How to add two variables without using a '+' operator?

Question

I was asked this question in an interview:

How to add two variables without using a '+' operator?

The interviewer asked me and I could not answer, even though I am a good C programmer!

Voting to close a C question as a duplicate of a C# question is not correct. — 2501, Oct 14 '16 at 11:33
@coredump did the same thing but he insisted me not to do that !! — Anjaneyulu, Oct 14 '16 at 12:01
@Anjaneyulu Unfortunately, this position required you to be able to perform mind reading. — coredump, Oct 14 '16 at 12:46
@Matt That is undefined behaviour, as it has there is unsequenced side-effects on variables that are read on multiple subexpressions. Or to put it short: your expression is broken and illegal. — user694733, Oct 14 '16 at 13:13
See: [sum (adding 2 numbers ) without plus operator](http://stackoverflow.com/q/17450959/253056) and [Adding two numbers without using operators](http://stackoverflow.com/q/12331328/253056) — Paul R, Oct 14 '16 at 13:49

Stubborn · Answer 1 · 2016-10-14T13:42:47.183

Using bitwise operators you can add two numbers. Try below:

   int Sum(int a, int b)
    {
        // Iterate till there is no carry  
        while (b != 0)
        {
            // now carry contains common set bits of a and b
            int carry = a & b;  

            // Sum of bits of a and b where at least one of the bits is not set
            a = a ^ b; 

            // Carry is shifted by one so that adding it to a gives the required sum
           b = carry << 1;
        }
        return a;
    }

Using Increment and decrement operators you can add two numbers. The other way may be like:

int Sum(int a, int b)
{
    // Iterate till there b becomes zero
    while (b--)
    {
        a++;
    }
    return a;
}

How to add two variables without using a '+' operator?

1 Answers1