Re: How to check a Float for NaN

["Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>]

On Thu, 22 May 2014 08:28:33 -0700 (PDT), Adam Beneschan wrote:

> On Thursday, May 22, 2014 2:48:47 AM UTC-7, Dmitry A. Kazakov wrote:
>> On Thu, 22 May 2014 10:24:35 +0100, Simon Wright wrote:
>> 
>>> "Dmitry A. Kazakov" writes:
>> 
>>>> On Thu, 22 May 2014 00:27:02 -0700 (PDT), jan.de.kruyf wrote:
> 
>>>>>       if Cos_Theta /= Cos_Theta then
>> 
>>>>> 	 Gct.Trace (Debug_Str, "cos_theta is NaN******");
>> 
>>>>>       else
>> 
>>>>> 	 Gct.Trace (Debug_Str, "cos_theta : " & Long_Float'Image (Cos_Theta)); 
>> 
>>>>>       end if;
>> 
>>>>> 
>> 
>>>>> gives this in the log:
>> 
>>>>> 
>> 
>>>>> [MATH3D.DEBUG] 1/372 cos_theta is NaN****** (2014-05-22 09:23:24.155)(loc: math3d.adb:129)
>> 
>>>>> 
>> 
>>>>> Hope it helps someone;
>> 
>>>>
>> 
>>>> You could simply use range check:
> 
>>>>    X in Long_Float'Range
> 
>>>> NaN is not a number and thus outside the range.
> 
>>> This would also return False for +/-Inf, so if the OP wants specifically
>>> to check for NaN it wouldn't do. Can't think why they would, though.
> 
>>> GNAT says about "X in Long_Float'Range"
>> 
>>>    inf.adb:10:60: warning: explicit membership test may be optimized away
> 
>> Looks like a bug, it may not be optimized away if Long_Float is IEEE 754.
> 
>>>    inf.adb:10:60: warning: use 'Valid attribute instead
>  
>> Hmm, reading RM 13.9.2 makes me believe that NaN's 'Valid should yield
>> True.
> 
> No, I don't think so.  3.5.7(8) says "The set of values for a floating
> point type is the (infinite) set of rational numbers."  This excludes NaN
> and infinities, which are not rational numbers (nor any kind of number).

These are not values of Long_Float. I guess that values of Long_Float are
"machine numbers" defined in the following sentence.

Why doesn't it preclude NaN returned and accepted by arithmetic operations?
Either 3.5.7 lies or the operations do.

-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de