10-10-2013 01:58 PM
You have an infinite loop. There is no reason to expect the DO UNTIL loop to ever terminate.
In fact, if you let it run for 10,000 iterations, you get a plot that looks like the one I've attached. For these iterations |z| is usually in [-0.5, 0.5].
A good programming practice is to put a maximum number of iterations on the DO/UNTIL loop, like this:
do rl = 1 to 10000 until(zz>h|zz<-h);
That way, you never have an infinite loop.