compare two hashes inefficiently.

[deprecated]

Description:

Yeah, I know this is the wrong way to do it, and it could be done by iterating over keys and values, but I didnt want to do it that way.

I also could have done this in storable, but storable's freeze and thaw are broken on ultrasparc/solaris. So I used Data::Dumper. You get the idea, and could switch over to Storable to make it faster if you were so inclined.

Suggested usage:

my @newarray = grep { hcmp( \%goodhash, $_ ) } @somebadhashes;

sub hcmp {
  use Digest::MD5 qw{ md5_hex };
  use Data::Dumper;
  my ($a, $b) = @_;
  my $astr = md5_hex( Dumper( $a ) );
  my $bstr = md5_hex( Dumper( $b ) );
  return ($astr eq $bstr) ? 1 : 0;
}

[MrNobo1024]

You don't need to use MD5 just to compare two strings. :-) I would have written it like this:

sub hcmp {
  use Data::Dumper;
  return(Dumper(shift) eq Dumper(shift));
}

-- MrNobo1024

s]]HrLfbfe|EbBibmv]e|s}w}ciZx^RYhL}e^print

[tilly]

This approach is unreliable. On my machine the script:

use Data::Dumper;
print Dumper({
  1007 => "Hello",
  1195 => "World",
});

print Dumper({
  1195 => "World",
  1007 => "Hello",
});

produces the output:

$VAR1 = {
          1195 => 'World',
          1007 => 'Hello'
        };
$VAR1 = {
          1007 => 'Hello',
          1195 => 'World'
        };

The issue is that Data::Dumper just returns keys in the order that keys gave it, which returns keys that are in the same bucket in the insertion order. With a 2 key hash your odds of hitting this are 1/8. But when the hashing algorithm works right, a fixed portion of the keys wind up in a bucket with a neighbour. Therefore for larger hashes you are virtually guaranteed that the set of keys alone does not determine what order they come back in.

If this explanation confuses, then Re (tilly) 4: Flip Flop III - Musical Buckets may help.

[jmcnamara]

Some of issues involved in comparing hashes are discussed in How to test equality of hashes?.

John.
--

[OfficeLinebacker]

Hmm, I think I will find this useful in a specific program I wrote, but it turns out my algorithm isn't quite right. Thanks. ++

[deprecated]

Hi, please don't use this code. I wrote it ages and ages ago. The above replies were correct in that there are better ways to do this. Upon thinking back to when I wrote some of the code I've posted to perlmonks, I realize that I had a lot more perl knowledge than I had common sense. In this case, I had a solution for the problem ahead of me, but I didn't understand that I was solving the wrong problem. This may be one of the most important things I've learned over the years; I'm sure somebody here (or even myself) could help you come up with a better, and certainly more elegant, solution to the problem you're trying to solve with the above hack(s).

–dep

[OfficeLinebacker]

That's OK. I don't even remember what I was talking about in my post. I think I was also writing a file comparison routine (basically handrolling my own stripped-down rsync). Since then I discovered FreezeThaw, which has worked very nicely for me for hash comparisons.