Re: NaNs

From:

Pete Becker <pete@versatilecoding.com>

Newsgroups:

comp.lang.c++.moderated

Date:

Tue, 29 Jul 2008 10:21:43 CST

Message-ID:

<2008072907331050073-pete@versatilecodingcom>

On 2008-07-28 19:28:18 -0400, Jiang <goo.mail01@yahoo.com> said:

On Jul 29, 8:35 am, "Alf P. Steinbach" <al...@start.no> wrote:

* Alan McKenney:

On Jul 28, 4:34 am, Thomas Richter <t...@math.tu-berlin.de> wrote:

Tom wrote:

[...]

More to the point, there are a very large
number of bit patterns that count as NaN.

Both Thomas and Alan and OP Tom, please note that (1) number of bitpatterns is
irrelevant, because (2) an IEEE NaN is required to be unequal to anything,
including itself.

Correct testing, after having established IEEE floating point, is

if( x != x )
{
// This is a NaN
}

Unfortunately, the above test itself will trigger the floating point
trap
if x is signalling NaN. Also it is quite possible optimizer will just
ignore/remove such test.

It own't trigger a trap if no trap handler is installed, and by
default, no trap handlers are installed. It will trigger a
floating-point invalid operation exception. By default, that exception
sets a status bit to indicate that the exception occurred, and converts
the value to a quiet NaN.

--
  Pete
Roundhouse Consulting, Ltd. (www.versatilecoding.com) Author of "The
Standard C++ Library Extensions: a Tutorial and Reference
(www.petebecker.com/tr1book)

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