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Hashtbl.remove legal within Hashtbl.iter for the same hash table?
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Date: -- (:)
From: Florent Monnier <fmonnier@l...>
Subject: Re: [Caml-list] Hashtbl.remove legal within Hashtbl.iter for the same hash table?
> Hello,
Hello David,

> Probably a newbie question but anyway: is it allowed to do a
> Hashtbl.remove while doing a Hashtbl.iter on the same hash table?
I don't know if it is legal, but at least it works:

# let h = Hashtbl.create 8 ;;

# for i = 0 to pred 8 do
    Hashtbl.add h i (char_of_int((int_of_char 'A') + i))
  done ;;

# Hashtbl.iter (fun i v ->
      if i = 3 then Hashtbl.remove h 5;
      Printf.printf " %d  %c\n" i v) h ;;
 0  A
 1  B
 2  C
 3  D
 4  E
 6  G
 7  H

But perhaps is it implementation dependant, in which case another 
implementation could react in a different way...

The ExtLib reacts in the same way than the standard one.

> More precisely, at one point while doing a "Hashtbl.iter f h" my
> function "f" is called with something like "f k v". Can I do a
> "Hashtbl.remove h k" within the body of "f"?

it seems to work too when we remove the current itered key
(with both implementations)

I don't know if this module was written with in mind to allow this behavior, 
but for what I understand from the manual : "in-place modification" should 
mean that this structure is purely imperative and that this behavior is 
"legal/allowed".
______

Perhaps a more sure method could be to get an enum from the hash table, and 
then iter on this enum (with the ExtLib), see below.
With this method you are sure that *all* the keys will be itered, including 
the hidden contents, which is different than the previous example.
Then you don't need to worry about a perhaps implementation dependant 
behavior.

ocaml -I +/site-lib/extlib extLib.cma
# open ExtLib ;;
# open ExtHashtbl ;;
# let h = Hashtbl.create 8 ;;
# for i = 0 to pred 8 do
    Hashtbl.add h i (char_of_int((int_of_char 'A') + i))
  done ;;

(* this one would not appear with the sdt Hashtbl.iter ! *)
# Hashtbl.add h 6 'Z' ;;

# let iter_all_hashtbl f h =
    let keys = Hashtbl.keys h
    and vals = Hashtbl.values h in
    Enum.iter2 f keys vals
  ;;

# let f i v =
    if i = 3 then Hashtbl.remove h 3;
    Printf.printf " %d  %c\n" i v
  ;;

# iter_all_hashtbl f h ;;
 0  A
 1  B
 2  C
 3  D
 4  E
 5  F
 6  Z
 6  G
 7  H

# iter_all_hashtbl f h ;;
 0  A
 1  B
 2  C
 4  E
 5  F
 6  Z
 6  G
 7  H

-- 
Florent