Difficulty: ★ ★ ★ ☆

The classic paradigm for swapping values between two variables involves using a third variable. But is the third variable even necessary?

For example, to swap a and b, you use variable c:

c = a;
a = b;
b = c;

At the end of the operation, the values of variables a and b are swapped, with variable c left with nothing else to do. But is variable c even necessary?

It turns out you need only two variables to swap their values — odd as it sounds. The process involves a bit of math and some head scratching, but it works. Rather than rush to StackOverflow for the answer, try on your own to think of a way to swap two variables’ values, which is the challenge for this month’s Exercise.

Here’s a skeleton to get you started:

2023_06_01-Lesson.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 */

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

    return(0);
}

As the code stands now, two values are fetched from standard input. Both are output, then comes the swap (the part you write) and the results are output:

Enter value A: 10
Enter value B: 45
Before: A=10, B=45
After: A=10, B=45

Your challenge is to add code after the comment, fancy math stuff that swaps the values without adding a third variable. In fact, as a hint, you need three lines of code (three statements) just as if you were using a third variable.

Here is the output from my solution:

Enter value A: 10
Enter value B: 45
Before: A=10, B=45
After: A=45, B=10

Please O please do try this exercise on your own. The solution is widely available on the interwebs, but I had a good time trying to make the swap happen without cheating. You can view my solution here.