Array of references in Perl

Question

I’ve read the reference documentation, but I can’t figure out how to dereference the array references inside my array. Don’t understand why @{$HoA{$cols[0]}} prints only the length of the array either. Any clarification is much appreciated.

file.txt:

    aa      bb
    bb      cc

The program:

#!/usr/bin/perl
use strict;
use warnings;
use Data::Dumper;

# my $filename = "file.txt";
my @newarray;

my %HoA = (
        aa        => [ "GAT_1", "GAT_2", "GAT_3", "GAT_4" ],
        bb        => [ "GAT_6", "GAT_1", "GAT_5", "GAT_4", "GAT_2" ],
        cc        => [ "GAT_6", "GAT_4", "GAT_3", "GAT_1", "GAT_2" ],
      );

open (FILE, '<' ,"$filename") or print "$filename does not exist\n";


while (<FILE>) {
    my @cols = split;
    $cols[0] = $HoA{ $cols[0] };
    #$cols[0] = @{$HoA{ $cols[0]} };
    $cols[1] = $HoA{ $cols[1] };
    #$cols[1] = @{$HoA{ $cols[1] }};
    push ( @newarray, join( " ", @cols ));
}

close FILE;

print Dumper(\@newarray);

This is my expected output:

$VAR1 = [
          [
            [
              'GAT_1',
              'GAT_2',
              'GAT_3',
              'GAT_4'
            ],
            [
              'GAT_6',
              'GAT_1',
              'GAT_5',
              'GAT_4',
              'GAT_2'
            ],
        [
              'GAT_6',
              'GAT_1',
              'GAT_5',
              'GAT_4',
              'GAT_2'   
            ],
            [
              'GAT_6',
              'GAT_4',
              'GAT_3',
              'GAT_1',
              'GAT_2'
            ],

          ]
        ];

This is my actual output:

$VAR1 = [
          'ARRAY(0x7f80110060e8) ARRAY(0x7f801102eb58)',
          'ARRAY(0x7f801102eb58) ARRAY(0x7f801102f308)'
        ];

Re "I can’t figure out how to dereference the array references inside my array", Technically, are you are deferencing it. What output are you expecting? — ikegami
– ikegami, Commented Nov 25, 2015 at 19:29
Re "Don’t understand why @{$HoA{$cols[0]}} prints only the length of the array", @{$HoA{$cols[0]}} doesn't print anything. @a in scalar context evaluates to the number of elements in the array. As such, @{ REF } in scalar context evaluates to the number of elements in the referenced array. — ikegami
– ikegami, Commented Nov 25, 2015 at 19:31
what is your expected output? your join is treating array references as strings, but without expected output it is hard to know what you should be doing instead — ysth
– ysth, Commented Nov 25, 2015 at 19:40
I was expecting that the keys in my file were completely replaced by the corresponding arrays in the values of my hash of arrays, but what I got instead were the references pointing to those arrays. Probably not well explained, my apologies. — cmvdi01
– cmvdi01, Commented Nov 25, 2015 at 19:44
It's still hard to see exactly what you wanted the output to look like. Why not edit your post and add sections to show the complete "Expected Output" and "Actual Output"? — George Adams
– George Adams, Commented Nov 25, 2015 at 19:54

ruakh · Accepted Answer · 2015-11-25 20:15:01Z

4

The big problem is this line:

    push ( @newarray, join( " ", @cols ));

join is inherently a string operation: @cols is an array of references, which join then dutifully stringifies so that it can join them with " ".

It seems like what you really want is probably this:

    push ( @newarray, [@cols] );

where the [ ... ] notation creates a new anonymous array (in this case populated with the values in @cols) and returns a reference to to it.

Additionally, instead of this:

    $cols[0] = $HoA{ $cols[0] };

(which causes @newarray and %HoA to end up containing references to the same underlying arrays), you may want this:

    $cols[0] = [ @{$HoA{ $cols[0] }} ];

(such that @newarray ends up with completely independent arrays that simply start out with the same data as what's in %HoA). This depends on whether you're planning to modify any of the arrays afterward.

answered Nov 25, 2015 at 20:15

ruakh

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1 Comment

cmvdi01 Over a year ago

Thank you, that's exactly it!

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