Path: utzoo!attcan!uunet!cs.utexas.edu!tut.cis.ohio-state.edu!mailrus!ames!uhccux!munnari.oz.au!mimir!hugin!augean!sirius!nt!levels!ccdn
From: ccdn@levels.sait.edu.au (DAVID NEWALL)
Newsgroups: comp.lang.c
Subject: Re: Obfuscated SWAP
Message-ID: <1267@levels.sait.edu.au>
Date: 25 Aug 89 10:55:06 GMT
References: <784@skye.ed.ac.uk>
Followup-To: comp.lang.c
Organization: Sth Australian Inst of Technology
Lines: 32

In article <784@skye.ed.ac.uk>, ken@aiai.ed.ac.uk (Ken Johnson) writes:
> Well, I tried it and it works
>
>       x ^= y ^= x ^= y;       /* Swap X and Y over */
>
> I don't understand how it works


The intent of this code is clear.  It is intended to work like this:

Given the variables x and y, which have initial values a and b respectively.
Then:

       x ^= y ^= x ^= y
       |    |    |    |
       |    |    |    x = a, y = b
       |    |    |
       |    |    x = a ^ b, y = b
       |    |
       |    y = b ^ (a ^ b) = (b ^ b) ^ a = a, x = a ^ b
       |    => y = a, x = a ^ b
       |
       x = (a ^ b) ^ a = (a ^ a) ^ b = b, y = a
       => x = b, y = a

(You realise, of course, that ^ is the exclusive or operator)


David Newall                     Phone:  +61 8 343 3160
Unix Systems Programmer          Fax:    +61 8 349 6939
Academic Computing Service       E-mail: ccdn@levels.sait.oz.au
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