OpenEuphoria

Re: Rob's going to hate me... (Remainder bug)

• Posted by kbochert at copper.net Nov 08, 2003
• 626 views

On 7 Nov 2003 at 18:07, Robert Craig wrote:

> 
> 
> kbochert at copper.net wrote:
> > It seems to me that given that fmod(a,b) is defined as returning a value
> > which is less than b, failure to do so is not just inaccurate but a real
> > bug.
> 
> I'm sure it *does* return a value less than b.
> But when you print a result such as 0.01 - 99 * epsilon
> to 10 significant digits, like Euphoria does by default,
> you see: 0.01
> 
> >>From a Google search:
> >  http://www.boic.com/b1mnum.pdf.  (dated Nov 3 2003 !!)
> > 
> > QUOTE:
> > Appendix 
> > Microsoft Modulo Bugs 
> > Over the years of testing of the Number Class, we've discovered nasty bugs
> > in
> > Microsoft's fmod() 
> > function, which you can easily confirm for yourself. For example, using the
> > Windows
> > 95 Calculator 
> > accessory (evidently built on the same fmod function) try this simple
> > calculation:
> > 5.5 mod 1.1 
> > Instead of giving the correct result, which should be 0, 
>  > it shows the result as 1.1.
>  > ...
> 
> This person obviously is not aware of the pitfalls of
> binary, limited-precision floating-point.
> 
> > We were unable to find any reference to this 
> > disturbing bug in the 
> > Microsoft Knowledge Base. 
> 
> Because it's not really a bug in the library.
> It's a limitation of the floating-point hardware.
> 
> > ... For some versions, the bug did NOT appear in the calculator 
> > program, but 
> > using fmod() ALWAYS produces this bug. 
> 
> No doubt because the calculator program uses
> *decimal* floating-point, where numbers like 0.1, 0.01 etc.
> *can* be represented with perfect accuracy (but then
> lots of other numbers can't, so this is not a
> perfect solution either.)
> 
> Regards,
>     Rob Craig
>     Rapid Deployment Software
>     http://www.RapidEuphoria.com
> 
Update:

I think you are right. Seems a pity though. Loss of accuracy is one thing
but flat out wrong results from ordinary inputs is bothersome.

Bach may add a default argument, the precision, and round the division
internally

KtB