Path: utzoo!utgpu!news-server.csri.toronto.edu!rutgers!att!dptg!ulysses!andante!alice!ark
From: ark@alice.UUCP (Andrew Koenig)
Newsgroups: comp.lang.c
Subject: Re: Floating point non-exactness
Message-ID: <11147@alice.UUCP>
Date: 6 Aug 90 04:19:01 GMT
References: <622@.tetrauk.UUCP> <5467@quanta.eng.ohio-state.edu> <4958@memqa.uucp>
Organization: AT&T Bell Laboratories, Liberty Corner NJ
Lines: 20

In article <4958@memqa.uucp>, r91400@memqa.uucp (Michael C. Grant) writes:

> For example, assuming a standard which
> requires normalization, this algorithm might work (note this is 
> just a near-equality test, not a complete comparison):

> 	if (exponent_a!=exponent_b) return false
> 	if abs(mantissa_a-mantissa_b)<error_limit return true
> 	   else return false


You can use frexp() to split numbers into mantissa and exponent
(though I forget whether frexp is actually part of the ANSI standard
or not).  However, the algorithm shown here doesn't work because
the two numbers being compared could straddle a power of 2,
which would give them different exponents even though they're
arbitrarily close together.
-- 
				--Andrew Koenig
				  ark@europa.att.com