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From: mnl@cernvax.UUCP (mnl)
Newsgroups: net.lang.c
Subject: Re: Boolean Operators Slighted in C (really swap with XOR)
Message-ID: <314@cernvax.UUCP>
Date: Thu, 15-May-86 22:11:08 EDT
Article-I.D.: cernvax.314
Posted: Thu May 15 22:11:08 1986
Date-Received: Sat, 17-May-86 04:13:12 EDT
References: <602@brl-smoke.ARPA> <2741@utcsri.UUCP>
Reply-To: mnl@cernvax.UUCP (Mark Nelson)
Organization: CERN, Geneva/Switzerland
Lines: 32

In article <2741@utcsri.UUCP> greg@utcsri.UUCP (Gregory Smith) writes:
>In article <602@brl-smoke.ARPA> rbj@icst-cmr (Root Boy Jim) writes:
>
>>3) Exchange operator. This has probably been discussed before, so pardon
>>	me if I blunder into that pit. BUT, DMR has stated that one of
>>	the reasons that exponentiation was left out was because no
>>	(or few, if any) machine could do it inline, and therefore
>>	all the compiler would be doing is a function call anyway.
>>	By the same reasoning, exchange CAN be done efficiently by
>>	some machines, and the hack of a temporary variable and
>>	three assignments is a rather stale idiom. The three
>>	statement XOR trick avoids the temporary, at the expense
>>	of execution time and clarity.
>>
>Good stuff. Even machines without an EXG can use a scratch register, which
>is better than using a user-supplied temp.
>What, by the way, is the XOR trick?
>
To swap the contents of A and B (no particular language):

A = A XOR B
B = A XOR B
A = A XOR B

On some machines, assuming A and B are in registers, this won't take
any more time than the normal three assignments using a temporary.
-- 
Mark Nelson

mnl@cernvax.bitnet or ...!seismo!mcvax!cernvax!mnl
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