Message-Id:	<m0ymNVA-000S3qC@inti.gov.ar>
Comments:	Authenticated sender is <salvador AT natacha DOT inti DOT gov DOT ar>
From:	"Salvador Eduardo Tropea (SET)" <salvador AT inti DOT gov DOT ar>
Organization:	INTI
To:	sassi AT biomed DOT polimi DOT it (Roberto Sassi), djgpp AT delorie DOT com
Date:	Wed, 17 Jun 1998 16:08:57 +0000
MIME-Version:	1.0
Subject:	Re: Another silly question about the double 0.1 ...
In-reply-to:	<3587fb67.19298603@news.polimi.it>

sassi AT biomed DOT polimi DOT it (Roberto Sassi) wrote:

> thank you very much to everyone who had spent time to answer to my
> previous silly question about the 0.1 double.
> Here I have another question...
> 
> Let's begin with the code ...
> 
> #include <stdio.h>
> 
> int main(void) {
> 
>  double r=0.1;
>  double inc=0.05;
> 
>  for(;r<=0.25;r+=inc) 
>     printf("%6.5f\n",r);
> 
>  }
> 
> I compile the source code, building cicla.exe (in italian "ciclo"
> means "loop")

The same is in spanish.
 
> D:\Roberto\djgpp\codice>gcc -o cicla.exe cicla.c
> 
> D:\Roberto\djgpp\codice>cicla
> 0.10000
> 0.15000
> 0.20000
> 0.25000
> 
> Now let's see what happen if I use the -O flag, when I compile with my
> djgpp compiler ..
> 
> D:\Roberto\djgpp\codice>gcc -O -o cicla.exe cicla.c
> 
> D:\Roberto\djgpp\codice>cicla
> 0.10000
> 0.15000
> 0.20000
> 
> D:\Roberto\djgpp\codice>
> 
> The floating point optimization error, I think, makes the loop one
> step shorter.

Use an integer for the loop variable. Or add a little bit to the problematic 
constant, for example:

#include <stdio.h>

#define sup_err 0.000000001

int main(void) {

 double r=0.1;
 double inc=0.05;

 for(;r<=0.25+sup_err;r+=inc)
    printf("%6.5f\n",r);

 }

This problem is very strange because it doesn't dissapear when you use 
-ffloat-store.

The problem is that 0.1 can't be encoded in a double, the closer value is:

0.10000000000000001
 
SET 
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Salvador Eduardo Tropea (SET). (Electronics Engineer)
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