Re: [Ocaml-lib-devel] Proposed addition to DynArray

[Brian H. <bri...@ql...>]

On Thu, 29 May 2003, William D. Neumann wrote:

> On Thu, 29 May 2003, Brian Hurt wrote:
> 
> Yeah, exposing phony nulls was one of the philisophical issues I was
> wrestling with as I added my expansion hack.  At the time I decided that
> the ability to do this outweighed the potential to do something silly (and
> I promised myself to always look out for null values (the null I'm using
> in this case would never be a valid value, so I can ignore them when I run
> across them).

One of the reasons I didn't want to use options as the array elements- for 
the cases where they weren't needed, and there really was a safe null.

> 
> And you're right, expanding the array the way I described it won't work
> like I wanted, you have to do something along the lines of:
> 
> let null = "";;
> let j = DynArray.make 2 null;;
> DynArray.add j "blah";;
> DynArray.add j "flah";;
> 
> (* Now I want to add something to slot 9 *)
> 
> ignore (Dynarray.append j (DynArray.init 8 8 null (fun x -> null)));;
> DynArray.set j 9 "flee";;

Given:

let enum_dup c val =
    (* returns an enumeration which is just val repeated c times *)
    let cref = ref c in
    let next () = if (!cref) > 0 then decr cref; val 
                  else raise No_more_elements
    and count () = !cref
    in
    Enum.make ~next:next ~count:count

You don't actually need to create the intermediate array.  You can just 
do:

DynArray.set_enum j (DynArray.length j) (enum_dup 8 "");
DynArray.set j 9 "flee";;

Mental note to self: need to add DynArray.append_enum.

> Sigh...why does doing things the right way always interfere with doing
> what I think I want...

Because what you think you wanted to do is not what you really wanted to 
do.  Perhaps instead of a Dynamic Array, you wanted a hash table mapping 
int->'a?

Brian