lookup.while

(* lookup.while

 This program in (extended) WHILE takes as input a list [X, Store] where

  X     is a number (in list form) that can be 0.
  Store is a list of bindings containing the variables' values (store)

  output is a value (in D i.e. binary tree)
  namely the value of the (X-1)-th variable in Store
	
  Exceptional case: 
  if X is greater than the length of  Store - 1,
  i.e. if one looks up a variable that has not been
  initialised, then the result is nil
  which means that uninitialised variables
  are implicitly initialised with nil

   (c) 2008-23 Bernhard Reus 
	
*)

lookup read XS {		(* argument XS is list [X, Store] *)
   X := hd XS;			(* X is number encoding variable we are looking for *)
   St := hd tl XS; 		(* Store contains list of bindings *)
				(* do the lookup; this destroys the Store *)
  while St {			(* run through store list *)
     bind := hd St; 		(* bind is current binding *)
     N := hd bind; 		(* N is var number of bind *)
     if N=X {			(* found right variable *)
        Res := hd tl bind; 	(* Res is value of X *)
        St := nil		(* reset St to exit loop *)
        }
     else { 			(* variable X not found yet *)
          St := tl St 		(* drop first binding off store *)
        } 			(* end of if-the-else *)
   } 				(* end of while loop *)
}
write Res