Path: utzoo!utgpu!water!watmath!clyde!bellcore!faline!ulysses!allegra!mit-eddie!uw-beaver!uw-june!pardo
From: pardo@uw-june.UUCP
Newsgroups: comp.lang.c
Subject: Re: "logical" xor
Message-ID: <3970@uw-june.UUCP>
Date: 11 Jan 88 23:25:14 GMT
References: <11207@brl-adm.ARPA> <20153@teknowledge-vaxc.ARPA>
Reply-To: pardo@uw-june.UUCP (David Keppel)
Organization: U of Washington, Computer Science, Seattle
Lines: 30
Posted: Mon Jan 11 18:25:14 1988


[ use "!=" for "^^" (logical xor) ]

Only works if the values you have are already 1/0 valued.  As with "^"
(bitwise xor)

int	x = 3,  y = 4;

	assert( x ^^ y == 0);
	assert( (x != y) == 1);


[ somebody else: logical xor is hard to optimize ]

True, in general, but the compiler (say, through flow analysis) may be
able to determine that one or both of the "^^" operands has just been
set by something that guarantees the result to be 0 or 1, in which case
the compiler can (safely!) reduce "^^" to "^".

I vote that

	if (a ^^ b) ...

is clearer than

	if ((a == 0) != (b == 0)) ...

or whatever the equivalent xor expression of choice happens to be.

	;-D on  (My favorite sintax: switch(x+=*a+++*b){...})  Pardo