From: "Chuck Allison" <cda@freshsources.com>
To: "Richard R. Malloy" <rrmalloy@attbi.com>,
"Randall R Schulz" <rrschulz@cris.com>
Cc: "Ross Smith" <rosss@pharos.co.nz>, <cygwin@cygwin.com>
Subject: Re: Strange behavior
Date: Sun, 03 Mar 2002 21:08:00 -0000 [thread overview]
Message-ID: <01b501c1c33a$96908e20$0300000a@FSHPXP> (raw)
In-Reply-To: <3C82FC59.2080700@attbi.com>
That's the point. They're always redued, so in both cases, the expression
2.0/3.0 is evaluated. How can that be non-deterministic?
----- Original Message -----
From: "Richard R. Malloy" <rrmalloy@attbi.com>
To: "Randall R Schulz" <rrschulz@cris.com>
Cc: "Ross Smith" <rosss@pharos.co.nz>; "'Chuck Allison'"
<cda@freshsources.com>; <cygwin@cygwin.com>
Sent: Sunday, March 03, 2002 9:47 PM
Subject: Re: Strange behavior
> OK. I'm no IA32 expert can someone explain the following results. (Do
> the floating point registers
> use guard bits, randomly initialized perhaps?)
>
> bool operator==(const Rational& r1, const Rational& r2)
> {
> double a=r1.toDouble(), b=r2.toDouble();
> cout << ?== a " << a << " " << ?== b " << b << endl;
> return a == b;
> // return r1.toDouble() == r2.toDouble();
> /* return ( r1.numerator == r2.numerator && r1.denominator ==
> r2.denominator ); */
> }
>
> 5/4
> == a 1.25 == b 1.25
> 1
> -1/4
> == a -0.25 == b -0.25
> 1
> 3/8
> == a 0.375 == b 0.375
> 1
> 2/3
> == a 0.666667 == b 0.666667
> 0 // return
> r1.toDouble() == r2.toDouble();
>
> 5/4
> == a 1.25 == b 1.25
> 1
> -1/4
> == a -0.25 == b -0.25
> 1
> 3/8
> == a 0.375 == b 0.375
> 1
> 2/3
> == a 0.666667 == b 0.666667
> 1 return a == b;
>
>
> But since the Rational are always reduced the "right" answer is
>
> return ( r1.numerator == r2.numerator && r1.denominator ==
> r2.denominator );
>
> No?
>
> Rich.
>
> Randall R Schulz wrote:
>
> > Ross,
> >
> > To call that result "pure luck" denies the fact that digital
> > computers, when properly functioning, are 100% deterministic.
> >
> > Of course, it's not proper floating-point programming, but that
> > doesn't mean "luck" is involved.
> >
> > Randall Schulz
> > Mountain View, CA USA
> >
> >
> > At 18:04 2002-03-03, Ross Smith wrote:
> >
> >> > From: Chuck Allison [mailto:cda@freshsources.com]
> >> >
> >> > I have a simple Rational number class and have discovered
> >> > weird behavior
> >> > with Cygwin's g++. If you look at the very short main program in file
> >> > rtest2.cpp, you will see by the output that g++ get's the
> >> > wrong answer for
> >> >
> >> > r1 / r2 == Rational(2,3); // should be true
> >> >
> >> > even though it prints as 2/3! Borland and Microsoft get it
> >> > right. Any ideas?
> >> > All code atached.
> >>
> >> [relevant bit of code]
> >>
> >> inline bool operator==(const Rational& r1, const Rational& r2)
> >> {
> >> return r1.toDouble() == r2.toDouble();
> >> }
> >>
> >> This is nothing to do with Cygwin, or g++ for that matter. You're
> >> comparing floating point numbers. Of course it's not reliable! If
> >> other compilers happened to give you an exact equality on that
> >> particular combination of arguments, it was pure luck.
> >
> >
> >
> > --
> > Unsubscribe info: http://cygwin.com/ml/#unsubscribe-simple
> > Bug reporting: http://cygwin.com/bugs.html
> > Documentation: http://cygwin.com/docs.html
> > FAQ: http://cygwin.com/faq/
> >
> >
>
>
>
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next prev parent reply other threads:[~2002-03-04 5:08 UTC|newest]
Thread overview: 8+ messages / expand[flat|nested] mbox.gz Atom feed top
2002-03-03 18:05 Ross Smith
2002-03-03 18:32 ` Randall R Schulz
2002-03-03 20:49 ` Richard R. Malloy
2002-03-03 21:08 ` Chuck Allison [this message]
-- strict thread matches above, loose matches on Subject: below --
2002-03-03 21:42 Gareth Pearce
2002-03-03 21:26 Robert Collins
2002-03-02 13:18 Chuck Allison
2002-03-02 15:09 ` David Means
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