use decimal

Mark S. Miller erights at google.com
Thu Sep 18 16:08:11 PDT 2008


On Thu, Sep 18, 2008 at 8:00 AM, Mike Cowlishaw <MFC at uk.ibm.com> wrote:

>
> >
> > Are -0 and 0 in the same cohort?
>
> In IEEE 754, no:
>
>   *2.1.10 cohort: *The set of all floating-point representations that
> represent a given
>     floating-point number in a given floating-point format. In this context
> −0 and +0
>     are considered distinct and are in different cohorts.
>
> (+0 and -0 are distinguishable in binary FP as well as in decimal FP; in
> fact this is the only case in binary where two finite numbers have the same
> value but different representations and encodings.)
>
>
I don't understand this definition of cohort. It seems to contradict itself.
The first sentence implies that -0 and 0 are in the same cohort -- since
they are different representations of the same number in the same given
floating point format. Should I read the second sentence as a clarification
-- in which case it seems inconsistent. Or should I read it as a
qualification on the first sentence, saying in effect "well, except for -0
and 0, which are just weird".

Given that -0 and 0 are in different cohorts, let me probe the extent to
which cohorts imply something like operational equivalence. For any
arithmetic operator OP and any decimal floating point values X1,X2,Y1,Y2, is
it the case that

    cohort(X1,X2) & cohort(Y1,Y2) implies cohort(X1 OP Y1,X2 OP Y2)

? I say "arithmetic" above in order to include operations like "+" but not
".toString()".

Clearly, if -0 and 0 were cohorts, this would not hold. Since they aren't,
does the above property hold for all decimal floating point values? If so, I
withdraw the proposal to generalize cohort. In which case I agree that Sam's
current proposal + an agreement never to supply compareTotal() (so that
decimal can have a single NaNm) is the best proposal on the table.

-- 
Cheers,
--MarkM
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