Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!wuarchive!uunet!abvax!iccgcc!browns
From: browns@iccgcc.decnet.ab.com (Stan Brown, Oak Road Systems)
Newsgroups: comp.std.c
Subject: Re: Why no logical XOR operator
Message-ID: <913.26ef9d0e@iccgcc.decnet.ab.com>
Date: 13 Sep 90 19:51:53 GMT
References: <1990Sep12.154515.18460@druid.uucp>
Lines: 60

In article <1990Sep12.154515.18460@druid.uucp>, darcy@druid.uucp (D'Arcy J.M. Cain) writes:
>>from: darcy@druid.uucp (D'Arcy J.M. Cain)
> 
> Recently I had occasion to write something like the following code:
> 
> 	if ((!x && (y & z)) || (x && !(y & z))
> 		something();
> 
> Which would have been simpler as:
> 
> 	if (x ^^ (y & z))
> 		something();
> 
> If there was such a thing as a '^^' operator.  Does anyone know why C left
> out the logical XOR operator?


BTW, did you mean y & z or y && z?


My first thought was, What about 
    if ( x ? !(y&z) : (y&z) )
Hmm, still pretty ugly, though at least it doesn't repeat both tests.
I could write it
    if ( (y&z) ? !x : x )
to repeat only the shorter test.  However ...

Buzzing around in the back of my head, like a bluebottle at a dusty
window, was the idea of capitalizing on the fact that ! returns 1 or 0
always.  Okay, I thought, y&z may as well be replaced by a single
variable, so set yz for y&z (or y&&z).  Now the expression is

    if ( (!x && yz) || (x && !yz) )

A quick truth table, using 1 to mean "anything non-zero".

      x   yz    !x   !yz    desired result
      1   1     0     0     0
      1   0     0     1     1
      0   1     1     0     1
      0   0     1     1     0

Hmm, looks mighty familiar.  In fact the desired result is !x ^ !yz, or in
the original terms of the problem,

     if ( !x ^ !(y & z) )

So the reason there's no "logical exclusive or" is that bitwise exclusive
ORing the negated quantities gives the same result.

BTW, (1) I'd probably comment that !^! construction, because it looks
weird, and (2) I'd ask somebody else to review it to make sure I didn't
have some dumb goof.  (3) On proofreading this posting, I think I'd
probably go back to my first thought, above, because IMHO it's clearer.  


Stan Brown, Oak Road Systems, Cleveland, Ohio, U.S.A.         (216) 371-0043
The opinions expressed are mine. Mine alone!  Nobody else is responsible for
them or even endorses them--except my cat Dexter, and he signed the power of
attorney only under my threat to cut off his Cat Chow!