From: Daniel Nugent <nugend@...>
Date: 2005-07-03T13:44:32+09:00
Subject: FlipFlop Voodoo

So I was reading through the retrospective on RedHanded today and saw
the piece of flipflops
(http://redhanded.hobix.com/inspect/hopscotchingArraysWithFlipFlops.html).

Reading it I realized that I didn't quite understand how the
flipfloperator worked exactly.  So I grabbed the Pickaxe2 and read
that it returns true when the state machine is in the set state at the
end of the call.

Looking at both why's examples, the ones in Pickaxe2, and through a
little experimentation of my own and once again reviewing the state
transition diagram in Pickaxe2, I figured there must be a typo.

See example 1 on pg 342:

a = (11..20).collect {|i| (i%4 == 0)..(i%3 == 0) ? i ? nil}
a -> [nil, 12, nil, nil, nil, 16, 17, 18, nil, 20]

If there isn't a typo and I'm reading it right, then 18 should not be
in the output array since the second part of the flipflop should have
triggered the state machine to go back to the unset state.  Now,
Pickaxe2 mentions that when the second condition evaluates to true on
the same call as the first condition, it will still output true
anyhow.

However, it does not explain why this happens or why it also outputs
18 even though 18%3==0 should evaluate to true and push the state
machine into the unset state.

If I'm missing something vital about this, I'd really appreciate
someone taking the time to explain it more thoroughly   Because I
*think* I understand what's going on here, but my reading of Pickaxe2
would seem to indicate that I don't.
-- 
-Dan Nugent