* double error
@ 2003-01-31 18:44 Bob Hou
2003-01-31 19:01 ` Bob Hou
2003-01-31 19:07 ` Andreas Conz
0 siblings, 2 replies; 4+ messages in thread
From: Bob Hou @ 2003-01-31 18:44 UTC (permalink / raw)
To: gcc, gcc, yhou
Hi,
I have the following simple code and expect the results are all the
same.
However the output is following: (The code is compiled with gcc under
Redhat8.0. )
The result is : 0
The result is : 0
The result is : -2.22045e-15
The code is also tested on MS VS6.0
and it give the same answers (zero).
Could you tell me whether it's a bug or I did sth wrong?
Thanks,
/*--------------- test.c---------------------------*/
#include<stdio.h>
int main()
{
double fi = 0.7509;
double fj = 100.0;
double fk, fk1, fk2 ;
fk = fj*fi - fj*fi;
fk1 = 0.7509*100.0 - 0.7509*100.0;
fk2 = fj*fi - 0.7509*100.0;
printf("The result is : %g\n", fk);
printf("The result is : %g\n", fk1);
printf("The result is : %g\n", fk2);
return 0;
}
/*-----------------end-----------------------------*/
--
__________________________________________________________
Dr. Bob (Yuqian) Hou
Certre for Vision Research
Rm0002B, CSB
York University
4700 Keele Street
North York, Ontario
Canada M3J 1P3
Tel: (416) 736-2100 ext. 66117
Fax: (416) 736-5814
^ permalink raw reply [flat|nested] 4+ messages in thread* double error
2003-01-31 18:44 double error Bob Hou
@ 2003-01-31 19:01 ` Bob Hou
2003-01-31 19:07 ` Andreas Conz
1 sibling, 0 replies; 4+ messages in thread
From: Bob Hou @ 2003-01-31 19:01 UTC (permalink / raw)
To: gcc, gcc, yhou
Hi,
I have the following simple code and expect the results are all the
same.
However the output is following: (The code is compiled with gcc under
Redhat8.0. )
The result is : 0
The result is : 0
The result is : -2.22045e-15
The code is also tested on MS VS6.0
and it give the same answers (zero).
Could you tell me whether it's a bug or I did sth wrong?
Thanks,
/*--------------- test.c---------------------------*/
#include<stdio.h>
int main()
{
double fi = 0.7509;
double fj = 100.0;
double fk, fk1, fk2 ;
fk = fj*fi - fj*fi;
fk1 = 0.7509*100.0 - 0.7509*100.0;
fk2 = fj*fi - 0.7509*100.0;
printf("The result is : %g\n", fk);
printf("The result is : %g\n", fk1);
printf("The result is : %g\n", fk2);
return 0;
}
/*-----------------end-----------------------------*/
--
__________________________________________________________
Dr. Bob (Yuqian) Hou
Certre for Vision Research
Rm0002B, CSB
York University
4700 Keele Street
North York, Ontario
Canada M3J 1P3
Tel: (416) 736-2100 ext. 66117
Fax: (416) 736-5814
^ permalink raw reply [flat|nested] 4+ messages in thread* Re: double error
2003-01-31 18:44 double error Bob Hou
2003-01-31 19:01 ` Bob Hou
@ 2003-01-31 19:07 ` Andreas Conz
2003-01-31 19:12 ` Andreas Conz
1 sibling, 1 reply; 4+ messages in thread
From: Andreas Conz @ 2003-01-31 19:07 UTC (permalink / raw)
To: Bob Hou; +Cc: gcc, gcc
On Fri, 31 Jan 2003, Bob Hou wrote:
Hi Bob,
you have hit a typical floatingpoint user error.
This are doubles, not real numbers!
for doubles a,b,c:
it could be that a+b+c != c+b+a
Take a look at the following link from the "GCC Further Reading: page:
http://www.validlab.com/goldberg/paper.pdf
> Hi,
> I have the following simple code and expect the results are all the
> same.
> However the output is following: (The code is compiled with gcc under
> Redhat8.0. )
>
> The result is : 0
> The result is : 0
> The result is : -2.22045e-15
This is the same.
> The code is also tested on MS VS6.0
> and it give the same answers (zero).
You could get different results at different optimisation levels at
different compilers.
> Could you tell me whether it's a bug or I did sth wrong?
> Thanks,
mit freundliche Gruessen / kind regards
Andreas Conz
WWW http://www.neuro.informatik.uni-kassel.de/~andreasc
e-mail andreasc@neuro.informatik.uni-kassel.de
^ permalink raw reply [flat|nested] 4+ messages in thread
* Re: double error
2003-01-31 19:07 ` Andreas Conz
@ 2003-01-31 19:12 ` Andreas Conz
0 siblings, 0 replies; 4+ messages in thread
From: Andreas Conz @ 2003-01-31 19:12 UTC (permalink / raw)
To: Bob Hou; +Cc: gcc, gcc
On Fri, 31 Jan 2003, Bob Hou wrote:
Hi Bob,
you have hit a typical floatingpoint user error.
This are doubles, not real numbers!
for doubles a,b,c:
it could be that a+b+c != c+b+a
Take a look at the following link from the "GCC Further Reading: page:
http://www.validlab.com/goldberg/paper.pdf
> Hi,
> I have the following simple code and expect the results are all the
> same.
> However the output is following: (The code is compiled with gcc under
> Redhat8.0. )
>
> The result is : 0
> The result is : 0
> The result is : -2.22045e-15
This is the same.
> The code is also tested on MS VS6.0
> and it give the same answers (zero).
You could get different results at different optimisation levels at
different compilers.
> Could you tell me whether it's a bug or I did sth wrong?
> Thanks,
mit freundliche Gruessen / kind regards
Andreas Conz
WWW http://www.neuro.informatik.uni-kassel.de/~andreasc
e-mail andreasc@neuro.informatik.uni-kassel.de
^ permalink raw reply [flat|nested] 4+ messages in thread
end of thread, other threads:[~2003-01-31 18:02 UTC | newest]
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2003-01-31 18:44 double error Bob Hou
2003-01-31 19:01 ` Bob Hou
2003-01-31 19:07 ` Andreas Conz
2003-01-31 19:12 ` Andreas Conz
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