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Date:	Sun, 03 Mar 2002 23:47:21 -0500
From:	"Richard R. Malloy" <rrmalloy AT attbi DOT com>
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To:	Randall R Schulz <rrschulz AT cris DOT com>
CC:	Ross Smith <rosss AT pharos DOT co DOT nz>, "'Chuck Allison'" <cda AT freshsources DOT com>,
	cygwin AT cygwin DOT com
Subject:	Re: Strange behavior
References:	<5 DOT 1 DOT 0 DOT 14 DOT 2 DOT 20020303181544 DOT 02707180 AT pop3 DOT cris DOT com>

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 AT freshsources DOT 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.
>
>
>
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