compare two hashes inefficiently.

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;
[download]

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;
}
[download]

Comment on compare two hashes inefficiently. Select or Download Code

Replies are listed 'Best First'.
Re: compare two hashes inefficiently. by MrNobo1024 (Hermit) on Sep 08, 2001 at 03:17 UTC
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)); }` [download] -- MrNobo1024 `s]]HrLfbfe\|EbBibmv]e\|s}w}ciZx^RYhL}e^print`	[reply] [d/l] [select]
Re (tilly) 1: compare two hashes inefficiently. by tilly (Archbishop) on Sep 08, 2001 at 19:05 UTC
This approach is unreliable. On my machine the script: `use Data::Dumper; print Dumper({ 1007 => "Hello", 1195 => "World", }); print Dumper({ 1195 => "World", 1007 => "Hello", });` [download] produces the output: `$VAR1 = { 1195 => 'World', 1007 => 'Hello' }; $VAR1 = { 1007 => 'Hello', 1195 => 'World' };` [download] 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.	[reply] [d/l] [select]
Re: compare two hashes inefficiently. by jmcnamara (Monsignor) on Sep 08, 2001 at 15:13 UTC
Some of issues involved in comparing hashes are discussed in How to test equality of hashes?. John. --	[reply]
Re: compare two hashes inefficiently. by OfficeLinebacker (Chaplain) on Jan 11, 2007 at 13:21 UTC
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. ++ I like computer programming because it's like Legos for the mind.	[reply]
Re^2: compare two hashes inefficiently. by deprecated (Priest) on Feb 10, 2007 at 10:50 UTC
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 -- Tilly is my hero.	[reply]
Re^3: compare two hashes inefficiently. by OfficeLinebacker (Chaplain) on Feb 10, 2007 at 15:31 UTC
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. I like computer programming because it's like Legos for the mind.	[reply]


The stupid question is the question not asked
	PerlMonks