Count the number of set bits in an integer [duplicate]

Answer 1

Conceptually this works:

int numones(int input) {
    int num = 0;
    do {
        num += input % 2;
        input = input / 2;
    } while (input > 0);
    return num;
 }

A more optimized way (from commenters link above):

unsigned int v; // count the number of bits set in v
unsigned int c; // c accumulates the total bits set in v
for (c = 0; v; c++)
{
  v &= v - 1; // clear the least significant bit set
}