Re: [boost] is_nan

From: Johan Råde (rade_at_[hidden])
Date: 2006-07-25 06:32:05

Paul Giaccone wrote:
> Johan Råde wrote:
>> Floating point numbers can be classified into four categories:
>>
>> 1. finite numbers
>> 2. infinity
>> 3. negative infinity
>> 4. not-a-number (nan)
>>
>> So maybe we need functions:
>>   
>> [snip]
>> template<class T> bool is_infinity(T a)
>> {
>> 	return std::numeric_limits<T>::has_infinity
>> 		&& a == std::numeric_limits<T>::infinity();
>> }
>>
>> [snip]
>>
>> You test for negative infinity by calling is_infinity(-a).
>>   
> 
> ....for which it would be easy enough to provide a convenience function 
> (perhaps "is_minus_infinity"), which would just be a wrapper around 
> "is_infinity".
> 
> Another useful convenience function would be "is_infinite", which would 
> return true if and only if the number was equal to plus or minus 
> infinity (making use of either "is_finite" or both "is_infinity" and 
> "is_minus_infinity")
> 
> The similarity of the names of "is_infinity" and "is_infinite" might be 
> a problem, but the former could be changed to "is_plus_infinity" to give 
> symmetry with "is_minus_infinity".
> 
> Paul
> 
> _______________________________________________
> Unsubscribe & other changes: http://lists.boost.org/mailman/listinfo.cgi/boost
> 

I agree that having the three functions

     is_infinite (or maybe is_infinity)
     is_plus_infinity
     is negative_infinity

is a good idea.

John's proposal has the function is_infinity, in the sense of both plus 
and minus infinity, but not the functions is_plus_infinity and 
is_negative_infinity.

Maybe John's intention is that one should test for plus infinity by writing
     is_infinity(x) && x >= 0
That would work, but is probably inefficient.

--Johan