[NTLUG:Discuss] The wrong computation example from the newsgroup

[Peter Koren]

Oh well, I've seen others have nailed this but here was my approach to
the problem:

 
 int main(void)
 {
     int    a = 60, b = 6, c = 10;
     double x;
     
     x = 7.0 - (c*0.7);
     printf("(int)(c*0.7)=%d\n\n", (int)(c*0.7));
     printf("(float)(0.7)=%26.19e\n\n", (0.7));
     printf("x=7.0-(c*0.7)=%26.19e\n\n", x);
     
     exit(1);
 }
 
which yields:

(int)(c*0.7)=6
 
(float)(0.7)= 6.9999999999999995559e-01
 
x=7.0-(c*0.7)= 4.4408920985006261617e-16
                                                                                    
As Steve Baker said, if you depend on exact floating point results, you
are apt to get screwed. Some base ten numbers like 0.3 and 0.7 can not
be exactly represented in binary. Then there is the semantics issue of
what should the result be. BTW, The Python community is debating
changing the semantics of integer arithmetic and there is no consensus
yet.

 
Chris Cox wrote:
> 
> Here's the original problem... for those NTLUGgers who want a challenge.
> 
> >  From: John Smith (jmsith at generic.com)
> >  Subject: Can linux be trusted?
> >  Newsgroups: comp.lang.c, comp.os.linux.development.apps, comp.os.linux.development.system, comp.sys.be.programmer,
> >  comp.sys.mac.programmer.misc, comp.unix.bsd.freebsd.misc, comp.unix.bsd.misc, gnu.gcc
> >  Date: 2001-02-28 17:52:12 PST
> >
> >
> >
> > "
> > If you've got a minute, and one of those 'bleeding edge' OSes, try compiling
> > with no optimizations and running this for fun:
> >
> > int main(void)
> > {
> > int a = 60, b = 6, c = 10;
> >
> > printf("%d = %d\n", (int) (((60/6)*0.3) + (10*0.7)), (int) ((( a/b)*0.3) +
> > ( c*0.7)));
> >
> > exit(1);
> > }
> >
> > (BTW the lameness filter defies logic, that was the best I could do with the
> > C snippet)
> >
> > A friend was tortured for a few hours doing an assignment until I took a
> > look at the code and realized the problem boiled down to something that can
> > be reduced to this snippet.
> >
> > I compiled this with default compiler settings on every platform I could
> > find. This means Digital Unix 4.0, OpenVMS 7.2, Solaris 8, IRIX 6.4, HP-UX
> > 10.20, FreeBSD 4-STABLE, OpenBSD 2.8 and various Linux distros, from ancient
> > to cutting edge - both with gcc and any commercial compilers that happened
> > to be available at each box.
> >
> > On all Linux distros, and only on Linux distros, ranging from an ancient
> > Slackware setup to the latest Red Hat, I get 9=10. On everything else, I get
> > 10=10. Go figure, and remember that the whole OS is compiled with that.
> >
> > I think I'll just stick to FreeBSD as far as my intel boxes are concerned.
> > "
> > http://slashdot.org/comments.pl?sid=01/02/28/1712232&cid=93
> >
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