Path: csiph.com!v102.xanadu-bbs.net!xanadu-bbs.net!news.glorb.com!npeer01.iad.highwinds-media.com!news.highwinds-media.com!feed-me.highwinds-media.com!border3.nntp.dca.giganews.com!Xl.tags.giganews.com!border1.nntp.dca.giganews.com!nntp.giganews.com!local2.nntp.dca.giganews.com!nntp.earthlink.com!news.earthlink.com.POSTED!not-for-mail
NNTP-Posting-Date: Wed, 01 Aug 2012 11:26:21 -0500
Message-ID: <501958AC.5261@mindspring.com>
Date: Wed, 01 Aug 2012 12:26:20 -0400
From: pete <pfiland@mindspring.com>
Reply-To: pfiland@mindspring.com
Organization: PF
X-Mailer: Mozilla 3.04Gold (WinNT; I)
MIME-Version: 1.0
Newsgroups: comp.programming
Subject: Re: problem 2.1.2 from "how to solve it by computer, Dromey"
References: <e531d211-e0ad-47d5-8b68-542b3fef915f@googlegroups.com> <5014B5FE.2FC3@mindspring.com>
Content-Type: text/plain; charset=us-ascii
Content-Transfer-Encoding: 7bit
Lines: 86
X-Usenet-Provider: http://www.giganews.com
NNTP-Posting-Host: 4.154.221.138
X-Trace: sv3-XXh5XrA57P1Jer3pSiIOdqKY7jKAwfSGeWWfAsdLsLK+Jdj9O2K/G42ZBKaN4/65rzNlGLCTpYWKh/l!kHcLWDLSqSWw7Pi7ck/TErOv7PJ1HoSwdPFpufx3dQVtvjgTwGbwMHFz6LTY4PWrbYtd+2OBv2kk!VkYdoCjUaCE=
X-Abuse-and-DMCA-Info: Please be sure to forward a copy of ALL headers
X-Abuse-and-DMCA-Info: Otherwise we will be unable to process your complaint properly
X-Postfilter: 1.3.40
X-Original-Bytes: 2647
X-Received-Bytes: 2788
Xref: csiph.com comp.programming:2029

pete wrote:
> 
> sathyashrayan wrote:
> >
> > I am a beginner in learning algo..So please bear with me..
> >
> > 2.1.2 Design an algorithm
> > that makes following exchanges of 3 varioubles..
> >
> > a-->b-->c--|
> > ^          |
> > ^----------|
> >
> > So
> > a_temp = a;
> > b_temp = b;
> > c_temp = c;
> >
> > a=c_temp;
> > c=b_temp;
> > b=a_temp;
> >
> > I know this process looks ugly.
> > Can anyone suggest a good one so that I could learn.
> 
> temp = c
>    c = b
>    b = a
>    a = temp

If you were in a situation where you might
not be guaranteed to have a temp variable
suitable for the values of the variables in question,
but if you had a way to swap values between variables,
then something like this would work:

SWAP(b, c);
SWAP(a, b);

In C, that could look like this:

/* BEGIN new.c */

#include <stdio.h>

#define SWAP(X, Y) (swap(&X, &Y, sizeof X))

void swap(void *arg1, void *arg2, size_t size)
{
    unsigned char temp;
    unsigned char *a1 = arg1;
    unsigned char *a2 = arg2;

    while (size-- != 0) {
        temp  = *a1;
        *a1++ = *a2;
        *a2++ = temp;
    }
}

int 
main(void)
{
    int a = 1;
    int b = 2;
    int c = 3;

    printf("(a) is %d\n", a);
    printf("(b) is %d\n", b);
    printf("(c) is %d\n", c);

    putchar('\n');
    
    SWAP(b, c);
    SWAP(a, b);

    printf("(a) is %d\n", a);
    printf("(b) is %d\n", b);
    printf("(c) is %d\n", c);
    return 0;
}

/* END new.c */

-- 
pete