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Problem Copying Hash of Arrays (REUSED_ADDRESS)


Consider this
Expand|Select|Wrap|Line Numbers
  2. use strict;
  3. use Data::Dumper;
  5. my %hash1=(a=>[1,1],
  6.         b=>[2,2],
  7.         c=>[3,3],);
  9. my %hash2=%hash1;
  11. $hash1{a}[0]=10;
  13. print Dumper(%hash2);
The problem is that when copying %hash1 into %hash2, the values of %hash2 are just a references to
the SAME arrays in %hash1, so changing an array element in %hash1 changes the respective element
in the new %hash2. But I want to store the old value of %hash1 into %hash2.
In a first attempt to solve that problem, I come with that :
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  1. use Data::Dumper;
  2. use strict;
  3. my %hash1=(a=>[1,1],
  4.         b=>[2,2],
  5.         c=>[3,3],);
  7. my %hash2=();
  8. while ( my ($key,$val)= each %hash1) {
  9.   my @arr=@{$val};
  10.   $hash2{$key}=\@arr;
  11. }
  13. $hash1{a}[0]=10;
  14. print Dumper(%hash2);
but it's quite ugly.
I've also found that can surround the problem by assigning not a new value to the array (see: $hash1{a}[0]=10; )
but assigning a whole new array like this : $hash1{a}=[10,10];

But I still can't understand why the arrays are "copied" with the same address into %hash2 and
how I can store a %HoA into another %Hoa ...

please help
Feb 5 '07 #1
3 6353
1,089 Expert 1GB
This is basically functioning as it shoud. If you want complete clones of each of the data structures inside the %hash, then you'll have to copy each of them. The easiest way to accomplish this would be to make a subroutine that recursively operated on arrays and unreferenced each element. The only function that it's important to be familiar with to accomplish this is ref.


Here is the code and function that I quickly through together from your example.

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  1. use strict;
  2. use Data::Dumper;
  4. my %hash = (
  5.     a    => [1,1],
  6.     b    => [2,2],
  7.     c    => [3,3],
  8. );
  10. my %copy = %hash;
  11. my %clone = clone(%hash);
  13. $hash{a}[0]=10;
  15. print 'copy - ' . Dumper(\%copy);
  16. print 'clone - ' . Dumper(\%clone);
  19. sub clone {
  20.     map { ! ref() ? $_ : ref eq 'HASH' ? {clone(%$_)} : ref eq 'ARRAY' ? [clone(@$_)] : die "$_ not supported" } @_;
  21. }
Also, just as a stylistic note, I find it much better if I only pass scalars to Dumper. In this instance that means passing a reference to the hashes you want to explore instead of the hashes themselves. You'll find the output to be much cleaner in my opinion.

Good luck.
Feb 5 '07 #2
4,059 Expert 2GB
another possibility:

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  1. use Data::Dumper;
  2. use Storable qw/dclone/;
  3. my %hash1=(a=>[1,1],
  4.         b=>[2,2],
  5.         c=>[3,3],);
  7. my $hash2 = dclone(\%hash1);
  9. $hash1{a}[0]=10;
  11. print Dumper(\$hash2,\%hash1);
$hash2 will be a copy only of %hash1, but the structure of $hash2 is a little different in that $hash2 is a reference to a hash of arrays instead of a hash of arrays like %hash1 is.
Feb 5 '07 #3
Really appreciate your help... Thanks
Feb 5 '07 #4

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