use decimal

Mark S. Miller erights at google.com
Thu Sep 18 18:03:16 PDT 2008


On Thu, Sep 18, 2008 at 5:22 PM, Maciej Stachowiak <mjs at apple.com> wrote:

>
> On Sep 18, 2008, at 5:13 PM, Mark S. Miller wrote:
>
> On Thu, Sep 18, 2008 at 4:52 PM, Brendan Eich <brendan at mozilla.org> wrote:
>
>> -0 and 0 are not the same "given floating point number". 1/-0 vs. 1/0 and
>> Math.atan2(-0,0) vs. 0,0 are but two examples.
>>
>
> Yes, I understand their operational difference. Whether that difference
> means they are not the same number depends on how you define "number". I
> would have defined "representing the same number" by correspondence to the
> reals. YMMV.
>
>
> But the mathematical behavior of floating point numbers is not the same as
> that of the reals. Not just because of -0. See <
> http://docs.sun.com/source/806-3568/ncg_goldberg.html>
>
> Floating point numbers are a useful rough approximation of the reals for
> many practical calculations, but treating them as if they were in fact a
> subset of the reals is not mathematically sound.
>


Long long ago I actually had read that document carefully, and I had also
looked at I think the [Brown 1981] which it cites. (But the doc has no
bibliography. Anyone have a pointer?) My memory of the theory of floating
point is that the numbers are exact but the operations are approximate.

* A given finite floating point value exactly represents a given real
number.

* The subset of real numbers which are exactly representable in a given
format are its "representable numbers".

* The standards require various well behaved properties like:

    Let Tr = B2R(Xb) +r B2R(Yb) // Tr stands for True sum as a real number
    If representable(Tr) then Tr ===r B2R(Xb +b Yb)
    Else (greatest representable number <r Tr) <r B2R(Xb +b Yb) <r (smallest
representable number >r Tr)

Since we have many people on this list much more expert on this than I, if
the above summary is mistaken, I'm sure someone will correct.

-- 
Cheers,
--MarkM
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