Mean for grouped data

I understand that when finding the mean for grouped data,
with class interval, e.g. , we use the midpoint of the interval, then multiply by frequency, and so on...

My question is this,

If x is discrete, for the range , then x takes values 0, 1, 2, 3, 4. Should we choose 2 for the midpoint? or 2.5?

Which one better?

Thanks!!!

In this case the range http://www.mathhelpforum.com/math-he...3e7f2d4c-1.gif is actually equal to 0 < x < 4 (those should be less than-or-equal signs but not sure how to do them on here), and the midpoint of that range is 2. So use 2.