jess195 has asked for the wisdom of the Perl Monks concerning the following question:

Why would the following happen:

%mapp = ();
@arr = ("R", "T", "HH", "M");
$mapp{"FirstKey"} = \@arr;
@val = $mapp{"FirstKey"};
print join (",", @{$mapp{"FirstKey"}}) . "\n";   #The array will be pr
+inted 
push(@val, "RBB");
$mapp{"FirstKey"} = \@val;
print join (",", @{$mapp{"FirstKey"}}) . "\n";  #an array reference wi
+ll be printed

[download]

Output

R,T,HH,M
ARRAY(0x7ffafc013bf8),RBB

[download]
How to solve this issue and print the new array instead of its reference?

Replies are listed 'Best First'.
Re: Map a string to an array
by runrig (Abbot) on Nov 01, 2013 at 22:36 UTC 
    @val = $mapp{"FirstKey"};

    [download]
    This assigns a reference to @arr to $val[0], because $mapp{FirstKey} contains a reference to the array @arr. When you stringify the reference, you get something like "ARRAY(0x7ffafc013bf8)". If you wanted to make @val a copy of @arr, then you can:
    @val = @{$mapp{FirstKey}};

    [download]
[reply]
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[select]

      What would be the case if this is a 2d array? i.e, the map is from a string to 2d array.

      Is there an easy way to print the content?

[reply]
        Not sure what you're asking. Do you want to make a copy of a 2D array? Or if you don't really need a copy, can you just use the reference.
        my @arr = (
[1,2],
[3,4],
);
my %hash = ( FirstKey => \@arr );

my $aref = $hash{FirstKey};
my $row0 = $aref->[0];
push @$row0, 99;

use Data::Dumper;
print Dumper \%hash;

        [download]
[reply]
[d/l]

        Further to runrig's replies:
        It's important to understand at each step how the structures of the hash and arrays are changing and evolving. Here's a kind of "running commentary" on that evolution, and a possible way to access and print what you want. Liberal use of data structure (for that's what you're dealing with; see perldsc) dump statements can be very enlightening about the changes that are going on. (And BTW, congratulations on being able to work with a system having at least 128 Terabytes of system RAM — or do I misinterpret the reference address of 0x7ffafc013bf8?) 

        >perl -wMstrict -MData::Dump -le
"my %mapp;
 my @arr = qw(R T HH M);
 ;;
 $mapp{k} = \@arr;
 my @val = $mapp{k};
 ;;
 dd \%mapp;
 dd \@val;
 ;;
 print join q{,}, @{$mapp{k}};
 ;;
 push @val, 'RBB';
 dd \@val;
 ;;
 $mapp{k} = \@val;
 dd \%mapp;
 ;;
 print join q{,}, @{$mapp{k}};
 print join q{,}, @{ $mapp{k}->[0] }, $mapp{k}->[1];
"
{ k => ["R", "T", "HH", "M"] }

[["R", "T", "HH", "M"]]

R,T,HH,M
[["R", "T", "HH", "M"], "RBB"]

{ k => [["R", "T", "HH", "M"], "RBB"] }

ARRAY(0x1e14b3c),RBB
R,T,HH,M,RBB

        [download]
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[d/l]

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