (Pilgrim) on Nov 27, 2007 at 21:27 UTC
Okay, I will use the first version. Thanks for the quick response. I got 99 problems, but a @%$()_ ain't one.	[reply]
Re: Using OO hash by meraxes (Friar) on Nov 27, 2007 at 21:53 UTC
As moritz said, this isn't OO, but the second line will work with one small modification: `print "$BldEnv{TTT2}->{VersionTwo}\n";` [download] Your version didn't work because the first `->` was trying to dereference $BldEnv as though it were a reference to a hash. But WAIT! `$BldEnv` doesn't exist! You declared %BldEnv. Different animal! Despite using the `$` sigil to get at hash values, when you added the `->` after it, Perl was looking for a SCALAR variable and ignoring the HASH that you defined! Perl's syntactic sugar allows you to get around derefencing the 'TTT2' hash using `->`. As such, the first one is better. Further to this, you really don't need to do string interpolation... can get a little messy with multi-level data structures. You could just do: `print $BldEnv{TTT2}{VersionTwo},"\n";` [download] That way the variable is easier to see and prevents a number of interpolation pitfalls. If you're feeling adventurous, you could install the Perl6::Say module so you don't have to append that `"\n"` to the end like so: `use Perl6::Say; say $BldEnv{TTT2}{VersionTwo};` [download] The `"\n"` is auto-appended by `say`. What you should _definately_ always do do is begin each script with: `use strict; use warnings;` [download] This would have thrown an error on your last print statement telling you that you that `$BldEnv` hadn't been declared, cluing you in that you're doing something wrong. Happy Perl-ing! update: expanded on difference between `$BldEnv{key}` and `$BldEnv->{key}`. update the second: fixed errorneous variable naming. -- meraxes	[reply] [d/l] [select]