f[x_] := 
      If[x > 0, "positiv", "nichtpositiv", "unentscheidbar"]

f[2]

f[-3]

f[x]

x = 1;  Do[x = k*x, {k, 100}];  x

Apply[Times, Table[k, {k, 100}]]

(∏_(k = 1)^100) k

100!