Shell programmers are probably the most aware of the concept of a schwartzian transform. They get the results from something like ls -l, sort it somehow, and then use awk to extract a specific field.
Thanks to Perl's built-in hash data type, many idioms (such as intersection, union, and difference computation) are a snap:
@union = union(\@a, \@b);
@inter = intersection(\@a, \@b, \@c);
@diff = difference(\@a, \@b);
@in_a = unique_to(\@a, (\@b, \@c));
sub union {
my %seen;
@seen{@$_} = () for @_;
return keys %seen;
}
sub intersection {
my %seen;
for (@_) { $seen{$_}++ for @$_ }
return grep $seen{$_} == @_, keys %seen;
}
sub difference {
my %seen;
for (@_) { $seen{$_}++ for @$_ }
return grep $seen{$_} == 1, keys %seen;
}
sub unique_to {
my %seen;
@seen{@{ shift() }} = ();
delete @seen{@$_} for @_;
return keys %seen;
}
Finding unique elements in a list is as easy as converting the list to the keys of a hash, and then extracting the keys again.
japhy --
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