Xref: news2.mv.net comp.lang.c:54740 comp.lang.c++:70278 comp.os.msdos.djgpp:726 gnu.g++.help:4465 gnu.gcc.help:5670
Newsgroups: comp.lang.c,comp.lang.c++,gnu.gcc.help,gnu.g++.help,comp.os.msdos.djgpp
Subject: Re: float != float and floats as return types
Message-ID: <TANMOY.96Feb1164022@qcd.lanl.gov>
From: tanmoy AT qcd DOT lanl DOT gov (Tanmoy Bhattacharya)
Date: 01 Feb 1996 23:40:22 GMT
References: <4ej9lb$mpc AT fu-berlin DOT de>  
	<4elnjj$er4 AT server2 DOT rz DOT uni-leipzig DOT de>	<4eqc7l$ugh AT godzilla DOT zeta DOT org DOT au>  
	<4eqtu3$ddo AT der DOT twinsun DOT com>
Organization: Los Alamos National Laboratory
NNTP-Posting-Host: qcd.lanl.gov
In-reply-to: eggert@twinsun.com's message of 1 Feb 1996 09:40:19 -0800
Lines: 35
To: djgpp AT delorie DOT com
DJ-Gateway: from newsgroup comp.os.msdos.djgpp

In article <4eqtu3$ddo AT der DOT twinsun DOT com> eggert AT twinsun DOT com (Paul
Eggert) writes: 
<snip>
   bde AT zeta DOT org DOT au (Bruce Evans) writes:

   >    a = 1.0;
   >    b = FLT_EPSILON / 4.0;
   >    c = a + b;
   >    assert(c == (float) (a + b));

   As far as I can tell the C Standard allows the assertion to either
   succeed or fail, depending on the implementation.  The key phrase in
   the standard is ``The values of floating operands and of the results of
   floating expressions may be represented in greater precision and range
   than that required by the type''.  So `c' might be represented in more
   bits than `(float) (a + b)' (or vice versa! though this is more
   controversial since it contradicts the Rationale, and it may be
   corrected by the committee soon), and the assertion might fail.

I do not agree with this interpretation, as the footnote (technically,
not part of the standard) clarifies: an assignment and cast both must
truncate the result to the required precision. It can be subsequently
widened to more precision: but that is a different matter.

Now whether `a+b' must always give the same answer (for constant a and
b) may be debatable: but certainly not outside comp.std.c (or c++) :-)

Cheers
Tanmoy
--
tanmoy AT qcd DOT lanl DOT gov(128.165.23.46) DECNET: BETA::"tanmoy AT lanl DOT gov"(1.218=1242)
Tanmoy Bhattacharya O:T-8(MS B285)LANL,NM87545 H:#9,3000,Trinity Drive,NM87544
Others see <gopher://yaleinfo.yale.edu:7700/00/Internet-People/internet-mail>,
<http://alpha.acast.nova.edu/cgi-bin/inmgq.pl>or<ftp://csd4.csd.uwm.edu/pub/
internetwork-mail-guide>. -- <http://nqcd.lanl.gov/people/tanmoy/tanmoy.html>