Behaviour of 2D array

[SimonB]

You can also create the data structure on-the fly, without need to initialize it beforehand

I see! How? (I come back to programming for the first time after 30 years - used to be a C programmer where the notion of "on the fly" is peculiar). In this particular case it is probably best to declare it for readability as this array is involved almost everywhere (it is a toroid surface for my own variation of Conway's game of Life).