How to swap two variables without a temp variable?

This was one of those weird questions that got popped at me during a recent recruitment process. I tried to attempt it using XOR logic which I had read at a c programming tutorial page. The XOR truth table is unique in the sense that when both inputs are opposite (i.e a=true/ON and b=false/OFF), then the output is true or ON. Otherwise, the output is false or OFF.

The truth table for XOR (Exclusive OR) is shown below:

A	B	O/P
0	0	0
1	0	1
0	1	1
1	1	0

The solution is as follows.

Assume int a = 5; and int b = 3;

Now to swap them using XOR, one has to do the following

a = a xor b;  // 5 ^ 3 = 6

b = a xor b; // 6 ^ 3 = 5

a = a xor b; //6 ^ 5 = 3

That’s it! we have swapped the two variables without using a temp variable. We saved resources when we could have easily wasted a few more bytes! Now, there’s an other way to do it. Why would you need a XOR when you could do it with the common ‘+’? Well, I needed some assistance to figure out it was the same logic to do a swap with the common and more preferred additive ‘+’ operator. Here is the solution.

a = a + b;  //a = 8 (sum of the two variables)

b = a - b; // b = 8 - 3 = 5

a = a - b; //a = 8 - 5 = 3

Phew! At the end of it, I could not but wonder what weird questions get popped at you during an interview. Hope you enjoy your next one!

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This entry was posted on Tuesday, February 10th, 2009 at 1:59 am and is filed under C Programming. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.

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