Question

I am not very familiar with Map module in Ocaml.

I have a simple map m: string Map.Make(Int).t from int to string, and I know that every binding (:string) is unique. I would like to write a funciton find_from_string_to_int: string -> string Map.Make(Int).t -> int, could anyone help? Thank you very much!

Answer 1

If this is an infrequent operation, the easiest way is to fold over all the (key,value) bindings of the map SMap.fold:

# module SMap = Map.Make(String);;
# let reverse v t =
  SMap.fold (fun k v' acc -> if v = v' then Some k else acc) t None;;
val reverse : 'a -> 'a SMap.t -> SMap.key option = <fun>

If this is a common operation, what you’re looking for is a bidirectional map (a map that can be queried in both “directions”). The easiest way is to carry around a pair of maps: int IMap.t * string SMap.t (accesses in both direction are logarithmic, while the reverse above is linear in the size of the map). You could also implement a dedicated data structure, but that’s probably not worth the added complexity.

PS: reverse could be optimized to quit the iteration early if the value is found, but if it’s an uncommon operation it’s not really important anyway.