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Ayan Banerjee
Ayan Banerjee

Posted on • Edited on • Originally published at csposts.com

Count Number of Set Bits of an Integer using Brian-Kernighan Method

#cpp #python #interview #career

Count the number of set bits of an integer.

Examples:

Input: 31
Output: 5
Explanation: Binary representation of 31 is 11111

Input: 42
Output: 3
Explanation: Binary representation of 42 is 101010

Approach-1: Naive

Naive algorithm is to use the binary representation of the number and count the number of set bits.

C++ code:

#include<iostream>
using namespace std;

int countSetBits(int n) {
    int count = 0;
    while (n > 0) {
        count += n & 1; // check if the last bit is set
        n = n >> 1; // right shift by 1 is equivalent to division by 2
    }
    return count;
}


int main() {
    cout << "Number of set bits of " << 31 << " is " << countSetBits(31) << "\n";
    cout << "Number of set bits of " << 42 << " is " << countSetBits(42) << "\n";
    return 0;
}
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Python code:

def count_set_bits(n):
    count = 0
    while n > 0:
        count += n & 1
        n = n >> 1
    return count

if __name__ == '__main__':
    print('Number of set bits of', 31, 'is', count_set_bits(31))
    print('Number of set bits of', 42, 'is', count_set_bits(42))
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Time Complexity: O(logN) where N is the number
Space Complexity: O(1) as we are not using any extra space

Approach-2: Brian Kernighan Algorithm

n = n & (n - 1) clears the rightmost set bit. Let us take a look at some
examples.

n           => 101010
n - 1       => 101001
---------------------
n & (n - 1) => 101000
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n is updated to 101000 now.

n           => 101000
n - 1       => 100111
---------------------
n & (n - 1) => 100000
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n is updated to 100000 now.

n           => 100000
n - 1       => 011111
--------------------------
n & (n - 1) => 000000
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n is now 0.

Thus, we need only 3 iterations to find the count of set bits.

C++ code:

#include<iostream>
using namespace std;

int countSetBits(int n) {
    int count = 0;
    while (n > 0) {
        n = n & (n - 1); // clear the right-most bit
        ++count;
    }
    return count;
}


int main() {
    cout << "Number of set bits of " << 31 << " is " << countSetBits(31) << "\n";
    cout << "Number of set bits of " << 42 << " is " << countSetBits(42) << "\n";
    return 0;
}
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Python code:

def count_set_bits(n):
    count = 0
    while n > 0:
        n = n & (n - 1)  # clear the right most bit
        count += 1
    return count

if __name__ == '__main__':
    print('Number of set bits of', 31, 'is', count_set_bits(31))
    print('Number of set bits of', 42, 'is', count_set_bits(42))
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Time Complexity: O(logN) when N has all of its bit set
Space Complexity: O(1) as we are not using any extra space

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