The % Operator
This is probably the most useful but underused operator. It very often happens that we want something to count up... but only to a certain point, and then start again at the bottom. Like cycling through screens, or colors. I see lots of code with things like “var_x=var_x+1 ; if var_x==10 then var_x=0 end”... it's a waste. We have an operator that does this for us already.
The % sign is the modulus operator. Basically, A%B means “the remainder when A is divided by B”, or equivalently, “the number that you get when you subtract B from A repeatedly until you get a non-negative number less than B.” This is exactly what you need for this kind of thing:
This counts up from 0 to 9, and back to 0. In lua (unlike most other languages), we are frequently counting from 1 to n, not 0 to n-1, so the formula would be a little different for the 1 to n case:
Don't use those long-winded functions; it's just clunky.