First, look at what you're doing here:

my %lookup;
@lookup{@in} = ();
# For every $elem in @db ... 
foreach my $elem (@db) {
  # see if it exists in the lookup hash ...
  if (exists $lookup{$elem}) { 
    # and if it does then search @db (*again*) looking for
    # items that do not appear in the lookup hash. You're
    # also modifying the array you're iterating over (@db),
    # which can yield unpredictable results, so don't do it ;)
    @db = grep {not exists $lookup{$_}} @db;
  }
}
[download]

In the second case, you could utilize a grep very similar to the one above, and simplify the if/push syntax.

How about this as a solution:

my (%db_lookup, %in_lookup);
@db_lookup{@db} = ();
@in_lookup{@in} = ();

if ($condition1) { # Dunno what your test conditions are, so...
  @db = grep { not exists $in_lookup{$_} } @db;
} elsif ($condition2) {
  push @db, grep { not exists $db_lookup{$_} } @in;
}
[download]

Hope that helps.

After much fiddling around I have arrived at the following:

#!/usr/bin/perl -w
#use strict;
use CGI ':standard';

my @db = (1, 3, 5, 7, 9, 11);
my @in = (1, 2, 5, 8, 9, 10, 13);
my $var = 7; # I have this variable available to indicate the length o
+f the array
my (%db_lookup, %in_lookup);
@db_lookup{@db} = ();
@in_lookup{@in} = ();



$count = 0;
while ($count < $var) {
     if (exists $in_lookup{$count}, @db) {
      @db = grep { not exists $in_lookup{$_} } @db;
      $count += 1;
      } elsif (not exists $in_lookup{$count}, @db) {
      push @db, grep { not exists $db_lookup{$_} } @in;
      $count += 1;
}
}
      
      print "view: @db";
[download]

That does what I need when strings match. However when strings don't match the db is unchanged from the original, wheras I need the contents of @in pushed into @db:

my @db = (1, 3, 5, 7, 9, 11);
my @in = (21, 22, 25, 28, 29, 210, 213);
[download]

Where am I going wrong?

A couple of problems here. First, $count will need to be specified in a more maintainable manner, such as using scalar on your arrays and finding the greater value. Second, you're looking at the value of $count in your evaluations, not the value of the index in your arrays that $count points to. Finally, by adding @db to your conditionals i.e.,if (exists $in_lookup{$count}, @db) you are ensuring that the test will return 'true' always unless the array is empty. So, the way you've got it now it executes the first conditional branch 7 times and then exits.

I'm still struggling to see what exactly you're attempting -- if element A in array @in exists in array @db, remove A from @db. Else, if element A does not exist in array @db, append it to @db. Is this right?

This is actually a reply to fever.

The reference to $count is actually pure guesswork on my part. The problem is that I cannot find a way to say "if the input exists - take it out, if it doesn't - put it in". Obviously inclusion of the $count mechanism is more problematical than I'd anticipated but I can't find another approach which comes close to letting me say, "while values of @in match values of @db or the converse".

That, I think is the crux of the problem!

One additional note, You can also add 1 to %data for the input and 2 to %data for the db.
Now your hash would look as followes:

%data = { 1=>1, 3=>3, 5=>3, 6=>2}
As you can see, now you can distinguish between keys from each array as
well as the union between the two.

needles

Thanks for your input. I have actually gone with fever's grep solution but I will give this a try to see if I can get my head round it!!