From: Vincent Poirriez <Vincent.Poirriez@univ-valenciennes.fr>
To: Pierre Weis <Pierre.Weis@inria.fr>
Cc: Pascal Zimmer <zimmer@easynet.fr>, caml-list@inria.fr
Subject: Re: Instruction return
Date: Tue, 03 Jun 1997 09:37:40 +0100 [thread overview]
Message-ID: <3393D7D4.2F33@univ-valenciennes.fr> (raw)
In-Reply-To: <199706021415.QAA21013@pauillac.inria.fr>
>
> > Serait-il possible d'ajouter une instruction return au langage CAML afin
> > de pouvoir sortir directement d'une boucle,
> ...
Je souscrit a la réponse de Pierre Weis
>C'est pourquoi on pre'fe`re alors conside'rer que la
>boucle est trop complexe pour une boucle for ou while et qu'il est
>temps d'utiliser des outils adapte's a` e'crire des boucles
>arbitrairement complexes, c'est-a`-dire les fonctions re'cursives.
Cependant, j'ai rencontré la situation suivante dans un contexte
où l'efficacité (à l'exécution) est essentielle.
Trouver l'indice du premier élément d'un tableau qui vérifie
un prédicat p, s'il y en a un.
Ce code étant compile avec l'option -unsafe, je ne peux pas
compter sur les test de débordement des primitives Array.get ...
Le choix d'utiliser Array.unsafe_get serait inutile si j'inclus
ce test de débordement dans la condition
du while ou de la fonction récursive ainsi:
let find p a =
let l = Array.length a in
let i = ref 0 in
while !i<l & not( p (Array.unsafe_get a !i) do
incr i
done;
!i
J'ai donc prefere le coût d'un try ... with:
exception Exit_for of int
let find p a =
let l = Array.length a in
try
for i = 0 to l-1 do
if p (Array.unsafe_get a i) then raise Exit_for i
done;
l
with Exit_for i -> i
Si l'on veut sortir d'une boucle for avant sa fin normale, en
admettant que ce soit justifié (while ou fonction récursive exclue)
, la valeur de sortie du compteur suffit.
English short version:
I agree with Pierre Weis:
>In fact the best way of writing complex loops is via recursive
>function definitions:
But, due to runtime efficiency considerations I had to make
the following choice.
I want to find the index of the first item of an array which
verifies a predicat p, if one exists.
It is useless to prefer Array.unsafe_get if I add the test
in the while (or recursive) condition as below:
let find p a =
let l = Array.length a in
let i = ref 0 in
while !i<l & not( p (Array.unsafe_get a !i) do
incr i
done;
!i
I prefer to pay the cost of one try ... with ...
exception Exit_for of int
let find p a =
let l = Array.length a in
try
for i = 0 to l-1 do
if p (Array.unsafe_get a i) then raise Exit_for i
done;
l
with Exit_for i -> i
When I want to exit a loop for before its natural end, when I really
need it, it is sufficient to return the value of the count
when I exit
Vincent Poirriez
--
Tel: (33) {0}3 27 14 13 33 Fax: (33) {0}3 27 14 12 94
mailto:Vincent.Poirriez@univ-valenciennes.fr
http://www.univ-valenciennes.fr/limav/poirriez
ISTV Université de Valenciennes Le Mont Houy BP 311 F59304 Valenciennes
CEDEX
next prev parent reply other threads:[~1997-06-03 10:04 UTC|newest]
Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top
1997-05-31 17:30 Pascal Zimmer
1997-06-02 14:15 ` Pierre Weis
1997-06-03 8:37 ` Vincent Poirriez [this message]
1997-06-03 10:24 ` Pierre Weis
1997-06-03 14:17 ` Vincent Poirriez
1997-06-02 16:23 Dwight VandenBerghe
1997-06-02 21:41 ` Pierre Weis
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