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Getting an element of a hashtable: simple ... or is it?
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Date: -- (:)
From: Peng Zang <peng.zang@g...>
Subject: Re: [Caml-list] Getting an element of a hashtable: simple ... or is it?
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You can do something similar using options:

let getone h = 
  let res = ref None in
  try
    Hashtbl.iter (fun k v -> res := Some (k,v); raise Exit) h;
    !res
  with Exit -> !res
;;

Peng
  

On Friday 08 August 2008 11:46:41 am Ludovic Coquelle wrote:
> If the type of hashtbl key is known (if the hashtbl module has been
> created by the functor),
> the same code as initial can avoid a reference and use exception
> propagation mechanism:
>
> type key = MyHashtbl.key
> exception One of key
> let get_one h = try (MyHashtbl.iter (fun k _ -> raise (One k)) h;
> raise Not_found) with One x -> x
>
> Can't we have polymorphic exception? like
> 'a exception One of 'a
>
> On Wed, Aug 6, 2008 at 5:47 AM, Brighten Godfrey <pbg@cs.berkeley.edu> 
wrote:
> > On Aug 5, 2008, at 5:25 AM, blue storm wrote:
> >> With Extlib you can use :
> >> let get_one hashtbl = Enum.peek (Hashtbl.enum hashtbl)
> >> val get_one : ('a, 'b) Hashtbl.t -> ('a * 'b) option
> >
> > Ah, thanks.
> >
> > On Aug 5, 2008, at 6:21 AM, Peng Zang wrote:
> >> I think this is pretty standard.  At least, I see it in ExtLib and I do
> >> it on
> >> a regular basis.  In fact I have a function to do this for me so I don't
> >> have
> >> to do it over and over again.  Eg.
> >>
> >>  let get_one ht = mkGetOne Hashtbl.iter ht
> >
> > OK -- so you're saying ExtLib also implements it by breaking out of the
> > loop with an exception.  Interesting.
> >
> > On Aug 5, 2008, at 2:02 PM, Chris Kauffman wrote:
> >> I'm curious what sort of scenario calls for retrieving any single
> >> element of a hash table (which is potentially empty?). It seems most
> >> of the cases I deal with involve simply storing or iterating over all
> >> the elements.
> >
> > Yes, nearly all cases are like that for me too.  But in this case, I want
> > to decompose a graph into its connected components, roughly according to
> > the following pseudocode:
> >
> >    unprocessed_nodes : (node_t, unit) Hashtbl.t = all nodes
> >    while unprocessed_nodes not empty do
> >        let one_node = choose any one node from unprocessed_nodes
> >        let cc = find_connected_component_containing one_node
> >        Do some sort of processing on cc. Then:
> >        for each node v in cc
> >            remove v from unprocessed_nodes
> >        done
> >
> > Thanks,
> > ~Brighten Godfrey
> >
> > _______________________________________________
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