This month’s Exercise challenges you to swap two variables’ values without using a third variable. It’s a solution widely available on the Internet, but your job is to figure out the technique without looking elsewhere.

I confess that I saw this solution months ago and marveled at it. But I forgot the specifics. Rather than look it up again, I set out to devise it on my own, using only my vague memory of the mathematical operations used on the two variables to swap values. Here are the three statements I use:

b = b + a;
a = b - a;
b = b - a;

Yes, it took me a while to hone this result, which works for both signed and unsigned values. Figure 1 helps illustrate how the operation works.

Figure 1. Swapping two variables’ values by using only two variables.

Effectively, variable b becomes what would otherwise be swapping variable c. First it holds the sum of a and b: b = b + a

When original a is subtracted, what’s left over is b, which is assigned to a: a = b - a

Finally, the new value of a (original b) is subtracted from new b, which yields the original value of a, assigned to b: b = b - a

It took my brain a few minutes to accept this solution. I even tried to condense it to only two statements, but either I’m not that smart or such a solution isn’t possible. Regardless, here is the full solution:

2023_06-Exercise.c

#include <stdio.h>

int main()
{
    int a,b;

    printf("Enter value A: ");
    scanf("%d",&a);
    printf("Enter value B: ");
    scanf("%d",&b);

    printf("Before: A=%d, B=%d\n",a,b);

    /* swap variables */
    b = b + a;
    a = b - a;
    b = b - a;

    printf("After: A=%d, B=%d\n",a,b);

    return(0);
}

After writing this code, I checked the interwebs to see what I found earlier, the inspiration for this Exercise. Yep, I got it right. I hope you did as well.