Date: Tue, 7 Feb 1995 20:08:36 +0900
From: Stephen Turnbull <turnbull AT shako DOT sk DOT tsukuba DOT ac DOT jp>
To: djgpp AT sun DOT soe DOT clarkson DOT edu
Subject: How to use Inf and NaN?

   > Another example:
   >   x= 0.0 / 0.0
   > also generates floating point example, but I need NaN in x.
   > 
   > Is there any solution for this problem?
   > 
   > Thanx in advance,
   > Pisti

   I haven't seen any replies posted, so, with much trepidation, I decided 
   to take a look at this problem.

   That leaves assembly language, and modifying the FPU contol word to mask 
   off certain exceptions, including divide by zero and whatever other 
   conditions could result in a NaN.  Then no exceptions will be generated.  

I think that this has been discussed on the list.  Try searching the
archives (see the URL below if you don't want to download the 8MB
monster from Clarkson).  Sure, it's dangerous, but what choice does he
have if he wants efficient code (ie, having the FPU trap it)?

   state is, I believe, affine, but if you want to use the result as above 
   maybe projective would be better????

I believe that affine means that there are plus and minus infinity;
projective (like complex) only has one infinity.  (That's what it
means in math, IEEE definitions may be something else again.)  So this
is not going to return NaN.

-- 
Stephen Turnbull  /  Yaseppochi-gumi  /  <turnbull AT shako DOT sk DOT tsukuba DOT ac DOT jp>
http://turnbull.sk.tsukuba.ac.jp/      anon FTP: turnbull.sk.tsukuba.ac.jp
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