RE: Strange behavior

["Gareth Pearce" <tilps at hotmail dot com>]

>Floating point comparisons should _always_ be done via a confidence
>interval, not bitwise equality. As for determinism, I don't know what
>the logic circuits look like, so can't and won't comment :}.
>
>Rob

confidence intervals are indeed the only way...

but as to determinism ... its a function of your compiler I believe.  
Differences will likely arrise with or without optimisation.
Which coincidently - is also the difference between the 2 situations below, 
1 is an optimisation of the other, the assembly produced, even without an 
optimisation flag, will be different, which means different results (one 
case needs temporary variables - other one does not).  So it should be 
deterministic for a single set of assembly, for a given system type... 
although someone could easily of made a processor in which I am wrong.

Gareth
>
>
>
> > -----Original Message-----
> > From: Chuck Allison [mailto:cda@freshsources.com]
> > Sent: Monday, March 04, 2002 4:08 PM
> > To: Richard R. Malloy; Randall R Schulz
> > Cc: Ross Smith; cygwin@cygwin.com
> > Subject: Re: Strange behavior
> >
> >
> > 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 ); */ }
>
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