Consider \(f(x) = x/10\) over R = {1,2,3,4}. Then, f(x) is obviously positive for each of the values in R and certainly
\begin{equation*}
\sum_{x \in R} f(x) = f(1) + f(2) + f(3) + f(4) = 1/10 + 2/10 + 3/10 + 4/10 = 1.
\end{equation*}
Therefore, f(x) is a probability mass function over the space \(R\text{.}\)