Interview Question for Software Engineer In Test at Google:

int numBits(int num)
{
   int numBits = 0;
   while(num > 0)
   {
      if(num % 2) numBits++;
      num /= 2;
   }
   return numBits;
}

The question stipulates "a number", but the code assumes "a positive integer". (It also assumes the number expressed in binary has a C compiler's default number of bits for type int (e.g. 32); that's probably acceptable since a real-world case would likely specify that type. ) Consider:

/* return n of set bits in a signed int */
int numBits(int num) {
    int numBits = 0;
    while (num != 0) {
        if (num < 0) ++numBits;
        num <<= 1;
    }
    return numBits ;
}