Re: reference initialization

Pierre Weis
 Hongwei Xi
 John Max Skaller
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Date:  20000514 (21:12) 
From:  Hongwei Xi <hwxi@e...> 
Subject:  Re: reference initialization 
> 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.(ialen) 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.(i1) * (ni+1) / i; result.(ni) < 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 \~~~~/ \\ // \\ // @ Mail: hwxi@ececs.uc.edu Co^o, ))__ \\__//_ // \\ Url: http://www.ececs.uc.edu/~hwxi ( ^ ) ))__ \/\\ \\ Tel: +1 513 871 4947 (home) / \V\ ))  // \\ \\ Tel: +1 513 556 4762 (office)  //  o // \\ \\//Fax: +1 513 556 7326 (department) Rhodes Hall 811D Department of ECE & CS University of Cincinnati P. O. Box 210030 Cincinnati, OH 452210030 On Fri, 12 May 2000, Pierre Weis wrote: > > > (2) If you really want to make sure that 'c' is wellinitialized, > > 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 MLlike 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 higherorder > 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/ > >