How define iteration interval increment

[ichabod801]

% is the modulus operator. That is, the remainder after division. So 1 % 3 = 1, 2 % 3 = 2, 3 % 3 = 0, 4 % 3 = 1, and so on. So every third time it's equal to one (you could choose 0 or 2 to get a different starting row). The other times you use continue, which skips to the next iteration of the loop.