Path: utzoo!mnetor!uunet!husc6!cmcl2!rutgers!mtune!codas!killer!jfh
From: jfh@killer.UUCP (The Beach Bum)
Newsgroups: comp.lang.c
Subject: Re: "logical" xor
Message-ID: <2796@killer.UUCP>
Date: 11 Jan 88 19:43:22 GMT
References: <2946@zeus.TEK.COM> <195@fxgrp.UUCP>
Organization: Big "D" Home for Wayward Hackers
Lines: 51
Summary: Here it is ...
         ... the LOGICAL XOR macro.

In article <195@fxgrp.UUCP>, ljz@fxgrp.UUCP (Lloyd Zusman, Master Byte Software) writes:
> In article <2946@zeus.TEK.COM> dant@tekla.UUCP (Dan Tilque) writes:
> > ...
> >Tell me, what is the ^^ supposed to add that is not already done by ^ ?
> >The answer is nothing.  The "bitwise" xor does exactly what you would want
> >a "logical" xor to do.  In fact, the terms "bitwise" and "logical" are
> >misnomers.  They should be "unoptimized" and "optimized".  And there's no
> >way to optimize an exclusive-or operation.

Only half true.  Actually, not true at all.  | and & operate on bits.  Calling
boolean operators an "optimized" version of a bit operator is a lot like
calling a type definition an "optimized" procedure call.

> >The only reason to add ^^ is esthetic.  ...
> > ...

True here.  The correct result can be had with little effort.  I would say
leaving out ^^ is at most being incomplete.

> I'm sorry, but I disagree.  Try running this program:
> 
[ eaten by the line eater ]
>  Lloyd Zusman

Here is the macro for you impatient people:

#define	XOR(a,b)	(((a)==0)!=((b)==0))

Unless I am missing something very subtle in the casting that will go on
if the macro is call with floating point expressions, this thing should
work like a dream.  The != operator has the same truth table as ^^ would,
if handed boolean values (ie, 0 and 1).

Proof:

	a	b	^^	!=
	0	0	0	0
	0	1	1	1
	1	0	1	1
	1	1	0	0

Testing a and b for equality against 0 should produce two boolean values,
or, given a good compiler, a couple of test and branch statements.  My only
question is about the casting.

- John.
-- 
John F. Haugh II                  SNAIL:  HECI Exploration Co. Inc.
UUCP: ...!ihnp4!killer!jfh                11910 Greenville Ave, Suite 600
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 ... Then Buy One!"                       (214) 231-0993 Ext 260