Path: utzoo!attcan!uunet!lll-winken!lll-lcc!ames!vsi1!daver!lynx!m5
From: m5@lynx.uucp (Mike McNally)
Newsgroups: comp.lang.c
Subject: More ANSI float.h stuff
Summary: Epsilon confusion
Message-ID: <5198@lynx.UUCP>
Date: 18 Jan 89 16:39:51 GMT
Reply-To: m5@lynx.UUCP (Mike McNally)
Distribution: na
Organization: Lynx Real-Time Systems Inc, Campbell CA
Lines: 33

The <float.h> file of the ANSI library contains two constants, FLT_EPSILON
and DBL_EPSILON.  These are (according to K&R II) supposed to be the 
smallest x such that 1.0 + x != 1.0.  Should I interpret this as:

    The smallest x such that the statement
        
        i = (1.0 + x != 1.0);

    assigns 1 to the variable i.

or should it be:

    The smallest x such that the sequence:

        val1 = 1.0;
        val2 = 1.0 + x;
        i = val1 != val2;

    results in assignment of 1 to the variable i.

Seems to me that there is a difference, because intermediate results during
expression evaluation may involve greater precision than available for
storage of values.

On a related note, the qualification that x be the "smallest" value bothers
me: shouldn't x be positive?  Most likely the actual ANSI draft is more
specific; it probably clears up the first question as well.

-- 
Mike McNally                                    Lynx Real-Time Systems
uucp: {voder,athsys}!lynx!m5                    phone: 408 370 2233

            Where equal mind and contest equal, go.