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tech / sci.electronics.design / Re: 0.0 / 0.0 = -NAN ?

Re: 0.0 / 0.0 = -NAN ?

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From: robin_li...@es.invalid (Carlos E.R.)
Newsgroups: sci.electronics.design
Subject: Re: 0.0 / 0.0 = -NAN ?
Date: Tue, 3 May 2022 23:11:08 +0200
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by: Carlos E.R. - Tue, 3 May 2022 21:11 UTC

On 2022-04-30 17:31, Skybuck Flying wrote:
> When exception masks are all enabled to stop the processor from throwing floating point exceptions the following calculation produces a somewhat strange result:
>
> 0.0 / 0.0 = -nan
>
> (At least in Delphi).
>
> For now I will assume this is the case in C/C++ as well and with that I mean on x86/x64 which should and seems to be following IEEE 754 floating-point format.
>
> I am a little bit surprised by this and I want/need to know more. Where is this defined that 0.0 / 0.0 should be -NAN ?!?
>
> Problem is with the code, example:
>
> T := 0;
> D := 0.0 / 0.0;
> P := T * D;
>
> This screws up P. instead of P being zero, P is now also -NAN ?!?

I have not read the entire thread, so maybe someone has already said it;
but yes, NAN is correct in math, because 0/0 is "undefined" (think
limits). It can be zero, one, any number, even infinite. So multiplying
T·D can actually be 0·∞, which is again "undefined", so NAN.

>
> I find this very strange but ok.
>
> I guess a simple solution could be to set D to 0 explicitly for this case, is there perhaps another solution ? Maybe some kind of mask or rounding mode so that additional branch is not necessary ???
>
> Bye for now,
> Skybuck.

--
Cheers, Carlos.