We are going to start off by looking at somethng we have all done. The given example comes fron a great article on adding search functionality to Perl Applications

my %stopwords;
@stopwords{(qw(a i at be do to or is not no the 
               that they then these them who where 
               why can find on an of and it by))} = 1 x 27;
[download]

What's he doing here? He is building a hash so that later on he can check for stopwords like so:

my @salient_word = grep { not $stopwords{$_} } @word ;
[download]

Now, what is wrong with what he did? Well, nothing, but there is a module on CPAN, Set::Scalar that makes this tasks more DWIM instead of DWIS.

Here is how the same coding of stop words would be done using it:

use Set::Scalar;
$stopwords = Set::Scalar->new;
$stopwords->insert( qw(a i at be do to or is not no the 
               that they then these them who where 
               why can find on an of and it by));
@word = split /\s+/, "Oh say can you see by the dawn's early light";
[download]

And then we check it like this:

my @salient_word = grep { not $stopwords->has($_) } @word ;
[download]

or like this

$word = Set::Scalar->new(@word);
my $salient_word = $word - $stopwords;
# or this:
my $salient_word = $word->difference($stopwords);
[download]

Other Goodies

• $s->invert(@members); : insert if it isnt in set, delete if it is. Or, codewise:

  $members = Set::Scalar->new(@members);
  
  my $not_in = $s - $members;
  my $in     = $s->intersect($members);
  
  $s->delete($in->members);
  $s->insert($not_in->members);
  [download]

• union, intersection, difference : you know what these do.
• symmetric_difference : the symmetric difference of sets $a and $b is performed like this:

   $N = ($a - $b); 
   $N->insert($b - $a);
  [download]

  In other words, it is ($a - $b) UNION ($b - $a)
• unique : I don't understand this one.
• complement : I don't understand this one.

Carter's compass: I know I'm on the right track when by deleting something, I'm adding functionality.