Arrays and Coordinates

I have to make a program using classes and arrays to assign X Y coordinate numbers that will later be added up and put into a slope equation and then will produce an equation to predict what either X or Y would be having just one or the other. I think I know what to do with everything except how to have the coordinates the user enters be put into the arrays in the right positions so the proper X and Y coordinates correspond. I know how to do it if I made like 50 different assignment statements, but I'd really like to use something that just stops when the user is done (which would probably involve nested loops and other stuff that I'm not so very good with). Could someone please help me? It would be much appreciated.