[Numpy-discussion] What is the logical value of nan?

Bruce Southey bsouthey@gmail....
Wed Mar 11 09:24:31 CDT 2009


Sturla Molden wrote:
> Charles R Harris wrote:
>   
>>     #include <math.h>
>>     #include <stdio.h>
>>
>>     int main() {
>>        double nan = sqrt(-1);
>>        printf("%f\n", nan);
>>        printf("%i\n", bool(nan));
>>        return 0;
>>     }
>>
>>     $ ./nan
>>     nan
>>     1
>>
>>
>> So resolved, it is True.
>>     
> Unless specified in the ISO C standard, I'd say this is system and 
> compiler dependent.
>
> Should NumPy rely on a specific binary representation of NaN?
>
> A related issue is the boolean value of Inf and -Inf.
>
> Sturla Molden
> _______________________________________________
> Numpy-discussion mailing list
> Numpy-discussion@scipy.org
> http://mail.scipy.org/mailman/listinfo/numpy-discussion
>   
This is one link that shows the different representation of these 
numbers in IEEE 754:
http://www.psc.edu/general/software/packages/ieee/ieee.php
It is a little clearer than Wikipedia:
http://en.wikipedia.org/wiki/IEEE_754-1985

Numpy's nan/NaN/NAN, inf/Inf/PINF, and NINF are not nothing so not zero. 
Also, I think that conversion to an integer should be an error for all 
of these because there is no equivalent representation of these floating 
point numbers as integers and I think that using zero for NaN is wrong.

Now for the other two special representations, I would presume that 
Numpy's PZERO (positive zero) and NZERO (negative zero) are treated as 
nothing. Conversion to integer for these should be zero.

However, I noticed that the standard has just been revised that may 
eventually influence Numpy:
http://en.wikipedia.org/wiki/IEEE_754r
http://en.wikipedia.org/wiki/IEEE_754-2008

Note this defines the min/max behavior:

    * |min(x,NaN) = min(NaN,x) = x|
    * |max(x,NaN) = max(NaN,x) = x|


Bruce



More information about the Numpy-discussion mailing list