This is not the role of the Piecewise function: it is more appropriate to simply use patterns in constructing your function. Here is an example:
f[n_Integer?EvenQ] := n
f[n_Integer?OddQ] := (n^2)
and take a look at a ListPlot of a table of values
ListPlot[Table[f[i], {i, 0, 10}]]
