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

I'm using Text::CSV_XS and it requires an array reference when printing output. I have a hash that I'd like to create an array reference for in order to create the output. I found this post (Perl packages to convert hash refs to array refs and vice-versa?) that shows a simple way to do it, but when I execute the following code I receive an error on the final line. Code: 
my %errors = ();
my $array_ref=[%$errors];
my $csvout = new IO::File "> csvoutput.csv";
my $status = $csv->print ($csvout, [%$errors])

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Error: 
Global symbol "$errors" requires explicit package name

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Thanks for any help!

Replies are listed 'Best First'.
Re: hashref to arrayref
by jethro (Monsignor) on Jul 30, 2009 at 20:58 UTC
    Just use %errors instead of %$errors in this script. The syntax %$errors means "the hash $errors points to". Since you don't have a variable $errors initialized with anything resembling a pointer it is no surprise perl doesn't know what you want

    Note that $errors and %errors are totally different variables and have nothing but the name in common

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Re: hashref to arrayref
by ssandv (Hermit) on Jul 30, 2009 at 21:07 UTC
    You're starting with a hash, not a hash ref, so you never declared $errors. 
    my $errors = {}; #or my $errors=\%existing_hash;
my $array_ref=[%$errors];
my $csvout = new IO::File "> csvoutput.csv";
my $status = $csv->print ($csvout, [%$errors])

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    or 
    my %errors = ();
my $array_ref=[%errors];
my $csvout = new IO::File "> csvoutput.csv";
my $status = $csv->print ($csvout, [%$errors])

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    should work.

    Edit: this was a block copy to show what changed. I did not test it, and the last line in each case should be 
    my $status = $csv->print ($csvout, $array_ref);

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      Thanks much for clarifying the use of the brackets and the references. I found part of my problem was my understanding of the square brackets. I thought they were also returning a reference (which they are) but only as evaluated at the time of the creation. I was using %errors before the hash was populated. I expected that the array reference would point to the populated list, but it did not. I guess it makes sense considering the brackets evaluate in list context and *then* return the reference. It seems to be working now, and I appreciate all your time! Thanks!
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      Regarding creating the array reference, is it possible to completely output the contents of the hash rather than the reference value? The hash is being accessed like this: 
      $errors{$key}{'date'}
$errors{$key}{'time'}

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      If I do the following I get a hash reference followed by the key value. 
      my $array_ref = [%errors]

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      If I do this, 
      my $array_ref = [keys %errors]

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      then I get only the keys, as you'd expect. Is there a way to generate the array to contain each element or will I have to resort to a looping construct? Thanks again!
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        I'm not clear on what you want the array to contain. Just the bottom-level values? If I understand correctly, your structure looks like 
        %errors=(key1 => { date => 'some date',
                   time => 'some time',
                 },
         key2 => { date => 'some other date',
                   time => 'some other time',
                 },
         ...
         );

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        so when you read it in list context, you get a list of keys and hashrefs. What exactly are you trying to get back?
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