http://www.perlmonks.org?node_id=167849

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

I'm ignornant when it comes to some of the more basic concepts of programming, so I'm tackling references. I've come across an example that I can't get to work and would love some insight as to why... 

#!/usr/bin/perl -w
%animals = ('donald'=>'duck', 'mickey'=>'mouse', 'cheshire'=>'cat');

#set reference for hash
$fer = \%animals;
print "The value of \$fer is $fer\n";
print "The value for the key \"mickey\" is $$fer['mickey']\n";

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... Only when I run this snippet, I get "Not an ARRAY reference at foo.cgi line 7"

Am I blind? Why isn't this correct?

Replies are listed 'Best First'.
Re: Reference Example
by VSarkiss (Monsignor) on May 20, 2002 at 15:44 UTC

    Well, it's not an array reference, as your comment indicates:#set reference for hashTry this: print "The value for the key \"mickey\" is $$fer{'mickey'}\n";That is, use curly braces {}, not square ones [].

    A slightly cleaner way to write that would be: print "The value for the key \"mickey\" is $fer->{mickey}\n";

    Just as a general comment: if you're trying to get your hands around the basics, references may be a little too much too soon. But I can't really say that applies to you specifically since I don't know. Good luck, HTH.

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Re: Reference Example
by Ovid (Cardinal) on May 20, 2002 at 15:50 UTC

    For a good understanding of references, type "perldoc perlreftut" at the command line. It's a great start for learning references.

    Cheers,
    Ovid

    Join the Perlmonks Setiathome Group or just click on the the link and check out our stats.

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Re: Reference Example
by rinceWind (Monsignor) on May 20, 2002 at 15:48 UTC
    Only when I run this snippet, I get "Not an ARRAY reference at foo.cgi line 7"
    $fer is a HASH reference not an ARRAY reference. You want curly brackets '{' not square brackets.

    Some more advice: learn how to use Data::Dumper. This will enable you to see inside references and deep structures. The first line you print out will not be very meaningful. Data::Dumper's Dumper function will be far more useful.

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      ...this was the most concise statement of the answer I saw here.

      The problem was the form of referenced used in the example. (use square brackets for array elements, curly braces for hash elements.)

      I would also second the suggestion made above that references are something to defer until you are certain that you have a good grasp of the basic data structures. But if you feel that you are ready, it's a very powerful mechanism. (just like pointers are in C programming.)

      ---v

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Re: Reference Example
by CukiMnstr (Deacon) on May 20, 2002 at 17:11 UTC
    The other replies already have covered where the error is in your code, as well as some style advice (arrow operator and such), so I won't repeat any of that. But I think you might want to check References quick reference by tye, it will help you understand the relationship between $$foo{$bar}, $foo->{$bar}, ${$foo}{$bar}... (I know it helped me ;)

    hope this helps,

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Re: Reference Example
by Beatnik (Parson) on May 20, 2002 at 15:47 UTC
    You have to It's better if you dereference the ref outside the quotes :) and ofcourse do it properly
    print "The value of \$fer is ",%{$fer},"\n";
print "The value for the key \"mickey\" is ",${$fer}{'mickey'},"\n";

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    There's ofcourse always Perl's Data Structures Cookbook.

    Greetz
    Beatnik
    ... Quidquid perl dictum sit, altum viditur.
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      Nitpicky time: one of the reasons for Perl's line-noise rep is lines like:

      print "The value for the key \"mickey\" is ",${$fer}{'mickey'},"\n";

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      Using qq instead of the double-quotes and the arrow as the dereferencing operator, you'd get (IMO) a much cleaner: 
      print qq(The value for the key "mickey" is $fer->{mickey}\n);

      [download]

      Chris
      M-x auto-bs-mode

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        Actually, the escaped double quotes were there in the root node. Since the question was about references, I didn't bother to edit THAT part :)

        Greetz
        Beatnik
        ... Quidquid perl dictum sit, altum viditur.
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Re: Reference Example
by DamnDirtyApe (Curate) on May 21, 2002 at 02:50 UTC
    For the purposes of your example, the first two steps can be condensed into one. Rather than create a hash and then create a reference to it: 
    %animals = ('donald'=>'duck', 'mickey'=>'mouse', 'cheshire'=>'cat');
$fer = \%animals;

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    Create a reference to an anonymous hash like so: 
    $fer = {'donald'=>'duck', 'mickey'=>'mouse', 'cheshire'=>'cat'};

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    This will accomplish the same thing.
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Re: Reference Example
by Anonymous Monk on May 21, 2002 at 16:49 UTC
    This should be $$fer{'mickey'}, not $$fer['mickey']. You want curlies instead of angle brackets, otherwise perl assumes you are dereferencing an array.
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