Rhamphoryncus wrote:
On Dec 7, 4:20 pm, Steven D'Aprano <[EMAIL PROTECTED]
cybersource.com.au> wrote:
On Sun, 07 Dec 2008 15:32:53 -0600, Robert Kern wrote:
Rasmus Fogh wrote:
Current behaviour is both inconsistent and counterintuitive, as these
examples show.
x = float('NaN')
x == x
False
Blame IEEE for that one. Rich comparisons have nothing to do with that
one.
There is nothing to blame them for. This is the correct behaviour. NaNs
should *not* compare equal to themselves, that's mathematically
incoherent.
Mathematically, NaNs shouldn't be comparable at all. They should
raise an exception when compared. In fact, they should raise an
exception when *created*. But that's not what we want. What we want
is a dummy value that silently plods through our calculations. For a
dummy value it seems a lot more sense to pick an arbitrary yet
consistent sort order (I suggest just above -Inf), rather than quietly
screwing up the sort.
Well, there are explicitly two kinds of NaNs: signalling NaNs and quiet NaNs, to
accommodate both requirements. Additionally, there is significant flexibility in
trapping the signals.
Regarding the mythical IEEE 754, although it's extremely rare to find
quotations, I have one on just this subject. And it does NOT say "x
== NaN gives false". It says it gives *unordered*. It is C and
probably most other languages that turn that into false (as they want
a dummy value, not an error.)
http://groups.google.ca/group/sci.math.num-analysis/browse_thread/thread/ead0392e646b7cc0/a5bc354cd46f2c49?lnk=st&q=why+does+NaN+not+equal+itself%3F&rnum=3&hl=en&pli=1
Table 4 on page 9 of the standard is pretty clear on the subject. When the two
operands are unordered, the operator == returns False. The standard defines how
to do comparisons notionally; two operands can be "greater than", "less than",
"equal" or "unordered". It then goes on to map these notional concepts to
programming language boolean predicates.
--
Robert Kern
"I have come to believe that the whole world is an enigma, a harmless enigma
that is made terrible by our own mad attempt to interpret it as though it had
an underlying truth."
-- Umberto Eco
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