brendan at mozilla.org
Thu Sep 18 16:52:21 PDT 2008
-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.
Sent from my iPhone
On Sep 18, 2008, at 7:08 PM, "Mark S. Miller" <erights at google.com>
> On Thu, Sep 18, 2008 at 8:00 AM, Mike Cowlishaw <MFC at uk.ibm.com>
> > 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.
> Es3.x-discuss mailing list
> Es3.x-discuss at mozilla.org
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