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Count number of bits to be flipped to convert A to B
  • Difficulty Level : Easy
  • Last Updated : 25 Mar, 2021

Given two numbers ‘a’ and b’. Write a program to count number of bits needed to be flipped to convert ‘a’ to ‘b’. 
Example : 
 

Input : a = 10, b = 20
Output : 4
Binary representation of a is 00001010
Binary representation of b is 00010100
We need to flip highlighted four bits in a
to make it b.

Input : a = 7, b = 10
Output : 3
Binary representation of a is 00000111
Binary representation of b is 00001010
We need to flip highlighted three bits in a
to make it b.

 

 

  1. Calculate XOR of A and B.      
        a_xor_b = A ^ B
  2. Count the set bits in the above 
     calculated XOR result.
        countSetBits(a_xor_b)

XOR of two number will have set bits only at those places where A differs from B. 
 

C++




// Count number of bits to be flipped
// to convert A into B
#include <iostream>
using namespace std;
 
// Function that count set bits
int countSetBits(int n)
{
    int count = 0;
    while (n > 0)
    {
        count++;
        n &= (n-1);
    }
    return count;
}
 
// Function that return count of
// flipped number
int FlippedCount(int a, int b)
{
    // Return count of set bits in
    // a XOR b
    return countSetBits(a^b);
}
 
// Driver code
int main()
{
    int a = 10;
    int b = 20;
    cout << FlippedCount(a, b)<<endl;
    return 0;
}

Java




// Count number of bits to be flipped
// to convert A into B
import java.util.*;
 
class Count {
     
    // Function that count set bits
    public static int countSetBits(int n)
    {
        int count = 0;
        while (n != 0) {
            count++;
            n &=(n-1);
        }
        return count;
    }
 
    // Function that return count of
    // flipped number
    public static int FlippedCount(int a, int b)
    {
        // Return count of set bits in
        // a XOR b
        return countSetBits(a ^ b);
    }
     
    // Driver code
    public static void main(String[] args)
    {
        int a = 10;
        int b = 20;
        System.out.print(FlippedCount(a, b));
    }
}
 
// This code is contributed by rishabh_jain

Python3




# Count number of bits to be flipped
# to convert A into B
 
# Function that count set bits
def countSetBits( n ):
    count = 0
    while n:
        count += 1
        n &= (n-1)
    return count
     
# Function that return count of
# flipped number
def FlippedCount(a , b):
 
    # Return count of set bits in
    # a XOR b
    return countSetBits(a^b)
 
# Driver code
a = 10
b = 20
print(FlippedCount(a, b))
 
# This code is contributed by "Sharad_Bhardwaj".

C#




// Count number of bits to be
// flipped to convert A into B
using System;
 
class Count {
     
    // Function that count set bits
    public static int countSetBits(int n)
    {
        int count = 0;
        while (n != 0) {
            count++;
            n &= (n-1);
        }
        return count;
    }
 
    // Function that return
    // count of flipped number
    public static int FlippedCount(int a, int b)
    {
    // Return count of set
    // bits in a XOR b
        return countSetBits(a ^ b);
    }
     
    // Driver code
    public static void Main()
    {
        int a = 10;
        int b = 20;
        Console.WriteLine(FlippedCount(a, b));
    }
}
 
// This code is contributed by vt_m.

PHP




<?php
// Count number of bits to be
// flipped to convert A into B
 
// Function that count set bits
function countSetBits($n)
{
    $count = 0;
    while($n)
    {
        $count += 1;
        $n &= (n-1);
    }
    return $count;
}
     
// Function that return
// count of flipped number
function FlippedCount($a, $b)
{
    // Return count of set
    // bits in a XOR b
    return countSetBits($a ^ $b);
}
 
// Driver code
$a = 10;
$b = 20;
echo FlippedCount($a, $b);
 
// This code is contributed by mits
?>

Javascript




<script>
// Count number of bits to be flipped
// to convert A into Bclass Count {
 
    // Function that count set bits
    function countSetBits(n) {
        var count = 0;
        while (n != 0) {
            count++;
            n &= (n - 1);
        }
        return count;
    }
 
    // Function that return count of
    // flipped number
    function FlippedCount(a , b) {
        // Return count of set bits in
        // a XOR b
        return countSetBits(a ^ b);
    }
 
    // Driver code
        var a = 10;
        var b = 20;
        document.write(FlippedCount(a, b));
 
// This code is contributed by shikhasingrajput
</script>

Output : 



4

Thanks to Sahil Rajput for providing above implementation.
 

To get the set bit count please see this post: Count set bits in an integer
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