Count bits used in int

Question

If you have the binary number 10110 how can I get it to return 5? e.g a number that tells how many bits are used? There are some likewise examples listed below:

Answer 1

Be careful what you ask for. One very fast technique is to do a table lookup:

int bittable [] = {0, 1, 1, 2, 1, 2, 2, 3, 1, 2, ... };
int numbits (int v)
{
    return bittable [v];
}

where bittable contains an entry for every int. Of course that has complications for negative values. A more practical way would be to count the bits in bitfields of the number

int bittable [16] = {0, 1, 1, 2,  1, 2, 2, 3,  1, 2, 2, 3,  2, 3, 3, 4};
int numbits (int v)
{
    int s = 0;
    while (v != 0)
    {
         s += bittable [v & 15];
         v >>= 4;
    }
    return s;
}