Excel Turing Machine

One of the biggest things to wrap your head around is that cells in Excel are write-once, read many. This forces you to unroll all of your loops, and you generally end up with a giant state machine, with each state occupying a row and depending on the state of the previous row.

I create a circular dependency and set Microsoft excel to evaluate circular dependencies through one iteration. Every time you tell it to calculate, it steps through another clock cycle. This way I got a cool animated tapehead.

An obvious question is whether this suffices to show Turing completeness. I think the answer is no: because Excel has a finite number of cells, there are programs that this Turing machine won't have enough cells for. To get around that, you'd have to do something clever with iterative calcs (i.e. circular references) that effectively reuse the same cells behind the scenes. VBA is out of the question, of course. :)

Nice work by the way. It's quite amazing the odd things you can do in Excel.