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Date: Fri, 12 May 2000 15:59:39 -0400 (EDT)
From: Hongwei  Xi <hwxi@ececs.uc.edu>
To: Pierre Weis <Pierre.Weis@inria.fr>
cc: caml-list@inria.fr
Subject: Re: reference initialization
In-Reply-To: <200005121707.TAA26883@pauillac.inria.fr>
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> Not exactly, you just have to find one element into a. For instance:
>  let alen = Array.length a in
>  if alen = 0 then b (or Array.copy b if you need a fresh array) else
>  let ? = a.(0) in
>  let c = ...
> 
>   for i = 1 to alen - 1 do
>    ...

But, why? Why should we first initialize c with such a value.
I don't think you have made a program safer by doing so.

> # Array.init;;
> - : int -> f:(int -> 'a) -> 'a array = <fun>
> 
> Using it, you don't need to bother with any dummy initialization value:
> 
> let combine_arrays a b =
>   let alen = Array.length a in
>   let blen = Array.length b in
>   let init i = if i < alen then a.(i) else b.(i-alen) in
>   Array.init (alen + blen) init

This is great in this case. But suppose that I want to compute
the combinatorial numbers 'n chooses k' for k= 0,..,n.

let chooses (n) =
  let result = Array.array (n+1) ? in begin
    result.(0) <- 1; result.(n) <- 1;
    for i = 1 to n/2 do
      result.(i) <- result.(i-1) * (n-i+1) / i;
      result.(n-i) <- result.(i);
    done;
    result
  end
(* code is not tested *)

Certainly, we can replace ? with 0. But what is really achieved?
I would say it is simply an illusion that a program is made safer
by initializing each array upon its allocation. Actually, a program
is likely made faster but unsafer by doing so (since no nullity
checking is needed but a wrong value may be supplied to continue
a computation that should be aborted)

Best,

--Hongwei

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On Fri, 12 May 2000, Pierre Weis wrote:

> 
> > (2) If you really want to make sure that 'c' is well-initialized,
> > you should probably check this after those two loops. The question
> > is how to incorporate the checking result into the type system.
> > (3) If you initialize 'c' with a (wrong) value, it seems to me
> > that nothing is achieved.
> > (4) Also, the problem cannot be solved using option type.
> > 
> > This is a precise senario that I had in mind, where the kind of
> > mandatory array initialization in ML-like langugages is simply
> > inappropriate, isn't it?
> 
> You should consider that there is a general initialisation function in
> the Array module, named Array.init, that allocates for you a fresh
> array then fill it with values obtained by calling an arbitrary
> supplied function:
> 
> 
> This code ensures the ``always initialized strategy'' of ML, and seems
> to me elegant and clear (but note that it uses higher-order
> functionality). Have I missed something ?
> 
> Best regards,
> 
> PS: An even shorter version of combine_arrays should be
>     let combine_arrays = Array.append
> 
> Pierre Weis
> 
> INRIA, Projet Cristal, Pierre.Weis@inria.fr, http://cristal.inria.fr/~weis/
> 
>