generic Hashtbl.to_array

generic Hashtbl.to_array

[oleg]

I wonder about the following solution. At least it traverses the
hashtable exactly once (and it does not ignore the result of the
fold).


let to_array9 t =
  let Some (a,_) =
    Hashtbl.fold (fun k v seed ->
      match seed with
	Some (a,i) -> a.(i) <- (k,v); Some (a,i+1)
      | None -> let a =  Array.make (Hashtbl.length t) (k,v) in
                Some (a,1))
      t None
  in a
;;

Re: [Caml-list] generic Hashtbl.to_array

[Damien Doligez]

On 2006-07-26, at 04:16, oleg@pobox.com wrote:

> let to_array9 t =
>   let Some (a,_) =
>     Hashtbl.fold (fun k v seed ->
>       match seed with
> 	Some (a,i) -> a.(i) <- (k,v); Some (a,i+1)
>       | None -> let a =  Array.make (Hashtbl.length t) (k,v) in
>                 Some (a,1))
>       t None
>   in a
> ;;

It fails whenever the hash table is empty, and the compiler
warns you about it.  Replace your "let Some (a,_) = ..."
with a pattern-matching and you have a nice solution.

-- Damien

generic Hashtbl.to_array

[Christoph Bauer]

Hi,

what is the best way to write Hashtbl.to_array?

Hashtbl.to_array : ('a, 'b) Hashtbl.t -> ('a * 'b) array

The simples idea has the problem, that you don't have
a initial value to make the result array:

let to_array t =
  let a =  Array.init make (Hashtbl.length t) ?init?  in
    ignore
      (Hashtbl.fold
         (fun k v i ->
            a.(i) <- (k, v); i + 1)
         t 0);
    a

The best solution I found is

let to_array t =
  let dummy =  Array.init 0 (fun _ -> raise Not_found) in
    fst
      (Hashtbl.fold
         (fun k v (a, i) ->
            if i = 0 then  
              let a = Array.make (Hashtbl.length t) (k, v) in
                (a, 0)
            else (a.(i) <- (k, v); (a, i + 1)))
         t (dummy, 0))

Is there a better one?

Thanks,
Christoph Bauer