If you're like me and read this comment and don't get the math explanation - plain english one is that if you add your points to the class average and your points cause that average to rise - that by default means your score was higher than the average of the class. That's why you don't need to add it in this particular example.

If n == 0, perhaps arr is the null pointer. You wouldn't want to try access it.

No, accessing that element is undefined behavior.