If you're just building a function, there's nothing stopping from simplify defining it as a function f : {1, 2, 3, 5, 7} -> {-1, 0, 1, 3} such that
[tex]f(x)=\begin{cases}3&\text{if }x=1\\1&\text{if }x=2\\-1&\text{if }x=3\text{ or }x=7\\0&\text{if }x=5\end{cases}[/tex]
In case it's not familiar to you, the notation f : A -> B simply means f is a function that maps elements in the set A to a single element in the set B ; I emphasize "single", because otherwise f would not be a function.
