Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!csd4.csd.uwm.edu!mrsvr.UUCP
From: davej@mrsvr.UUCP (David Johnson x4-6506)
Newsgroups: comp.lang.c
Subject: Re: swap(x,y)
Message-ID: <906@mrsvr.UUCP>
Date: 22 Aug 89 19:24:00 GMT
References: <MYwDEFy00WB842JEUx@andrew.cmu.edu>
Sender: news@mrsvr.UUCP
Lines: 34

From article <MYwDEFy00WB842JEUx@andrew.cmu.edu>, by tg1e+@andrew.cmu.edu (Timothy R. Gottschalk):
= 
=        To swap two variables x,y in C without using a temporary variable:
= 
=        /* begin swap */
=        x += y;
=        y = x - y;
=        x -= y;
=        /* end swap */
= 
=        Just curious...(this looks pretty valid) does anyone know an even
= simpler method?  I would never program this way -- it's more of a theory
= question.  I've been told that it can't be done (???).
= Tim Gottschalk
= Pgh, PA

This code should work fine for numeric x,y that aren't TOO large (so that
the first line does not cause overflow - though even that may work (depending
on your particular compiler)) or for float x,y that are not excessively
far apart in precision.  If you set x = 1.0e+09 and y = 1.0e-8, the swap code
above results in x = 0 and y = 1.0e+09.  Also, low-order bits could be
lost due to rounding/truncation of float's.

A more intriguing (and less obvious) way is to do:
  /* begin swap */
  x ^= y;
  y ^= x;
  x ^= y;
  /* end swap */

Alas, in C, the bitwise OR only works with int's and char's :-(.               
Of course, you can type-cast, but it doesn't look as neat.  

Dave Johnson - Computer People Unlimited @ GE Medical Systems.