I had an annoying bug a couple of weeks ago. I had an array of ongoing events, and an array of time windows (roughly, quarters or years). I wanted to generate t

I had an annoying bug a couple of weeks ago. I had an array of ongoing events, and an array of time windows (roughly, quarters or years). I wanted to generate the cross-product of events and windows, evaluate some metric for each combination, and total the metric per time window. So, i wrote a pair of nested loops over the two arrays, calculated the metric in the inner loop, and accumulated it into a third array with a slot per time window. Except i had called the loop variables i and j, forgot which way round i had nested the loops, and used the index into the event array to select a slot in the accumulation array. I was baffled as to why my results were completely wrong, but totalled to the right amount! Once i realised what i had done, i renamed the loop variables to eventIdx and windowIdx. This is far better.