Path: utzoo!attcan!uunet!husc6!uwvax!oddjob!uxc!uxc.cso.uiuc.edu!uxe.cso.uiuc.edu!mcdonald
From: mcdonald@uxe.cso.uiuc.edu
Newsgroups: comp.lang.c
Subject: Re: Floating point puzzle
Message-ID: <225800048@uxe.cso.uiuc.edu>
Date: 7 Aug 88 13:13:00 GMT
References: <3117@emory.uucp>
Lines: 34
Nf-ID: #R:emory.uucp:3117:uxe.cso.uiuc.edu:225800048:000:710
Nf-From: uxe.cso.uiuc.edu!mcdonald    Aug  7 08:13:00 1988


I tried running the following program on my IBM PC, using Microsoft C 5.1
with command line switch /Od - no optimization.

#include <stdio.h>

main()
{
union { 
    float fl;
    unsigned long  lo;
} x,y;
	x.fl = 1.0/10.0;
	y.fl = 1677721.0/16777216.0; 
	printf("x: %lx", x.lo);
	printf("%20.17f\n",x.fl);
	printf("y: %lx", y.lo);
	printf("%20.17f\n",y.fl);
}

/* output is:

x: 3dcccccd 0.10000000149011610
y: 3dccccc8 0.09999996423721313

/*

Which is exactly what I would expect. The first one is 0.1 to almost
8 places, while the second is a bit smaller - just as it should be.

Try running this program, or, if your compiler has 32bit ints, 
substitute "int" for "long" and "%x" for %lx".

Doug McDonald