type 'a alist = (string * 'a) list;;

let rec zip list1 list2 =
  match (list1,list2) with
    | ([],_) -> []
    | (_,[]) -> []
    | (x::xs,y::ys) -> (x,y) :: zip xs ys;;

let make_alist list1 (list2:('a list)) : ('a alist) = zip list1 list2;;

let l1 = ["Jim";"Steve";"Anne";"James";"Rachel"];;

let l2 = [21;32;22;19;20];;

let l3 = ["English";"Math";"Computer Science";"Biology";"Physics"];;

let a1 = make_alist l1 l2;;

let a2 = make_alist l1 l3;;

let rec lookup_alist key list =
  match list with
    | [] -> None
    | (k,v)::vs -> if key = k then Some v else lookup_alist key vs;;

lookup_alist "Mark" a1;;

lookup_alist "James" a1;;

lookup_alist "Rachel" a2;;

let print_major name alist =
  let result = lookup_alist name alist
  in match result with
    | None -> print_string "No record found!"
    | Some s -> print_string (name ^ "'s major is " ^ s);;

print_major "Mark" a2;;

print_major "Anne" a2;;