First off, I am not stupid but I have a hard time framing a question. But here is the situation

Take any number between 1 to 18
Add the numbers 9 or 18 based on user choice to get the result one less than the result
That is..1+9 = 9. I start on 1 and end on 9 or 18.

If the result is more than 18, it needs to start over from 1.
1,2,3...17,18,1,2,3.....

Is this a recursive function, or something else?
For the ones of you who know Golf, this makes hella more sense to you.