This is a topic that is quite easy and doesn't need much explanation but still many people manage to mess it up. Not going into a hyperbole, I'll get straight to the point...Basically, given any random number, if you take its modulus with a, you will obviously get a number between 0 and a -1. This is all fine and dandy, so now someone asks you to generate a random number between a and b (a < b). This one is also really simple but few people fumble out still. Just think of it this way.