Path: utzoo!mnetor!uunet!husc6!cmcl2!beta!unm-la!unmvax!nmtsun!hydrovax
From: hydrovax@nmtsun.nmt.edu (M. Warner Losh)
Newsgroups: comp.lang.c
Subject: Re: comma operator
Message-ID: <1239@nmtsun.nmt.edu>
Date: 22 Jan 88 00:42:05 GMT
References: <3819@sigi.Colorado.EDU> <5080013@hpfcdc.HP.COM> <7120@brl-smoke.ARPA> <3887@sigi.Colorado.EDU>
Organization: NMT Hydrology program
Lines: 27
Summary: It's in lots of books, OSs, etc.

In article <3887@sigi.Colorado.EDU>, swarbric@tramp.Colorado.EDU (SWARBRICK FRANCIS JOHN) writes:
> #define swap(a,b) ((a) = ((b) = ((a) = (a) ^ (b)) ^ (b)) ^ (a))
> 
> Try that, and tell me what you think.  A super-genius friend of mine figured
> it out.  He says it will work for anything (int, char *, etc.).  But it
> sure boggles my mind...
It is a simple mathematical propterty of XOR.  That is :
    a = a xor b
    b = a xor b
    a = b xor a

swaps the values of A and B.  My opsys book had that in it.  A systems
programming course offered here at NMT has that as one of the home
work problems (at least the time I took it).  You can find this device
in lots of places.  The first place that I saw it was in RSTS/E COMMOM.MAC
file, where it was used to swap two registers, w/o the need for a temp
storage.




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