Return the number of bits which are set to 1 inside a variable representation

Asked Mar 26 '17 at 16:01

Active Mar 26 '17 at 16:40

Viewed 44 times

Let's suppose I have an unsigned integer number represented in an 8bit variable. I want to write a function that returns the number of bits which are set to 1 inside the variable representation. For example the function should return 1 if the number is 1 and 2 if the number is.. let's say 5. How can I do this? I looked on this website to other questions but didn't find a clear answer for my question, or a similar question.

Thanks in advance.

Update:

#include <stdio.h>
#include <stdlib.h>
int NumberOfSetBits(int i)
{
i = i - ((i >> 1) & 0x55555555);
  i = (i & 0x33333333) + ((i >> 2) & 0x33333333);
 return (((i + (i >> 4)) & 0x0F0F0F0F) * 0x01010101) >> 24;
}

int main(){
printf("%d\n",NumberOfSetBits(4));
return 0;
}

edited Mar 26 '17 at 16:40

asked Mar 26 '17 at 16:01

Jack

What makes you think 4 would have 2 bits set to 1? – user2357112 Mar 26 '17 at 16:04
1

The operation is sometimes called "sideways addition" or "popcount". There are intrinsics for this and some well-known algorithms. – Raymond Hettinger Mar 26 '17 at 16:05
AKA finding the Hamming weight. – ad absurdum Mar 26 '17 at 16:06
Exactly, it's the Hamming weight. I didn't knew at first how it's called. – Jack Mar 26 '17 at 16:15
@user2357112 I edited, I wanted to write 5. Because 5 is 0101 – Jack Mar 26 '17 at 16:35
Done, I edited my answer. For 4 the result will be 2, and for 5 the result will be 2.. – Jack Mar 26 '17 at 16:43
no need to know about Hamming weight. You just need to google for "get number of set bits" to get the results immediately – phuclv Mar 26 '17 at 16:45
http://stackoverflow.com/q/21051663/1566187 seems to be of interest I think. It's taken from K&R - a great book still after so many years. I recommend you learn C with this book too. – Ely Mar 26 '17 at 17:03

Return the number of bits which are set to 1 inside a variable representation

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