Designing function f(f(n)) == -n

Question

A question I got on my last interview:

Design a function f, such that:

f(f(n)) == -n

Where n<

Answer 1

This Perl solution works for integers, floats, and strings.

sub f {
    my $n = shift;
    return ref($n) ? -$$n : \$n;
}

Try some test data.

print $_, ' ', f(f($_)), "\n" for -2, 0, 1, 1.1, -3.3, 'foo' '-bar';

Output:

-2 2
0 0
1 -1
1.1 -1.1
-3.3 3.3
foo -foo
-bar +bar