Perl data structure reference question

Factual Questions
1

I almost feel abusive asking for this here on the SDMB since it’s totally not a general interest question. On the other hand there’s an expert on everything here and we’re all generally much nicer to Perl newbies… can someone give me a hand with what I think is a simple, basic, God-I-feel-stupid-for-having-to-ask problem with Perl?

If I have this hash:



%HoA = (
        flintstones        => [ "fred",   "barney"         ],
        jetsons            => [ "george", "jane",  "elroy" ],
        simpsons           => [ "homer",  "marge", "bart"  ],
      );

Then something like print scalar (keys %HoA) gives me the number of keys. Easy enough – %HoA refers to the hash as a hash. If I do this:


my $flints = $HoA{'flintsones'};

Then $flints contains a reference to the array “contained” in the value for key “flintstones”, right? That is, key “flintstones” can only be scalar, so it contains the address of the array. So I should be able to use the array pointed to by $flints as @flints, isn’t that correct? So I should be able to do something like print scalar @flints to get the array size. That doesn’t work, though.
By extension, I think I should be able to


print $flints[0];

…in order to print “fred” – also fails.

I’ve read all kinds of documentation about references in Perl, and it’s just flying way over my head, which is odd because I do perfectly fine in Object Pascal, C, and Objective-C with pointers and handles. Weird.

Could someone provide me with an example on how to access the data in the structure above? Nothing I’ve read addresses such a structure. Also if the example isn’t totally obvious, could you break it down for me? I hate to sound like such a noob, but this is really frustrating me. This is Perl 5.003 BTW (old, old stuff on old, old machines).

2

You’ll have to dereference $flints, which is a scalar, in order to get at its underlying array. Think of the reference as being a pointer to something. To access an individual element you have to say something like $flints->[0]. To get the array size you have to tell perl that it’s actually an array, e.g., scalar @{ $flints }.

3

You can also do this: $$flints[0], which makes sense to my twisted mind because:

@array is to $array[0] as @{flints} is to {$flints}[0], or $$flints[0] for short.

4

In short, what they said. But in an attempt to be helpful:

$flints[0] = the zero element of array @flints
$flints->[0] = the zero element of the array referenced by $flints

5

** Balthisar**, the best document for this sort of thing is the Perl data-structure cookbook.

Also, the best place to ask Perl questions is Perl Monks. You will find that your favorite programmer is very active there.

6

Ah! It was the curly brackets that did the trick for me, coupled with the arrow notation. I don’t know why I thought changing the symbol would do the dereferencing for me. That is, I’d been thinking that
@variable[n] was the same as @ {$variable->[n]} which clearly isn’t the case. Seeing it in this fashioned has totally changed things for me. So thanks to Terminus Est for getting there first. Jawdirk, doing $$ gives me compiler errors, but part of the problem is I have an ancient version of Perl 5.003 on a Sun server with broken modules, so I don’t trust that $$ shouldn’t work or it’s another error. Not to exclude duality from the accolades for helping. And finally friedo, that’s a lot clearer than the official Perl documentation I stumbled across, but seeing where it’s hosted, I wonder why there’s duplicate documentation. I guess there are multiple ways to document Perl, too. :wink:

7

And, technically, shouldn’t your data be:



       flintstones        => [ "fred",   "wilma", "pebbles"  ],
       rubbles            => [ "barney", "betty", "bamm-bamm"]