Find nth SET bit in an int

Question

Instead of just the lowest set bit, I want to find the position of the nth lowest set bit. (I\'m NOT talking about value on the nt

Answer 1

def bitN (l: Long, i: Int) : Long = {
  def bitI (l: Long, i: Int) : Long = 
    if (i == 0) 1L else 
    2 * { 
      if (l % 2 == 0) bitI (l / 2, i) else bitI (l /2, i-1) 
    }
  bitI (l, i) / 2
}

A recursive method (in scala). Decrement i, the position, if a modulo2 is 1. While returning, multiply by 2. Since the multiplication is invoced as last operation, it is not tail recursive, but since Longs are of known size in advance, the maximum stack is not too big.

scala> n.toBinaryString.replaceAll ("(.{8})", "$1 ")
res117: java.lang.String = 10110011 11101110 01011110 01111110 00111101 11100101 11101011 011000

scala> bitN (n, 40) .toBinaryString.replaceAll ("(.{8})", "$1 ")
res118: java.lang.String = 10000000 00000000 00000000 00000000 00000000 00000000 00000000 000000