def calc_iterative(n)
	a = [0,1,0]
	for i in 2..n
		a[k] = a[k-1] + a[k-2]
	end
	return a[n%3]
end

I like this one, since it uses array wrapping.  So many ways to write
it that are all *slightly*  different... But this was to show someone,
so :)