Re: This calculation is just wrong / computer can't count!

From:

"David Ching" <dc@remove-this.dcsoft.com>

Newsgroups:

microsoft.public.vc.mfc

Date:

Sun, 7 Oct 2007 08:09:55 -0700

Message-ID:

<uG6Oi.4554$oA2.1091@nlpi068.nbdc.sbc.com>

"BobF" <rNfOrSePeAzMe@charter.net> wrote in message
news:e445Z8MCIHA.1188@TK2MSFTNGP04.phx.gbl...

David - You might try "guard bits". That's a method that's been used in
the past. It's the old trick where you divide 10 by 3, see .333333333,
then multiply by 3. Sometimes you would see .999999 and sometimes you
would see 10. Those that displayed 10 were said to employ guard bits.

The methods may have changed over the years, but it should get you on the
trail.

Thanks Bob. It seems guard bits are used to implement rounding of a result
to the precision supported by the hardware. I'm not sure it corrects the
fundamental problem of representing results exactly for a given number of
digits, like a calculator does, which seems to be what GT wants.

I have never had a problem with the float/double C++ types, so I've not
looked into the floating point inaccuracies in much detail. I've not even
followed this thread very closely. But it seems that what he is asking for
is how to emulate a $1 calculator in terms of accuracy. No more and no
less, and I've yet to see a simple answer like, "use this library and your
problem will be solved." Since I may well have this problem some day, I
would like to see such an answer.

Thanks,
David