RE: Floating point related question

John,

Thanks for your reply and suggestions.  The problem with
your suggestion is that a tolerance value is a single
number (the way I understand it) but the representation
error of floating point numbers changes depending on the
value of the number itself.  For example eps(10e-15) is
much smaller than eps(10e+15), and it will also depend on
whether the number is single or double precision.

>> eps(10e-15)
ans = 1.5777e-030

>> eps(10e+15)
ans = 2


Arpad
==========================================================

-----Original Message-----
From: John Shields [mailto:John_Shields@mentor.com] 
Sent: Wednesday, December 20, 2006 5:16 PM
To: Muranyi, Arpad
Cc: vhdl-ams@vhdl.org
Subject: Re: Floating point related question

Hi Arpad,

There is nothing in the language that I am aware of. One should only 
compare real numbers using a tolerance. The tolerance to use depends on 
the application and should be defined by the user/model writer.  I think 
it depends on the use case you have.  You should be able to write a 
user-defined function like eps, if you truly want to make comparisons 
within the tolerance that eps is defined to be or compute a tolerance 
based on some model of accumulated errors. There are others who may 
offer specific cautions, but tolerance errors can accumulate depending 
on how expressions are computed and compilers have some freedom in 
evaluation order of operands of an operator.

Regards,
John

Muranyi, Arpad wrote:
> Hello Everyone,
>
> We all know that using relational operators between
> floating point numbers is a risky proposition.  This
> code, for example, may never execute, believe it or
> not, (I tried it):
>
>       if (0.1+0.2 = 0.3) then
>         report "Test";
>       end if;
>
> However, there are times when taking care of such
> floating point errors may be needed.
>
> In Matlab there is a function, called "eps" which
> can help in incorporating such errors in equations.
> http://www.mathworks.com/access/helpdesk/help/techdoc/ref/index.html?/ac
> cess/helpdesk/help/techdoc/ref/eps.html&http://www.google.com/search?hl=
> en&q=eps+function
>
> I would like to find out whether there is a similar
> function in VHDL-AMS.  For a moment I thought I could
> use the T'succ(x) or related functions, but the problem
> is that this would only work on discrete or physical
> types.  The type "real" is not one of these.  Could
> someone give me some ideas how to go about this?
>
> Thanks, and Happy Holidays...
>
> Arpad
> ========================================================
>
>