__builtin_popcount() in STL C++ : Count Set bits

Before we describe this function, can you think of a solution to the problem: “Count Set bits in a given integer?“, In simple words, count the number of bit positions in binary representation of an integer which is set to 1?

How will you solve this problem?

// Here is a solution

int count_setbits(int N)
{ 
   int cnt=0;

   while(N>0)
   {
       cnt+=(N&1);
       N=N>>1;
   }

   return cnt;
}

Above is a simple solution to the problem.

The solution works as follows:

  • Until the given number is greater than zero(0), that is, until all bits in the number is not set to 0.
    • Keep on doing bitwise AND with the number 1, which has only first bit 1.
    • and, keep on shifting the bits by 1 place to right in the input number.
    • Simultaneously, keep on calculating count.

What if we could have solved this using a single line of code?

Yes, it’s possible using the function __builtin_popcount() in STL.

The function takes an unsigned integer as input parameter and returns the number of set bits present in that integer.

Syntax:

__builtin_popcount(int num);

Note: This function only works for unsigned or positive integers.

Code:

#include<iostream>
#include<vector>
#include<algorithm>

using namespace std;

int main() {
    
    int n = 7;
    
    cout<<__builtin_popcount(n);
    
    return 0;
}

Output:

3

What if the integer is of the type “long long” and not fits in the range of int?

In that case, there is a separate function with slight variation in function name but serves exactly the same purpose. The function is __builtin_popcountll(). Notice the ll in the end of function name.

Below is the implementation of the above function:

Code:

#include<iostream>
#include<vector>
#include<algorithm>

using namespace std;

int main() {
    
    long long n = 77777777777777;
    
    cout<<__builtin_popcountll(n);
    
    return 0;
}

Output:

23

Learn about other STL functions and containers in detail.