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

Dear Monks, I have a hash, where value to a key is a nested hash in itself. Somewhere down the line in process,I expand the single key, into multiple keys/exploded keys, and process the value of single key (nested hash) to introduce a minor change and assign it to exploded keys, such that values in key are unique. But the dumper gives me a reference as value, that too to a refering to a single value, instead of expanded hash each having unique value. Below is the truncated code and Dumper output. 
if ($padinstance =~/^\s*(\w+)\[\s*(\d+)\s*\:\s*(\d+)\s*\]\s*$/) {
         #Check and modify simultaneously
         my $padword = $1;
         my $msb = $2;
         my $lsb = $3;
         my %hashcopy = %{$topxls_hash_ref->{$hier}->{$padinstance}};

         for (my $counter = 0; $counter <= $msb - $lsb; $counter ++) {
            my %newhash = %hashcopy;
            foreach my $chk_key (keys (%hashcopy)) {
               if ($chk_key eq "MODE") {
                 ## For each mode check type and do action
                 foreach my $mode (keys (%{$hashcopy{MODE}})) {
                   if ($hashcopy{MODE}->{$mode}->{TYPE} eq "O") {
                     foreach my $chk_mode_key (keys (%{$hashcopy{MODE}
+->{$mode}})) {
                       if ($chk_mode_key eq "ENABLES") {
                          foreach my $enable_sig (@{$hashcopy{MODE}->{
+$mode}->{ENABLES}}) {
                             if ($enable_sig =~/\w+/) {
                                print Dumper(\%hashcopy);
                                if (defined ($hashcopy{MODE}->{$mode}-
+>{"EN\@$enable_sig"}) && $hashcopy{MODE}->{$mode}->{"EN\@$enable_sig"
+} =~/\w+/) {
                                    my $array_index = 0;
                                    foreach my $enable_sig_iter (@{$ha
+shcopy{MODE}->{$mode}->{"EN\@$enable_sig"}}) {
                          print "In mode for $mode for ENABLES $enable
+_sig_iter\n";
                                      if ($enable_sig_iter =~/^(\s*\w+
+)\s*\[\s*(\d+)\s*\:\s*(\d+)\s*\]/) {
                                         my $prefix_word = $1;
                                         my $local_msb = $2;
                                         my $local_lsb = $3;
                                         if ($local_msb - $local_lsb =
+= $msb - $lsb) {
                                            my $token = $local_lsb + $
+counter;
                                            my $newsignal = "$prefix_w
+ord\[$token\]";
                                            
                                            $newhash{MODE}->{$mode}->{
+"EN\@$enable_sig"}->[$array_index] = $newsignal;
                                         } else {
                                                                      
+               return -1;
                                        }
                                     } 
                                     $array_index ++;
                                  }
                                }
                             }
                          }

[download]
Similar to ENABLE key, I have many other keys on which I am doing the operation and at end, I am attaching back the value of %newhash 
 $topxls_hash_ref->{$hier}->{$padinstance_new} =\%newhash;

[download]
however, after first iteration, modification in %newhash also modifies %hashcopy. I am not able to understand the reason behind such behaviour in nested hashes. Please help! Please let me know, where I am going wrong. While searching after posting, I came across that this is more to do with shallow copy. I need to make a deep copy.

Replies are listed 'Best First'.
Re: Hash value, containing a reference to some value of hash, not the intended behaviour.
by cdarke (Prior) on Mar 14, 2011 at 11:34 UTC
    You need to do a deepcopy. Replace:
    my %newhash = %hashcopy;

    [download]
    With something like this:
    use Storable qw(dclone);

my %newhash = %{dclone(%hashcopy)};

    [download]
    We could use thaw and freeze, but a simpler interface is provided in dclone which carries out both operations. There is also a Clone module on CPAN.
[reply]
[d/l]
[select]
Re: Hash value, containing a reference to some value of hash, not the intended behaviour.
by Anonymous Monk on Mar 14, 2011 at 11:00 UTC
    however, after first iteration, modification in %newhash also modifies %hashcopy. I am not able to understand the reason behind such behaviour in nested hashes. Please help!

    Its simple, it boils down to 

    my %newhash;
my %hashcopy;
my %other;
$hashcopy{something} = \%other;
$newhash{something} = \%other;

    [download]
    $newhash{something} and $hashcopy{something} are the same thing, a reference to %other;
[reply]
[d/l]
      Got it! made a deep copy to make things work
[reply]

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