Finding trailing 0s in a binary number

Question

How to find number of trailing 0s in a binary number?Based on K&R bitcount example of finding 1s in a binary number i modified it a bit to find the trailing 0s.

Answer 1

int countTrailZero(unsigned x) {
    if (x == 0) return DEFAULT_VALUE_YOU_NEED;
    return log2 (x & -x);  
}

Explanation:

x & -x returns the number of right most bit set with 1.

e.g. 6 -> "0000,0110", (6 & -6) -> "0000,0010"

You can deduct this by two complement: x = "a1b", where b represents all trailing zeros. then

-x = !(x) + 1 = !(a1b) + 1 = (!a)0(!b) + 1 = (!a)0(1...1) + 1 = (!a)1(0...0) = (!a)1b 

so

x & (-x) = (a1b) & (!a)1b = (0...0)1(0...0)

you can get the number of trailing zeros just by doing log2.