The question is: how likely is it that I get the trash input in and do I care about the result if the input is trash? Modulo two works well for a lot of cases w

The question is: how likely is it that I get the trash input in and do I care about the result if the input is trash? Modulo two works well for a lot of cases where this is needed. (Which probably is size of a container (array) or alternating something in a loop.