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From: Hannah Schroeter on 13 Jul 2005 13:35
Hello!

Russell McManus <russell_mcmanus(a)yahoo.com> wrote:
>[...]

>I think this works:

>(defmacro xor (v1 v2)
> `(not (eq (not ,v1) (not ,v2))))

Use macros only if functions don't work.

(defun xor (v1 v2)
(not (eq (not v1) (not v2))))

>-russ

Kind regards,

Hannah.
From: Frank Buss on 13 Jul 2005 17:00
Christophe Rhodes wrote:

> 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.

a nice specification is given by the ext:xor function in CLisp:

| This function checks that exactly one of its arguments is non-NIL and,
| if this is the case, returns its value and index in the argument list
| as multiple values, otherwise returns NIL.

An implementation by me:

(defun xor (&rest list)
(when list
(destructuring-bind (first &rest rest) list
(if first
(when (every #'null rest) first)
(apply #'xor rest)))))

I'm sure it could be done easier :-)

--
Frank Buý, fb(a)frank-buss.de
http://www.frank-buss.de, http://www.it4-systems.de
From: Christophe Rhodes on 13 Jul 2005 17:04
Frank Buss <fb(a)frank-buss.de> writes:

> a nice specification is given by the ext:xor function in CLisp:
>
> | This function checks that exactly one of its arguments is non-NIL and,
> | if this is the case, returns its value and index in the argument list
> | as multiple values, otherwise returns NIL.

Why is this nice?

Christophe
From: Frank Buss on 13 Jul 2005 17:06
Frank Buss wrote:

>| This function checks that exactly one of its arguments is non-NIL and,
>| if this is the case, returns its value and index in the argument list
>| as multiple values, otherwise returns NIL.
>
> An implementation by me:

I forgot the index, so some standard loop magic:

(defun xor (&rest list)
(when list
(loop for (first . rest) on list
for i = 0 then (1+ i) do
(if first
(when (every #'null rest) (return (values first i)))))))

--
Frank Buý, fb(a)frank-buss.de
http://www.frank-buss.de, http://www.it4-systems.de
From: jtdubs on 13 Jul 2005 18:02
Philippe Lorin wrote:
> 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))))

Assuming v1 and v2 are boolean:
(xor v1 v2) == (not (eq v1 v2))
(xnor v1 v2) == (eq v1 v2)

So, in your case I would write:
(assert (eq (null v1) (null v2)))

As #'AND and #'OR are both macros that take any number of arguments,
I'm not sure that XOR really does fit in with them. However, it is
trivially implementable as a function because it must evaluate all of
its arguments.

Justin Dubs

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