float("nan") in set or as key

[Erik Max Francis]

Gregory Ewing wrote:
> Steven D'Aprano wrote:
>> Fair point. Call it an extension of the Kronecker Delta to the reals 
>> then.
> 
> That's called the Dirac delta function, and it's a bit different --
> instead of a value of 1, it has an infinitely high spike of zero
> width at the origin, whose integral is 1. (Which means it's not
> strictly a function, because it's impossible for a true function
> on the reals to have those properties.)
> 
> You don't normally use it on its own; usually it turns up as part
> of an integral. I find it difficult to imagine a numerical algorithm
> that relies on directly evaluating it. Such an algorithm would be
> numerically unreliable. You just wouldn't do it that way; you'd
> find some other way to calculate the integral that avoids evaluating
> the delta.

True, but that's the Dirac delta, which as you (and later he) said, is 
quite a different thing, not simply a Kronecker delta extended to the 
reals.  Kronecker deltas are used all the time over the reals; for 
instance, in tensor calculus.  Just because the return values are either 
0 or 1 doesn't mean that their use is incompatible over reals (as 
integers are subsets of reals).

-- 
Erik Max Francis && max at alcyone.com && http://www.alcyone.com/max/
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