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Re: Bug converting numbers?
- Pierre Weis
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Date: | 1998-09-14 (07:55) |
From: | Pierre Weis <Pierre.Weis@i...> |
Subject: | Re: Bug converting numbers? |
> Hello - Bonjour! > > I have encountered unexpected behaviour converting integers to floats: > > # let x = 7.;; > val x : float = 7 > # let y = float 7;; > val y : float = 7 > # x == y;; > - : bool = false > > The internal representation of "7." is obviously different to "float 7". > Is there some reason to it? This makes it hard to write things like: > > if a_float == float (truncate a_float) ... > > if one wants to check whether a float can actually be represented as > an integer... For a more exhaustive discussion on this problem, see the Caml FAQ: <http://caml.inria.fr/FAQ/FAQ_EXPERT-eng.html#egalite>. The problem here is that you missed the right equality predicate: you must use = instead of ==. Remember that == means unicity of representation, not semantical equality of values. For instance: # "ok" == "ok";; - : bool = false "ok" is not the same object as (another) "ok". In contrast, "ok" is equal to (any other) "ok": # "ok" = "ok";; - : bool = true So there is no problem of floating point numbers conversion, since the same (unexpected ?) behaviour still apply if we remove conversions: # 1.0 == 1.0;; - : bool = false (it means that floats may be allocated and then stored in different memory locations). Using = your predicate works fine: let represented_has_an_integer f = f = float (truncate f);; val represented_has_an_integer : float -> bool = <fun> And now: # represented_has_an_integer 7.0;; - : bool = true Remember that the == predicate is not for casual uses: you should know something about value representations in your Caml system to use it safely. In doubt, use the regular = predicate. > français: > La représentation de "7." et évidemment differente de "float 7". > Est-ce qu'il y a une raison? C'est un problème si on veut verifier si un > "float" peut être représenté comme "int"... Pour une discussion plus complète du problème voir <http://caml.inria.fr/FAQ/FAQ_EXPERT-fra.html#egalite>. Le comportement que vous observez vient de ce que les flottants sont des structures de données (généralement) allouées en mémoire. Or vous utilisez le prédicat == qui teste l'identité physique, au lieu de = qui teste l'égalité sémantique. Le test == donne alors des résultats surprenants: # 1.0 == 1.0;; - : bool = false Il suffit d'utiliser = pour que tout rentre dans l'ordre. Pierre Weis INRIA, Projet Cristal, Pierre.Weis@inria.fr, http://cristal.inria.fr/~weis/