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From: ark@alice.UUCP (Andrew Koenig)
Newsgroups: comp.lang.c
Subject: Re: Floating point non-exactness
Message-ID: <11117@alice.UUCP>
Date: 1 Aug 90 12:46:24 GMT
References: <622@.tetrauk.UUCP> <5467@quanta.eng.ohio-state.edu>
Organization: AT&T Bell Laboratories, Liberty Corner NJ
Lines: 23

In article <5467@quanta.eng.ohio-state.edu>, rob@baloo.eng.ohio-state.edu (Rob Carriere) writes:

> In other words, usually what you want is something like

> int fcmp( double a, double b, double eps )
> {
>    if (  fabs( a-b ) < eps  ) return 0;
>    if (  a > b  ) return 1;
>    return -1;
> }

Probably not.

The trouble is that if a and b are big enough, then a-b will either be
exactly 0 or will be larger than eps.  (Here, `big enough' is approximately
eps * 2^n, where n is the number of significant bits in a floating-point
freaction.)  There is also the possibility that a-b might overflow,
in which case the comparison would surely have been valid if done directly.

Floating point arithmetic is *hard*.
-- 
				--Andrew Koenig
				  ark@europa.att.com