@Carl Love sorry for bad english

 if z=0 then -1 else  v1:=z fi; I think this comment doesnt work. What i want is that when v=0 then v1 equal to -1. when v is not equal to 0 then v1=v. Moreover i want that in the interval (-18,18) . Even i tried this

while z=0 do v1:=-1 od; while z>0 do v1:=z od; while z<0 do v1:=z od;   

Still i have problems. I want to have this procedure

 f :=
  proc(x) if x < 0 then 0 elif x < 1 then x elif x < 2 then 2-x else 0 fi end

 > plot('f(x)',x=0..2);

i want loop  in interval u=(-18..18)

@Carl Love