Path: utzoo!attcan!uunet!aplcen!uakari.primate.wisc.edu!zaphod.mps.ohio-state.edu!swrinde!ucsd!ucbvax!hplabs!hpfcso!hplisa!hpislx!bayes
From: bayes@hpislx.HP.COM (Scott Bayes)
Newsgroups: comp.sys.mac.programmer
Subject: Re: Why can't the Mac add?
Message-ID: <9710005@hpislx.HP.COM>
Date: 26 Sep 90 23:43:03 GMT
References: <45060@apple.Apple.COM>
Organization: Measurement Systems Operation - Loveland, CO
Lines: 37

> Let's try this one more time, then I promise to go away and stop
> bothering all you nice people.  
> 
> I know what happens when you convert 0.2 to binary.  I know literally
> how precision is lost in the base conversion.  I know what a mantissa and
> an exponent are.  That's not my question.
> 
> I know that the sample program I posted is bad programming style,
> but it illustrates the question well. So stop flaming me about it.
> (I don't usually write code like that, I promise.)
> 
> huh-HUM (throat clearing noises.)
> The calculator DA can do this math correctly.  If you add 0.2 fifty
> times, you get 10.0.  Exactly.  Not 9.999999 or 10.0000001.  I tried
> changing my float variable to an extended, as someone suggested.  No dice.
> Does the calculator DA have its own special math package?  (If so, the
> dCad calculator does too.)  No one seems to think SANE has calls which
> take care of this.  So why does it work in the calculator?
> 
> David

So does the calculator DA work in base 10, maybe? Note that QuickBASIC for
the Mac hase a decimal base version, which should also give exact answers
for your case. It's just that often decimal base calculations are considered
enough more expensive than binary in a binary computer that they are only
used in specialized code, usually. Nice for business, where the world is
naturally decimal; not so nice in science, where it only seems to be
decimal based, due to the way we write things down.

Scott "binary" Bayes

Note: the decimal base trick is often done with 2 BCD fields per byte,
except in "infinite-precision" math, where it might be done with rational
numbers, or strings, or goodness knows what... I believe HP's old 9845
was a decimal base machine.

S