Perl: Finding non-duplicate elements

As always, there is more than one way to do it. I would think that using grep would be more efficient than using a hash table with flags. In that case, what you want is to get a list of all elements in a2 that don't exist in a1. I wish I rememberhow to alias the $_ parameter so I could do it in two nested grep's, but that might be less readable. The complementary common function does the same, but with one small switch in the logic.

@a1 = qw(one two three four five six);
@a2 = qw(one three five seven nine);

# what is in common -> @c
foreach $a (@a2) { push @c,$a if grep {$a eq $_} @a1; }
print "\@c = (",join(',',@c),")\n";

# what is different -> @d
foreach $a (@a2) { push @d,$a unless grep {$a eq $_} @a1; }
print "\@d = (",join(',',@d),")\n";

I used this technique once on a list of several thousand email recipients. I loaded their user_ids into a @found array so that I would not send the email twice to the same person. but by the time the list loaded into the array (it was being loaded from a flatfile), the server killed the process. I had to find another method since arrays are memory intensive. For short lists, it is an easy, effective method.