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Re: problem 2.1.2 from "how to solve it by computer, Dromey"

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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

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problem 2.1.2 from "how to solve it by computer, Dromey" sathyashrayan <sathyashrayan@gmail.com> - 2012-07-28 19:24 -0700
  Re: problem 2.1.2 from "how to solve it by computer, Dromey" pete <pfiland@mindspring.com> - 2012-07-29 00:03 -0400
    Re: problem 2.1.2 from "how to solve it by computer, Dromey" pete <pfiland@mindspring.com> - 2012-08-01 12:26 -0400

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