things change

I’ve spent well over a dozen hours trying to find a bug in my VBNET boxes program.

Debugging computer code can be time consuming and frustrating, but there’s also much pleasure to be gained by trying to unravel the puzzle. It’s a bit like struggling with a crossword clue where nothing fits the space and any existing letters. Something isn’t quite right but what is it? In the case of a crossword clue it’s often because one of the other completed words is incorrect and by putting in the right word one can solve the clue that was causing the problem. And so it was with my computer bug.

The code I had written was to play through all possible combination of moves in the boxes game in order to determine the best move to make. But sometimes my code came up with the worst move rather than the best. I studied the code for hours, went through the steps in my head, but I couldn’t see where the flaw in my logic was. I eventually resorted to stepping through the code as it was being executed, and watching as variables changed value. And then a little after midnight I found something strange happening, but it wasn’t in the code I was concentrating on.

My code called various functions – separate pieces of code I had written to perform specific tasks. And it was in one of these functions, which I thought was tested and working, that I found a poor piece of coding that resulted in the function sometimes returning a random result. An annoying mistake but a satisfying conclusion.

The original, buggy code is below – can you see the flaw?