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From: lins@Apple.COM (Chuck Lins)
Newsgroups: comp.sys.mac.programmer
Subject: Re: Why can't the Mac add?
Message-ID: <45084@apple.Apple.COM>
Date: 24 Sep 90 17:22:00 GMT
References: <45060@apple.Apple.COM>
Organization: Apple Computer Inc., Cupertino, CA
Lines: 44

In article <45060@apple.Apple.COM> das@Apple.COM (David Shayer) writes:
>I was running this simple program.
>
>main ()
>{
>	float	x; 
>	
>	for (x=0.0;x!=10.0;x+=0.2)
>		printf ("x=%f \n",x);
>}
>
>As you can see,  it ought to stop when x==10.0.  However, 
>it actually runs in an infinite loop.  This is because
>x never equals exactly 10.0.  The +0.2 always makes x
>equal to 9.999999 or 10.000001 or something.  Changing
>x from a float to a double makes x stay closer to the
>correct value, but it still isn't exactly correct, and
>as the loop runs longer, x gets off more and more.
>
>So my question is, why can't the Mac do simple math?
>What's going on here?  Is SANE insane?
>
>Please enlighten me.

Ok. I'll try :-)

Real arithmetic on computer tends to be inexact. Testing for
*exactly* some specific real number is a bad thing. It won't
matter what hardware you use. The folks who write numerical
software know this well. The proper test should be to test for
almost equality with a very small epsilon.

>
>David
>


-- 
Chuck Lins               | "Is this the kind of work you'd like to do?"
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The intersection of Apple's ideas and my ideas yields the empty set.