Re^2: Copy of multidimensional hash

[tommaso.fornaciari]

Dear All Monks,
Thanks a lot for your quick and clear answers!
In fact, storing elsewhere the same data, it works:

#!/usr/bin/env perl
use Modern::Perl 2011;
use autodie;
# I create a multidimensional hash
my %one = ('a' => [1, 2],
           'b' => [3, 4],
           'c' => [5, 6]);
# I create a copy:
# wrong way:
#my %two = %one;
# right way:
my %two;
for(keys %one)
   {$two{$_}[0] = $one{$_}[0];
    $two{$_}[1] = $one{$_}[1]}
# I print them:
say "Before:\n\%one:";
for(sort keys %one){say "$_\t$one{$_}[0]\t$one{$_}[1]"}
say '%two:';
for(sort keys %two){say "$_\t$two{$_}[0]\t$two{$_}[1]"}
# Then I modify the copy:
for(sort keys %two){$two{$_}[1] = 'two'}
# And I print again:
say "After:\n\%one:";
for(sort keys %one){say "$_\t$one{$_}[0]\t$one{$_}[1]\tOk!!!"}
say '%two:';
for(sort keys %two){say "$_\t$two{$_}[0]\t$two{$_}[1]"}

# I receive this output:
#Before:
#%one:
#a    1    2
#b    3    4
#c    5    6
#%two:
#a    1    2
#b    3    4
#c    5    6
#After:
#%one:
#a    1    2    Ok!!!
#b    3    4    Ok!!!
#c    5    6    Ok!!!
#%two:
#a    1    two
#b    3    two
#c    5    two

Is this the best way to create a copy of a multidimensional hash?
Thanks again for your helpfulness!
Tommaso

[muba]

Is this the best way to create a copy of a multidimensional hash?

I don't know if it's the best. At any rate, I would reformat the for loop as follows:

for (keys %one) {
    $two{$_}[0] = $one{$_}[0];
    $two{$_}[1] = $one{$_}[1];
}

I find this significantly more readable than the way you formatted it. Furthermore, I would generalize copying the arrray, and tell Perl to take out all the elements instead of just those elements with index 0 and 1.

for my $k (keys %one) {
    $two{$k}[$_] = $one{$k}[$_] for 0..scalar( @{$one{$k}} );
}

And then I would generalize it even further: take the whole array reference, dereference it in a single pass, and create and store a new reference to it. Sounds complex? Nah, not so much:

for my $k (keys %one) {
    $two{$k} = [ @{$one{$k}} ];
}

This was already suggested by johngg.</p

Step by step of [ @{$one{$k}} ]:

1. $one{$k}: this takes the element identified by key $k from hash %one. That element, in this case, is an array reference.
2. @{ ... } dereferences the array reference inside the curlies. So after @{ $one{$k} } we're working with a normal, regular, every day array (albeit an anonymous one).
3. [ ... ] stores whatever is between the square braces as an anonymous array and returns a reference to it.

But then I would want to generalize it even further and use Storable's dclone function, as suggested by davido.

use Storable qw(dclone);
...
my %two = %{ dclone(\%one) }    # dclone() takes a ref and returns a r
+ef
                                # So we'll have to put in \%one (not %
+one)
                                # and then we have to dereference ( %{
+...} )
                                # what comes out.

[tommaso.fornaciari]

Thanks muba, thanks johnGG,
great dissertation about the the topic :))

[Neighbour]

You could use Clone and copy the hash using my %two = %{ Clone::clone(\%one) };
Or use hashrefs all the way:

my $hr_one = {
  'a' => [1, 2],
  'b' => [3, 4],
  'c' => [5, 6]};
my $hr_two = Clone::clone($hr_one);