comp.lang.c FAQ list · Question 20.12

Q: What is the most efficient way to count the number of bits which are set in an integer?

A: Many ``bit-fiddling'' problems like this one can be sped up and streamlined using lookup tables (but see question 20.13). Here is a little function which computes the number of bits in a value, 4 bits at a time:

static int bitcounts[] =
	{0, 1, 1, 2, 1, 2, 2, 3, 1, 2, 2, 3, 2, 3, 3, 4};

int bitcount(unsigned int u)
{
	int n = 0;

	for(; u != 0; u >>= 4)
		n += bitcounts[u & 0x0f];

	return n;
}

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