Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!csd4.milw.wisc.edu!lll-winken!uunet!mcvax!ukc!etive!hwcs!zen!frank
From: frank@zen.co.uk (Frank Wales)
Newsgroups: comp.unix.questions
Subject: Re: Re^2: Bit Switching - How?
Message-ID: <1561@zen.co.uk>
Date: 11 Apr 89 09:10:16 GMT
References: <1138@uvm-gen.UUCP> <9977@smoke.BRL.MIL> <2345@van-bc.UUCP> <2255@solo5.cs.vu.nl> <1574@lznv.ATT.COM> <1558@zen.UUCP>
Reply-To: frank@zen.co.uk (Frank Wales)
Organization: Zengrange Limited, Leeds, England
Lines: 33

In article <1558@zen.UUCP> I wrote:
>In article <1574@lznv.ATT.COM> jlw@lznv.ATT.COM (J.L.WOOD) writes:
>>> sl@van-bc.UUCP (pri=-10 Stuart Lynne) writes:
>>> \For fast generic swapping I use:
>>> 
>>> \#define	swap(a,b) {a^=b;b^=a;a^= b;}
>>> 
>>
>>Also be careful how you use this algorithm if you are using pointers.
>>I once used this method in a sorting routine as my basic exchange and
>>got badly burned.
>>
>>If a and b point to the same location, you get zero as in 0, nada, goose egg.
>
>Well, this surprised me, 
[example of why I thought there was no problem deleted]

>I'd be interested to find out the machine on which this XOR trick
>fails to swap integral values correctly.  [I hardly ever use it, but
>I'd still expect it to work.]

And indeed it does, except when a and b are the *same* (not the same
value, but the same object), because the first a^=b toasts both a and b,
and you've lost your recovery information.

>Or is there some portability problem
>lurking here that my Monday morning mind has missed?

Well, it's Tuesday now -- thanks to those who mailed me with a wake-up
call [I did say I hardly used the trick myself; good job, too ;-)].
--
Frank Wales, Systems Manager,        [frank@zen.co.uk<->mcvax!zen.co.uk!frank]
Zengrange Ltd., Greenfield Rd., Leeds, ENGLAND, LS9 8DB. (+44) 532 489048 x217