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From: jwp@uwmacc.UUCP (Jeffrey W Percival)
Newsgroups: comp.unix.wizards,comp.lang.c
Subject: Incorrect subtraction in C program
Message-ID: <1573@uwmacc.UUCP>
Date: Mon, 1-Jun-87 19:13:04 EDT
Article-I.D.: uwmacc.1573
Posted: Mon Jun  1 19:13:04 1987
Date-Received: Wed, 3-Jun-87 02:48:12 EDT
Organization: UWisconsin-Madison Academic Comp Center
Lines: 34
Keywords: vax, C, 4.3BSD
Xref: mnetor comp.unix.wizards:2553 comp.lang.c:2293

I have a problem I have distilled down into a short C program.
The problem is that when I subtract a tiny number from 1,
I get 1 (the right answer) on a VAX 780 running 4.3BSD, but
I get 0 (the wrong answer) on a MicroVAX running 4.3BSD.  Here's
the program:
---------------------------------------------------
#include <stdio.h>
#include <math.h>

main()
{
    double a, b, c, d, e;

    a = 1.000000;
    b = 0.000001;
    c = 0.193113;

    d = (b * c);
    e = a - d;

    fprintf(stdout, "a=%f, b=%f, c=%f, d=%f, e=%f\n", a, b, c, d, e);

    exit(0);
}
----------------------------------------------------------
and here's the MicroVAX output followed by the VAX 780 output:
-------------------------------------------------------
a=1.000000, b=0.000001, c=0.193113, d=0.000000, e=0.000000
a=1.000000, b=0.000001, c=0.193113, d=0.000000, e=1.000000

So, the MicroVAX says that (a-d) is 0, not 1.
What's the deal here?
-- 
	Jeff Percival (jwp@salvax1)