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

I've got a situation where there are a hundred items, either true or false.

I've got the old list of which ones are true or false, and I've edited it using a web form, probably removing some from the "true" list and adding others to it.

Having submitted the form, what's the best way to reset the list correctly?

I did it like this:

I made a hash of 100 entries all set to zero, then did a foreach on the list of old items, and set the right values to 1.

Then I made a hash of 100 entries all set to zero and did a foreach loop on the list of new items, setting the right values to 1.

Then I went through the hash keys like this: 

foreach $entry(@a){
 if ($oldhash{$entry}==1 && $newhash{$entry}==0){
     #  push it into an array of deletions to be made
 } elsif($newhash{$entry}==1 && $oldhash{$entry}==0){
      # ditto array of additions to be made
 }
}

[download]

I've been staring at this code for a while and I can't figure out if that's a dumb way to do it or not, but it feels kind of dumb.

Is there a smarter way?
--

Edit: chipmunk 2002-02-24

Replies are listed 'Best First'.
Re: comparing two hashes
by chipmunk (Parson) on Feb 24, 2002 at 06:24 UTC
    What you're really trying to do is subtract one set from another set. Hashes are the right tool, but this can be done with slices instead of loops. 
    # find old values that are not in new values
my %deletions;
@deletions{@oldvalues} = ();
delete @deletions{@newvalues};
my @deletions = keys %deletions;

# find new values that are not in old values
my %additions
@additions{@newvalues} = ();
delete @additions{@oldvalues};
my @additions = keys %additions;

    [download]

    Update: fixed hash names.

[reply]
[d/l]

      chipmunk is right that hashes are the right tool to use here. However, there are some instances when hashes are not usable:

      • When you need to preserve the order of the data
      • If you have duplicate keys.

      I have had quite a lot of success with Algorithm::Diff. See also Re: Comparing Elements of Two Arrays.

[reply]

      That looks very much smarter and I'm sure it works, but can you (or anyone) please explain it a little more?

      What is 

      @deletions{@oldvalues} = ();

      [download]
      doing? I didn't know you could create hashes that way!

      Also, what's "%tmp"? --

[reply]
[d/l]
        Thats something called a hash slice.This is used to refer to (and/or operate on) multiple hash elements at the same time.
        hth
[reply]
Re: comparing two hashes
by Cody Pendant (Prior) on Feb 25, 2002 at 00:13 UTC
    Is it bad form to come back and answer my question? I looked at "comparing two arrays" in categorised Q and A, and dumbed it down a little to this: 
    # List pair which has one old item to be deleted,
# and one new item to be added:
@oldlist = (  1,3,5,7,9     );
@newlist = (    3,5,7,9,11  );

foreach $item(@oldlist){
  $old{$item}=1;
}
foreach $item(@newlist){
  $new{$item}=1;
}

# Find the stuff that's on both lists:
my @bothlists = grep {defined $new{$_}} @oldlist;
print "Stuff on both lists: @bothlists\n";

# Find the stuff that's on the old but not the new list:
my @deletions = grep {!defined $new{$_}} @oldlist;
print "Stuff on old but not on new (needs deleting): @deletions\n";

# Find the stuff that's on the new but not the old list:
my @additions = grep {!defined $old{$_}} @newlist;
print "Stuff on new but not on old (needs adding) @additions\n";

    [download]
[reply]
[d/l]

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