What would be most efficient way to find number of 1 in a variables?

Question

Input: 23678210
Output: 8

Answer 1

Use bit wise and operator (&) with 1 on you number. Do this on each bit of your number.
Here is my C program (Tested).

#include <stdio.h>

int num_ones_present(int num)
{
    int i = 0;

    while (num) {
        i += (num & 1);  //It will return 1 if num's 0th bit is 1 else return 0.
        num >>= 1;
    }

    return i;
}

int main()
{ 
    int num, next_num, i = 0;
    int num_ones = 0;

    printf("Enter the number : ");
    scanf("%d", &num);

    num_ones = num_ones_present(num);
    printf("Then given number is : %d which has %d num of 1's\n", num, num_ones);

    return 0;
}

Happy Coding :)

Answer 2

int main()
{
   int count = 0,x;
   printf("Enter Number\n");
   scanf("%d",&x);
   while(x)
   {
      count++;
      x = x & x-1;
   }
   printf("Number of on bits[%d]\n", count);
   return 0;
}