Re: [boost] is_nan

From: Johan Råde (rade_at_[hidden])
Date: 2006-07-25 05:35:49

Johan Råde wrote:
> Joe Gottman wrote:
>>    Several libraries have an is_nan() function buried deep in the code.  Are 
>> there any plans to make a general is_nan function available?  This would be 
>> quite useful, as well as similar functions like is_finite.
>>
>> Joe Gottman 
>>
>>
>>
>> _______________________________________________
>> Unsubscribe & other changes: http://lists.boost.org/mailman/listinfo.cgi/boost
>>
> 
> 
> 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:
> 
> 
> template<class T> bool is_finite(T a)
> {
> 	return a >= -std::numeric_limits<T>::min()
> 		&& a <= std::numeric_limits<T>::max();
> }
> 
> template<class T> bool is_infinity(T a)
> {
> 	return std::numeric_limits<T>::has_infinity
> 		&& a == std::numeric_limits<T>::infinity();
> }
> 
> template<class T> bool is_nan(T a)
> {
> 	return a != a;
> }
> 
> 
> You test for negative infinity by calling is_infinity(-a).
> 
> Three issues:
> 
> 1. Do these implementations work an all platforms?
> 
> 2. The category not-a-number can be divided into the sub-categories 
> quite not-a-number and signalling not-a-number. Does anyone need to 
> distinguish between them?
> 
> 3. Are these the most efficient implementations. (Important if you have 
> a large vector of numbers and need to verify that each element is finite.)
> 
> 
> --Johan Råde
> 
> _______________________________________________
> Unsubscribe & other changes: http://lists.boost.org/mailman/listinfo.cgi/boost
> 

Typo, should be

template<class T> bool is_finite(T a)
{
        return a >= -std::numeric_limits<T>::max()
                && a <= std::numeric_limits<T>::max();
}

--Johan Råde