You're exactly right about this. I knew this sort of thing had an official title, just didn't know what it was. I've seen this sort of algorithm called a "random number generator", but it is nearly the opposite. Having once issued a value in the space, there is 0% chance it will re-issue it. Well, until you start through the list a second time, in which case it will issue the very same list of numbers in exactly the same order. The "random" function in the BASIC interpreter I used 40+ years ago used something very much like this, and my first attempts at computer games were very disappointing until I figured that out ;)