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 00000111Binary 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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