Question

Given a binary number, what is the fastest way of removing the lowest order bit?

01001001010 -> 01001001000

It would be used in code to iterate over the bits of a variable. Pseudo-code follows.

while(bits != 0){
  index = getIndexOfLowestOrderBit(bits);
  doSomething(index);
  removeLowestOrderBit(bits);
}

The possible languages I’m considering using are C and Java.

Answer 1

Uh … In your example, you already know the bit’s index. Then it’s easy:

bits &= ~(1 << index);

This will mask off the bit whose index is index, regardless of its position in the value (highest, lowest, or in-between). Come to think of it, you can of course use the fact that you know the bit is already set, and use an XOR to knock it clear again:

bits ^= (1 << index);

That saves the inversion, which is probably one machine instruction.

If you instead want to mask off the lowest set bit, without knowing its index, the trick is:

bits &= (bits - 1);

See here for instance.