On 2009-07-15 13:29, Wayne Brehaut wrote:
On Tue, 14 Jul 2009 11:47:41 -0700 (PDT), Mark Dickinson
<dicki...@gmail.com> wrote:
On Jul 14, 7:25 pm, "Dr. Phillip M. Feldman"<pfeld...@verizon.net>
wrote:
Current Boolean operators are 'and', 'or', and 'not'. It would be nice to
have an 'xor' operator as well.
Hmm. I don't think 'nice' is sufficient. You'd need to make the case
that it's sufficiently useful to justify adding a new keyword 'xor' to
the language; I suspect that would be an uphill struggle. :)
I'll just note that:
(1) It's easy to emulate xor: 'x xor y'<-> bool(x) != bool(y)
(2) 'and' and 'or' are special in that they have useful short-
circuiting behaviour; xor doesn't have this property (that is, you
always need to evaluate *both* operands to determine the result).
I'd also guess that 'xor' would be much less used than 'and' or 'or',
but maybe that's just a reflection of the sort of code that I tend to
write.
You're right about that!. It's used everywhere in:
- coding and encryption theory (and practice) (e.g.,
http://www.mathcs.emory.edu/~whalen/Hash/Hash_Articles/IEEE/XOR-based%20hash%20functions.pdf)
- emulation and simulation of hardware (since all but the most trivial
logic circuits are likely to include XOR-gates)
- hence, for design of new architectures or simulators or virtual
machines and simplification of existing ones--(e.g.,
http://www.date-conference.com/archive/conference/proceedings/PAPERS/1999/DATE99/PDFFILES/05A_6.PDF)
and
http://bochs.sourceforge.net/Virtualization_Without_Hardware_Final.pdf
which includes:
"The answer relies on the observation that subtracting an integer
value from 0xFFFFFFFF gives the same result as XOR-ing that same value
to 0xFFFFFFFF."
And, perhaps the most useful use of all, for Bouton's solution of the
game of Nim--both for the proof that his strategy "solves" the game
and for an easy implementation of a Nim-playing program--and the only
operator needed is XOR (e.g., http://www.wordiq.com/definition/Nim).
All of those can use the bitwise XOR operator, not the boolean XOR. Python
already has the ^ operator for those purposes.
--
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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