I'm guessing it's the maximum number of points to consider in the data set. you've got 3 doubles for each index position from 0 up to MAX_POINT_NO - 1. There's probably a header file somewhere that defines that value. If you're using an IDE with your compiler, you may be able to search through each of the project files to see if it's declared somewhere. If not, just make one up and see what happens. const int MAX_POINT_NO = some_value;