perl: using push() on an array inside a hash

Question

Is it possible to use Perl's push() function on an array inside a hash?

Below is what I believe to be the offending part of a larger program that I am working on.

my %domains = ();
open (TABLE, "placeholder.foo") || die "cannot read domtblout file\n";
while ($line = <TABLE>)
{
    if (!($line =~ /^#/))                                                                 
    {
            @split_line = split(/\t/, $line);                                              # splits on tabs - some entries contain whitespace
        if ($split_line[13] >= $domain_cutoff)
        {
            push($domains{$split_line[0]}[0], $split_line[19]);                    # adds "env from" coordinate to array
            push($domains{$split_line[0]}[1], $split_line[20]);                    # adds "env to" coordinate to array
            # %domains is a hash, but $domains{identifier}[0] and $domains{$identifier}[1] are both arrays
            # this way, all domains from one sequence are stored with the same hash key, but can easily be processed iteratively
        }

    }

}

Later I try to interact with these arrays using

for ($i = 0, $i <= $domains{$identifier}[0], $i++)
        {
            $from = $domains{$identifier}[0][$i];
            $to = $domains{$identifier}[1][$i];
            $length = ($to - $from);
            $tmp_seq =~ /.{$from}(.{$length})/;
            print("$header"."$1");
        }

but it appears as if the arrays I created are empty.

If $domains{$identifier}[0] is an array, then why can I not use the push statement to add an element to it?

I'd recommend that you use strict; use warnings; at the top of your program. — Håkon Hægland
– Håkon Hægland, Commented Nov 30, 2015 at 20:21
Yes you need an array not a reference, see perldoc, for example push @{ $domains{$identifier}[0] }, .. ) — Håkon Hægland
– Håkon Hægland, Commented Nov 30, 2015 at 20:22
Yeah, I understand that I need an array not a reference, but I don't understand your suggested syntax. Do you mean push(@{$domains{$split_line[0]}[0]}, $split_line[19]) ? — user5399200
– user5399200, Commented Nov 30, 2015 at 20:32
Thanks, I think that solved my problem - but I won't know until I can test the whole thing (turns out I have a second problem - the supposedly machine-readable data that I accept as input is whitespace delimited for some reason, so I need to deal with that before I can test overall functionality) — user5399200
– user5399200, Commented Nov 30, 2015 at 20:40

ikegami · Accepted Answer · 2015-11-30 20:44:47Z

4

$domains{identifier}[0] is not an array.
$domains{identifier}[0] is an array element, a scalar.
$domains{identifier}[0] is a reference to an array.

If it's

@array

when you have an array, it's

@{ ... }

when you have a reference to an array, so

push(@{ $domains{ $split_line[0] }[0] }, $split_line[19]);

References:

answered Nov 30, 2015 at 20:44

ikegami

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