From: Richard M. Stallman <rms@gnu.org>
Date: Sat, 25 Jun 2005 02:29:46 +0000 (+0000)
Subject: (Float Basics): Explain how to test for NaN, and printing the sign of NaNs.
X-Git-Tag: emacs-pretest-22.0.90~8657
X-Git-Url: http://git.eshelyaron.com/gitweb/?a=commitdiff_plain;h=32d4834cc6c76b53925edc5bf8db7079c8932413;p=emacs.git

(Float Basics): Explain how to test for NaN, and printing the sign of NaNs.
---

diff --git a/lispref/numbers.texi b/lispref/numbers.texi
index 776251e4933..f7f88248ff1 100644
--- a/lispref/numbers.texi
+++ b/lispref/numbers.texi
@@ -183,21 +183,26 @@ there is no correct answer.  For example, @code{(/ 0.0 0.0)} returns a
 NaN.  For practical purposes, there's no significant difference between
 different NaN values in Emacs Lisp, and there's no rule for precisely
 which NaN value should be used in a particular case, so Emacs Lisp
-doesn't try to distinguish them.  Here are the read syntaxes for
-these special floating point values:
+doesn't try to distinguish them (but it does report the sign, if you
+print it).  Here are the read syntaxes for these special floating
+point values:
 
 @table @asis
 @item positive infinity
 @samp{1.0e+INF}
 @item negative infinity
 @samp{-1.0e+INF}
-@item Not-a-number
-@samp{0.0e+NaN}.
+@item Not-a-number 
+@samp{0.0e+NaN} or @samp{-0.0e+NaN}.
 @end table
 
-  In addition, the value @code{-0.0} is distinguishable from ordinary
-zero in @acronym{IEEE} floating point (although @code{equal} and
-@code{=} consider them equal values).
+  To test whether a floating point value is a NaN, compare it with
+itself using @code{=}.  That returns @code{nil} for a NaN, and
+@code{t} for any other floating point value.
+
+  The value @code{-0.0} is distinguishable from ordinary zero in
+@acronym{IEEE} floating point, but Emacs Lisp @code{equal} and
+@code{=} consider them equal values.
 
   You can use @code{logb} to extract the binary exponent of a floating
 point number (or estimate the logarithm of an integer):