Re: [Boost-users] double arithmetic precession and equality

From: Mike Marchywka (marchywka_at_[hidden])
Date: 2007-12-20 07:41:48

>So does boost have anything for this I don't know of?
> Or any other libraries?
>

Yeah, I think the question is, " what are you really trying to do?"
Even with Java's repeatable-to-the-last-bit floating point spec, this would have only
mattered to me once in writing an arithmetic encoder as , in this example, debug decoders
would not work with release encoders. Then, I guess the questions would be,
" what other abstractions would better address my objectives?"

Close is supposed to be good enough for floating point and I wouldn't
even rely on a given test branching reproducibly. For example, let's
say you are trying to follow a signal and have a locked and unlocked state
and look at behaviour as you try to obtain lock. 
Presumably, the output would vary a little if you chose unlocked for an extra
cycle or two but who would care? You really have to look at floating point
as not-so-deterministic. 

If you are tracking money or something, ints or strings would be better.

> Date: Wed, 19 Dec 2007 20:47:47 -0800
> From: mavdzee_at_[hidden]
> To: Boost-users_at_[hidden]
> Subject: [Boost-users] double arithmetic precession and equality
>
> Hi,
>
> I was wondering if boost provides any support for
> testing equality of two doubles. In principle in C/C++
> you cannot rely on equality because it uses bitwise
> comparison of doubles and in arithmetic expressions
> results may have lost some precession. An example will
> help:
>
> #include 
>
> int main(void)
> {
> double start = 3.123;
> double end = 4.124;
> double diff = end - start; // == 1.001 ???
>
> if (diff != 1.001)
> {
> std::cout << diff << " and " << 1.001 << " are not
> equal" << std::endl;
> }
> }
>
> On my machine, this example outputs:
> 1.001 and 1.001 are not equal
>
> I am using a lot of arithmetic expressions on
> floating-point values and also need to test for
> equality. Also, at compile-time I do not know the
> precession so comparing within the range of an epsilon
> seems impossible.
>
> So does boost have anything for this I don't know of?
> Or any other libraries?
>
> Thanks,
> Andrej
>
>
>
> ___________________________________________________________
> Yahoo! Answers - Got a question? Someone out there knows the answer. Try it
> now.
> http://uk.answers.yahoo.com/
> _______________________________________________
> Boost-users mailing list
> Boost-users_at_[hidden]
> http://lists.boost.org/mailman/listinfo.cgi/boost-users

_________________________________________________________________
Share life as it happens with the new Windows Live.
http://www.windowslive.com/share.html?ocid=TXT_TAGHM_Wave2_sharelife_122007