# Count number of bits to be flipped to convert A to B

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) ` ` ` `{ ` ` ` `count += n & 1; ` ` ` `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; ` `} ` |

*chevron_right*

*filter_none*

## 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 & ` `1` `; ` ` ` `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 ` |

*chevron_right*

*filter_none*

## 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 ` `+` `=` `n & ` `1` ` ` `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". ` |

*chevron_right*

*filter_none*

## 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 & 1; ` ` ` `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. ` |

*chevron_right*

*filter_none*

## 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` `+= ` `$n` `& 1; ` ` ` `$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 ` `?> ` |

*chevron_right*

*filter_none*

**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

If you like GeeksforGeeks and would like to contribute, you can also write an article using contribute.geeksforgeeks.org or mail your article to contribute@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.

Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above.

## Recommended Posts:

- Check if bits of a number has count of consecutive set bits in increasing order
- Count pairs (A, B) such that A has X and B has Y number of set bits and A+B = C
- Count total bits in a number
- Count unset bits of a number
- Count number of bits changed after adding 1 to given N
- Count pairs of elements such that number of set bits in their AND is B[i]
- Program to count number of set bits in an (big) array
- Toggle bits of a number except first and last bits
- Count set bits in an integer
- Count set bits in a range
- Count unset bits in a range
- Count total set bits in all numbers from 1 to n
- Sort an array according to count of set bits | Set 2
- Python | Count set bits in a range
- Range query for count of set bits