This question is very similar to my previous one. I have: $s \in [0;100]$ and $s \in Z$. The piece-wise definition is as follows: $20 \le s \le 100 \to 0\\ 10 \le s \lt 20 \to 1\\ s \le 9 \to 2\\ $
Following the previous question's answers my best try was this:
$\displaystyle f(s) = 2 - \left \lfloor \frac{s}{10} \right \rfloor + H \left (\left \lfloor \frac{s}{10} \right \rfloor - 3 \right ) \cdot \left ( \left \lfloor \frac{s}{10} \right \rfloor - 2 \right )$
where $H(x)$ is the Heaviside Step Function. That involves too many calculations. There must be something simpler, I'm just too blind to see it