Cannot dereference hash in subroutine....

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

Hi all, I have a PERL script in which I do the following: -call a subroutine to do some file initialization and stuff - return -initialize a hash (%hashname = (); ) -call a subroutine, passing a reference to the hash. In the subroutine I parse a bunch of data and eventually write to the hash thusly: $$hashname{$key1}{$key2}{$key3}{$key4} = $value; - call a second subroutine, passing a reference to the hash to it. Here's where the trouble starts..... Here's my subroutine:

sub GEN_HTML {                                                        
+                  # generate HTML for form
    $mp3hash = shift;                                                 
+                  # hash - parsed xml itunes data
    #print Dumper(\%{$mp3hash});
    foreach $player (sort keys %{$mp3hash}) {
        foreach $collection (sort keys %{$mp3hash{$player}}) {
print "DEBUG: the player is [$player] the collection is [$collection]\
+n";
            foreach $thetrack ( sort keys %{$mp3hash{$player}{$collect
+ion}}) {
                foreach $bute ( sort keys %{$mp3hash{$player}{$collect
+ion}{$track_name}}) {
                    print "$player - $collection - $thetrack - $mp3has
+h{$player}{$collection}{$track_name}{$bute}\n";
                }
            }
        }
    }
return;
}
[download]

If I uncomment the Dumper line, I get the expected data structure and data. If I put a print statement after the first foreach and print $player, I get the data I expect, however, I cannot get any of the data from the inner foreach loops to print. I'm pretty sure I just haven't figure out how to properly de-reference the keys in the inner loops, but I can't seem to figure out how to do this.... Oh, and if I put these loops up in main, everything works fine..... Any help? Thanks, bigtiny

Comment on Cannot dereference hash in subroutine.... Download Code

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Re: Cannot dereference hash in subroutine.... by davorg (Chancellor) on Jul 18, 2005 at 15:10 UTC
I think we'd need to see either the code where you build the hash or the output of the Data::Dumper call in order to know what your data structure _really_ is. It obviously isn't what you think it is (or you'd be getting the right results!) so I think you may be misreading the output from Data::Dumper. It's just a guess, but I wouldn't be surprised if you have one more or one less level of references than you think you do. Update: Looking at your code again, it looks like I'm right. You have (for example) `$mp3hash{$player}` where you should probably have `$mp3hash->{$player}`. This is a good example of where `use strict` is useful as it would have told you that you are trying to access a variable that does exist (the hash `%mp4hash`). -- <http://www.dave.org.uk> "The first rule of Perl club is you do not talk about Perl club." -- Chip Salzenberg	[reply]
Re^2: Cannot dereference hash in subroutine.... by bigtiny (Acolyte) on Jul 18, 2005 at 15:17 UTC
I appreciate the answer, however, if the data is not structured as I think it is, why do these statements work when issued against the hash in main (and they do....) ? bigtiny	[reply]
Re^3: Cannot dereference hash in subroutine.... by davorg (Chancellor) on Jul 18, 2005 at 15:19 UTC
At a guess, because outside of this subroutine you are accessing the hash, whereas in this subroutine you are using a reference to the hash. -- <http://www.dave.org.uk> "The first rule of Perl club is you do not talk about Perl club." -- Chip Salzenberg	[reply]
Re^4: Cannot dereference hash in subroutine.... by bigtiny (Acolyte) on Jul 18, 2005 at 15:24 UTC
Re^5: Cannot dereference hash in subroutine.... by davorg (Chancellor) on Jul 18, 2005 at 15:32 UTC
Re: Cannot dereference hash in subroutine.... by ikegami (Patriarch) on Jul 18, 2005 at 15:33 UTC
Change `%{$mp3hash{$player}...}` to `%{$mp3hash->{$player}...}` or to `%{$$mp3hash{$player}...}` because you want to use $mp3hash (a reference to a hash), but you're using %mp3hash (which doesn't exist). `use strict` would have found this error. `use strict` would also point out that all your variables are global (which is asking for trouble). By the way, `print Dumper(\%{$mp3hash});` is the same as `print Dumper($mp3hash);`	[reply] [d/l] [select]
Re: Cannot dereference hash in subroutine.... by artist (Parson) on Jul 18, 2005 at 15:16 UTC
Don't know much about your error, but you can try mp3. That may be able to eliminate some errors before happening. --Artist	[reply]
Re: Cannot dereference hash in subroutine.... by ysth (Canon) on Jul 19, 2005 at 06:35 UTC
`sub GEN_HTML { + # generate HTML for form $mp3hash = shift;` [download] That $mp3hash is a global variable, a strange thing to put a sub parameter in. Did you mean `my $mp3hash = shift;`?	[reply] [d/l] [select]