Question

How would i go about finding the number of ‘zero’ bits in C++.
Suppose I have an integer;

int value = 276; 

For which I have the bits 100010100, but how do I count the zeros?

Answer 1

The easiest most naive way is to just iterate over the bits and count:

size_t num_zeroes = 0;

for(size_t i = 0; i < CHAR_BIT * sizeof value; ++i)
{
  if ((value & (1 << i)) == 0)
    ++num_zeroes;
}

There are all number of better (for different values of “better”) ways, but this is quite clear, very terse (code-wise), and doesn’t require a bunch of setup.

One micro-optimization that might be considered an improvement is to not compute the mask to test each bit, instead shift the value and always test the rightmost bit:

for(size_t i = 0; i < CHAR_BIT * sizeof value; ++i, value >>= 1)
{
  if ((value & 1) == 0)
    ++num_zeroes;
}