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

completely new to Perl. I have been struggling all day with the concept of references in hashes (ie when to use it?). my current problem is I made a hash that stores 3 elements: x, y, and an array associated with that coordinate. 
 
$lengths{$line}= {x=> $site[0], y=>$site[1], z=> \@data};

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I don't have an issue assigning the hash and using Data::Dumper I see that z is a reference. However, I cannot seem to retrieve the array to manipulate. Say I wanted to print each element in this array: 
for my $coord (keys %lengths) {
   for (@{$lengths{$coord}{z}}) {
      print "$_\n";
   }
}

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This gives me nothing. Is it because I am forcing scalar? Sample output from Data::Dumper 
$VAR57 = '10:36759286';
$VAR58 = {
           'reads' => $VAR2->{'reads'},
           'x' => '10',
           'y' => '36759286'

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edit: forgot the sigil thanks.

Replies are listed 'Best First'.
Re: Help with array ref in hashes
by kennethk (Abbot) on Jan 09, 2015 at 21:26 UTC
    Your posted code fails compile with the error 
    Missing $ on loop variable

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    Please make sure that when you post code, it literally replicates your issue -- see How do I post a question effectively?.

    If I run the code: 

    my %lengths = (
    1 => {x => 1,
          y => 2,
          z => [1,2,3],
         },
    2 => {x => 2,
          y => 3,
          z => [2,3,4],
         },
    3 => {x => 4,
          y => 5,
          z => [5,6,7],
         },
);

for my $coord (keys %lengths) {
   for (@{$lengths{$coord}{z}}) {
      print "$_\n";
   }
}

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    I get the output 
    1
2
3
5
6
7
2
3
4

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    which seems to be your spec. My best guess is that you've got a typo in the code you are actually running, so that you aren't accessing the array. Try checking Data::Dumper after your loop to see if you are getting bitten by autovivification.

    #11929 First ask yourself `How would I do this without a computer?' Then have the computer do it the same way.

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Re: Help with array ref in hashes
by toolic (Bishop) on Jan 09, 2015 at 21:23 UTC
    Your loop looks ok, but maybe we need to see your dumper output: 
    use warnings;
use strict;

my %lengths = (c1 => {z => [1..5]});
for my $coord (keys %lengths) {
   for (@{ $lengths{$coord}{z} }) {
      print "$_\n";
   }
}

__END__

1
2
3
4
5

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    A good reference:

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Re: Help with array ref in hashes
by AnomalousMonk (Archbishop) on Jan 09, 2015 at 21:45 UTC

    I, too, seem to be getting the output I think you want to get from the OPed code (with missing  $ sigil restored): 

    c:\@Work\Perl\monks>perl -wMstrict -MData::Dump -le
"my $line = 'foo';
 my @data = qw(hi ha ho);
 my @site = (9, 8, 7);
 my %lengths;
 ;;
 $lengths{$line} = { x => $site[0],  y => $site[1],  z => \@data };
 dd \%lengths;
 ;;
 for my $coord (keys %lengths) {
   for (@{$lengths{$coord}{z}}) {
     print $_;
     }
   }
"
{ foo => { x => 9, y => 8, z => ["hi", "ha", "ho"] } }

hi
ha
ho

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    Give a man a fish:  <%-(-(-(-<

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      ok thanks to everyone for your help. i just realized i made a dumb mistake that I was passing the reference \@data to z but since it is a temporary variable I was using to pass data I was erasing it everytime. I guess I need to for loop the storage of @data into my hash.
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