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Message from discussion Pourquoi 10^16+1-10^16 = 0 ?
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Éric Lévénez  
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 More options Oct 14 2012, 3:21 pm
Newsgroups: fr.comp.lang.c
From: Éric Lévénez <use...@levenez.com>
Date: Sun, 14 Oct 2012 21:21:41 +0200
Local: Sun, Oct 14 2012 3:21 pm
Subject: Re: Pourquoi 10^16+1-10^16 = 0 ?
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Le 14/10/12 20:51, ZarkXe a écrit :

> Bonjour à tous,

> Je du mal a comprend pourquoi 10^16 + 1 - 10^16 = 0 ?

> Merci d'avance.

> ZarkXe

> ---------------%<-------------
> #include <stdio.h>
> #include <stdlib.h>
> #include <math.h>

> int main() {
>    double total3 = pow(10, 16) + 1 - pow(10, 16);
>    printf("pow(16) + 1 - pow(16) = %lf\n", total3);

>    return EXIT_SUCCESS;
> }
> ---------------%<-------------

Le double contient un flottant avec mantisse et exposant. La taille de
la mantisse ne peut contenir la valeur exacte de 10^16 + 1 et donc le +1
est en quelque sorte ignoré.

Pour avoir plus de chiffres significatifs, il faut passer à un truc du
genre :

#include <stdio.h>
#include <stdlib.h>
#include <math.h>

int main(void)
{
        long double total3 = powl(10, 16) + 1 - powl(10, 16);
        printf("pow(16) + 1 - pow(16) = %Lf\n", total3);

        return EXIT_SUCCESS;

}

--
Éric Lévénez
FAQ de fclc : <http://www.levenez.com/lang/c/faq/>

 
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