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From: ger@qtecmuc.UUCP
Newsgroups: comp.lang.c
Subject: Re: Floating point equality - (nf)
Message-ID: <16800001@qtecmuc.UUCP>
Date: Mon, 16-Mar-87 12:45:00 EST
Article-I.D.: qtecmuc.16800001
Posted: Mon Mar 16 12:45:00 1987
Date-Received: Wed, 18-Mar-87 03:50:24 EST
References: <4915@brl-adm.UUCP>
Lines: 57
Nf-ID: #R:brl-adm:-491500:qtecmuc:16800001:000:1703
Nf-From: qtecmuc!ger    Mar 16 18:45:00 1987

I had a similar problem today, with Microsoft C 4.0 on an IBM-AT compatible.
The two programs

#include <math.h>
double	a=0.01,
	b=0.01;
int	c;

main()
{	  
	a=floor(1.e5*a/0.5)/1.e5;
        if(a!=b)
        	printf("not equal");
        else
        	printf("equal");
}
/****/
and
/****/

/* declarations as above ... */

main()
{	  
	a=floor(1.e5*a+.5)/1.e5;
	c=1;				/* This is the only difference */
        if(a!=b)
        	printf("not equal");
        else
        	printf("equal");
}
/*****/

yielded different results. the first one says: 'Not equal',
while the other one prints out 'equal'.
With a debugger I found the reason for this strange behaviour:
The compiler tries to hold the result of the floor... statement in the
80 bit register of the 8087 or the emulator and loads just b into another 
register for the comparison, if there is no other statement inbetween. Thus 
the compiler compares the 80-bit value resulting from previous operations and 
b, which is extended from 64 bit double precision to the internal 80 bit
format, giving a!=b.
If there is another statement like 'c=1' or anything else between the
evaluation of floor .... and the comparison, the result of floor...
is written to the variable 'a' correctly  in 64 bit format, then loaded and 
extended again to 80 bit format, which results in a==b.
My question: is this behaviour legal C ????
I always thought, double variables could only be compared with double
precision, not with the precision of some internal format, depending
of other statements, like 'c=1'.
Too much optimization in my eyes.

Gerhard Pehland
Quantec Tonstudiotechnik, Munich W-Germany
UUCP: ....!mcvax!unido!qtecmuc!ger

Sorry for my english...