How to xor?
in [Lisp]
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From: Philippe Lorin on 13 Jul 2005 05:04 I'm looking for a XOR to use like I use AND and OR. But the only things I found in the CLHS are: BIT-XOR, which works on bit arrays BOOLE-XOR, which, when used with BOOLE, works on integers LOG-XOR, which works on integers CLisp seems to have XOR, but CMUCL and SBCL don't. Isn't there some standard function for this? Or am I wrong in wanting one? Background: I need to check that two values are either both null or both non-null, and not a mix of null and non-null. I'd like to write: (assert (not (xor (null v1) (null v2))))
From: Christophe Rhodes on 13 Jul 2005 05:10 Philippe Lorin <palpalpalpal(a)gmail.com> writes: > CLisp seems to have XOR, but CMUCL and SBCL don't. Isn't there some > standard function for this? No. > Or am I wrong in wanting one? You're not necessarily wrong, but note that AND and OR are macros, not functions, because they short-circuit; also, they take arbitrary numbers of arguments, not just two; so it's not clear that XOR actually fits in with AND and OR. > Background: I need to check that two values are either both null or > both non-null, and not a mix of null and non-null. I'd like to write: > (assert (not (xor (null v1) (null v2)))) (assert (if v1 v2 (not v2))) This is easily wrapped up into a function if you feel the need. Christophe
From: Frank Buss on 13 Jul 2005 05:41 Philippe Lorin wrote: > Background: I need to check that two values are either both null or both > non-null, and not a mix of null and non-null. I'd like to write: > (assert (not (xor (null v1) (null v2)))) what about (eq (null v1) (null v2)) ? -- Frank Buý, fb(a)frank-buss.de http://www.frank-buss.de, http://www.it4-systems.de
From: Philippe Lorin on 13 Jul 2005 07:55 Frank Buss wrote: > what about (eq (null v1) (null v2)) ? This one is nice, it conveys the intention perfectly. Thanks!
From: Russell McManus on 13 Jul 2005 08:02
Philippe Lorin <palpalpalpal(a)gmail.com> writes: > I'm looking for a XOR to use like I use AND and OR. But the only > things I found in the CLHS are: > BIT-XOR, which works on bit arrays > BOOLE-XOR, which, when used with BOOLE, works on integers > LOG-XOR, which works on integers > > CLisp seems to have XOR, but CMUCL and SBCL don't. Isn't there some > standard function for this? Or am I wrong in wanting one? I think this works: (defmacro xor (v1 v2) `(not (eq (not ,v1) (not ,v2)))) -russ |