Stefan Mahlitz <stefan / mahlitz-net.de> writes:

> I really do
> not need the value of any stored key.

Right.

> Why use a Hash?

Efficiency. Array#include? runs in linear time (i.e. the running time
is proportional to the size of the array because all elements have to
be tested sequentially), while Hash#[] runs in constant time (i.e. the
running time is independent of the size of the hash). So the
array-based solution will scale very badly for large numbers.