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). wayne -- http://mail.python.org/mailman/listinfo/python-list
