Re: [boost] The meaning of -nan (was [serialization] facets for non-finite numbers)

From: Johan Råde (rade_at_[hidden])
Date: 2006-08-18 12:01:25

Richard Hadsell wrote:
> Paul A Bristow wrote:
> 
>> But a NaN is a NaN, whatever the sign, only the exponent field determines
>> NaN-ness AFAIK.
>>
>> Apart from sign there are also lots of significand bits whose meaning is
>> officially undefined.
>>  
>>
> The exponent field is all 1's, just like infinity, but the mantissa 
> field has at least one 1 bit.  All 0's in the mantissa would be infinity.

For more information about negative nan, and related beasts, read the 
the section on the IEEE 754 standard in documentation of my library. If 
you still want to know more, follow the links listed there.

The library is in the vault: serialization/non_finite_num_facets.zip

Hubert Holin wrote:
> *Negative* Not-A-Number? What is such a beast, and what should
it be used for? Next, I vote for an Octonionic Not-A-Number!

I have good news for you Hubert ;-)
There already are octonionic not-a-number's.
The following program gives the output "false" when I run it on VC++ 
7.1. (On some compilers you may have to turn off the optimizations.)

   #include <boost/math/octonion.hpp>

   int main()
   {
     boost::math::octonion<double>
     oct(std::numeric_limits<double>::quiet_NaN());

     std::cout << std::boolalpha << (oct == oct) << std::endl;
   }

--Johan Råde