Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!mailrus!csd4.csd.uwm.edu!cs.utexas.edu!romp!shaffer!jon
From: jon@shaffer.UUCP (Jon Doran/60000)
Newsgroups: comp.lang.c
Subject: Re: swap(x,y)
Summary: exclusive-or will swap without temp variables
Message-ID: <2578@shaffer.UUCP>
Date: 24 Aug 89 15:01:04 GMT
References: <MYwDEFy00WB842JEUx@andrew.cmu.edu>
Organization: IBM AWD, Austin, TX
Lines: 25

In article <MYwDEFy00WB842JEUx@andrew.cmu.edu>, tg1e+@andrew.cmu.edu (Timothy R. Gottschalk) writes:
> 
>        To swap two variables x,y in C without using a temporary variable:
> 
>        /* begin swap */
>        x += y;
>        y = x - y;
>        x -= y;
>        /* end swap */

I don't know about that, seems that overflow is a possibility...

Something that I've used (mainly in assembly code) is:
	x ^= y;
	y ^= x;
	x ^= y;

To verify that the above works (x and y must be the same size of a variable...)
choose appropriate values for x and y, represent these in binary on paper,
then perform the exclusive ors manually.  This is perhaps the best explaination.



Jon Doran
IBM AWD,  Austin TX