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

[William D. N. <wne...@cs...>]

On Thu, 29 May 2003, Brian Hurt wrote:

> I don't have a problem with a pre-expand, but adding elements out of order
> doesn't work (at least the way I think they work).  If the dynarray has n
> elements, the elements are numbers 0..n-1 inclusive.
>
> The solution is to use a 'a option Dynarray.t.  You're null element then
> becomes None.  I dislike exposing the null elements as "valid entries"- to
> my mind, they're just filler.  This is especially important if you
> consider an int Dynarray.t - is that 0 a real, important 0, or a null
> element?  So I would would fake up a enum that returns n Nones and append
> it to the dynarray to increase the size.  Dynarray.set can then change the
> Nones to Some x.

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).

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";;

Where the length field is artificially pushed out to the end of the array,
so stuff can be added out of order.  But this does expose all those
not-really-null nulls in spots 2 - 8.  Now that I think about it this
might be best left out.  However, if it is added, it should be called
unsafe_expand or something similar so people realize that they're
willfully doing something "incorrect".

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

William D. Neumann

---

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