troubleshooting Question

How to compute the number of ones in a 32 bit integer ?

Avatar of HukkaHua
HukkaHua asked on
CAlgorithms
6 Comments1 Solution1003 ViewsLast Modified:
Hi Folks
             I am trying to figure out how to parallelly compute the number of bits. The solution from the Stanford site is as follows :

Counting bits set, in parallel

unsigned int v; // count bits set in this (32-bit value)
unsigned int c; // store the total here
static const int S[] = {1, 2, 4, 8, 16}; // Magic Binary Numbers
static const int B[] = {0x55555555, 0x33333333, 0x0F0F0F0F, 0x00FF00FF, 0x0000FFFF};

c = v - ((v >> 1) & B[0]);
c = ((c >> S[1]) & B[1]) + (c & B[1]);
c = ((c >> S[2]) + c) & B[2];
c = ((c >> S[3]) + c) & B[3];
c = ((c >> S[4]) + c) & B[4];

The B array, expressed as binary, is:

B[0] = 0x55555555 = 01010101 01010101 01010101 01010101
B[1] = 0x33333333 = 00110011 00110011 00110011 00110011
B[2] = 0x0F0F0F0F = 00001111 00001111 00001111 00001111
B[3] = 0x00FF00FF = 00000000 11111111 00000000 11111111
B[4] = 0x0000FFFF = 00000000 00000000 11111111 11111111


I am actually having trouble to decipher it, if any one can you explain it
it would be great.
Thanks for the help.

-H
Join the community to see this answer!
Join our exclusive community to see this answer & millions of others.
Unlock 1 Answer and 6 Comments.
Join the Community
Learn from the best

Network and collaborate with thousands of CTOs, CISOs, and IT Pros rooting for you and your success.

Andrew Hancock - VMware vExpert
See if this solution works for you by signing up for a 7 day free trial.
Unlock 1 Answer and 6 Comments.
Try for 7 days

”The time we save is the biggest benefit of E-E to our team. What could take multiple guys 2 hours or more each to find is accessed in around 15 minutes on Experts Exchange.

-Mike Kapnisakis, Warner Bros