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

• Difficulty Level : Easy
• Last Updated : 09 Sep, 2022

Given two numbers A and B. Write a program to count the number of bits needed to be flipped to convert A to B

Examples:

Input: A = 10, B = 20
Output: 4
Explanation: 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
Explanation: 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.

Recommended Practice

## Count the number of bits to be flipped to convert A to B using the XOR operator:

To solve the problem follow the below idea:

Calculate (A XOR B), since 0 XOR 1 and 1 XOR 0 is equal to 1. So calculating the number of set bits in A XOR B will give us the count of the number of unmatching bits in A and B, which needs to be flipped

Follow the given steps to solve the problem:

• Calculate the XOR of A and B
• Count the set bits in the above-calculated XOR result
• Return the count

Below is the implementation of the above approach:

## C

 `// C program for the above approach` `#include `   `// 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;` `  `  `      ``// Function call` `    ``printf``(``"%d\n"``, FlippedCount(a, b));` `    ``return` `0;` `}`   `// This code is contributed by Sania Kumari Gupta` `// (kriSania804)`

## C++

 `// C++ program for the above approach` `#include ` `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;` `  `  `      ``// Function call` `    ``cout << FlippedCount(a, b) << endl;` `    ``return` `0;` `}`   `// This code is contributed by Sania Kumari Gupta` `// (kriSania804)`

## Java

 `// Java program for the above approach` `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``;` `      `  `          ``// Function call` `        ``System.out.print(FlippedCount(a, b));` `    ``}` `}`   `// This code is contributed by rishabh_jain`

## Python3

 `# Python3 program for the above approach`   `# 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` `if` `__name__ ``=``=` `"__main__"``:` `  ``a ``=` `10` `  ``b ``=` `20`   `  ``# Function call` `  ``print``(FlippedCount(a, b))`   `# This code is contributed by "Sharad_Bhardwaj".`

## C#

 `// C# program for the above approach`   `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;` `      `  `          ``// Function call` `        ``Console.WriteLine(FlippedCount(a, b));` `    ``}` `}`   `// This code is contributed by vt_m.`

## PHP

 ``

## Javascript

 `// 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`

Output

`4`

Note: Set bits in (a XOR b) can also be computer using built in function __builtin_popcount() in C/C++

Below is the implementation of the above approach:

## C

 `//C program to Count number of bits to be flipped` `// to convert A into B` `#include `   `// Driver code` `int` `main()` `{` `    ``int` `a = 10;` `    ``int` `b = 20;` `  `  `      ``// Function call` `    ``printf``(``"%d\n"``,__builtin_popcount(a^b));` `    ``return` `0;` `}`   `// This code is contributed by Suruchi Kumari`

## C++

 `// C++ program to Count number of bits to be flipped` `// to convert A into B` `#include ` `using` `namespace` `std;`   `// Driver code` `int` `main()` `{` `    ``int` `a = 10;` `    ``int` `b = 20;` `  `  `      ``// Function call` `    ``cout <<__builtin_popcount(a^b) << endl;` `    ``return` `0;` `}`   `// This code is contributed by Suruchi Kumari`

Output

```4
```

Time Complexity: O(K) where K is the number of bits
Auxiliary Space: O(1)

## Count the number of bits to be flipped to convert A to B using the AND operator:

To solve the problem follow the below idea:

Start comparing the bits in A and B, starting from the least significant bit and if (A & 1) is not equal to (B & 1) then the current bit needs to be flipped, as the value of bits is different at this position in both the numbers

Follow the given steps to solve the problem:

• Declare variable flips equal to zero
• Run a loop, while a is greater than zero and b is also greater than zero
• Calculate values of (A AND 1) and (B AND 1)
• If these values are not equal then increase the flip value by 1
• Right shift a and b by 1
• Return flips

Below is the implementation of the above approach:

## C++

 `// C++ program for the above approach` `#include ` `using` `namespace` `std;`   `int` `countFlips(``int` `a, ``int` `b)` `{`   `    ``// initially flips is equal to 0` `    ``int` `flips = 0;`   `    ``// & each bits of a && b with 1` `    ``// and store them if t1 and t2` `    ``// if t1 != t2 then we will flip that bit`   `    ``while` `(a > 0 || b > 0) {`   `        ``int` `t1 = (a & 1);` `        ``int` `t2 = (b & 1);`   `        ``if` `(t1 != t2) {` `            ``flips++;` `        ``}` `        ``// right shifting a and b` `        ``a >>= 1;` `        ``b >>= 1;` `    ``}`   `    ``return` `flips;` `}`   `int` `main()` `{` `    ``int` `a = 10;` `    ``int` `b = 20;` `    ``cout << countFlips(a, b);` `}`   `// this code is contributed by shivanisinghss2110`

## Java

 `// Java program for the above approach`   `// CONTRIBUTED BY PRAVEEN VISHWAKARMA`   `import` `java.io.*;`   `class` `GFG {`   `    ``public` `static` `int` `countFlips(``int` `a, ``int` `b)` `    ``{` `        ``// initially flips is equal to 0` `        ``int` `flips = ``0``;`   `        ``// & each bits of a && b with 1` `        ``// and store them if t1 and t2` `        ``// if t1 != t2 then we will flip that bit`   `        ``while` `(a > ``0` `|| b > ``0``) {`   `            ``int` `t1 = (a & ``1``);` `            ``int` `t2 = (b & ``1``);`   `            ``if` `(t1 != t2) {` `                ``flips++;` `            ``}` `            ``// right shifting a and b` `            ``a >>>= ``1``;` `            ``b >>>= ``1``;` `        ``}`   `        ``return` `flips;` `    ``}`   `    ``// Driver code` `    ``public` `static` `void` `main(String[] args)` `    ``{` `        ``int` `a = ``10``;` `        ``int` `b = ``20``;` `        `  `          ``// Function call` `        ``System.out.println(countFlips(a, b));` `    ``}` `}`

## Python3

 `# Python3 program for the above approach`     `def` `countFlips(a, b):`   `    ``# initially flips is equal to 0` `    ``flips ``=` `0`   `    ``# & each bits of a && b with 1` `    ``# and store them if t1 and t2` `    ``# if t1 != t2 then we will flip that bit` `    ``while``(a > ``0` `or` `b > ``0``):` `        ``t1 ``=` `(a & ``1``)` `        ``t2 ``=` `(b & ``1``)` `        ``if``(t1 !``=` `t2):` `            ``flips ``+``=` `1`   `        ``# right shifting a and b` `        ``a >>``=` `1` `        ``b >>``=` `1`   `    ``return` `flips`     `# Driver code` `if` `__name__ ``=``=` `"__main__"``:` `  ``a ``=` `10` `  ``b ``=` `20`   `  ``# Function call` `  ``print``(countFlips(a, b))`   `# This code is contributed by shivanisinghss2110`

## C#

 `// C# program for the above approach` `using` `System;`   `class` `GFG {`   `    ``public` `static` `int` `countFlips(``int` `a, ``int` `b)` `    ``{`   `        ``// initially flips is equal to 0` `        ``int` `flips = 0;`   `        ``// & each bits of a && b with 1` `        ``// and store them if t1 and t2` `        ``// if t1 != t2 then we will flip that bit` `        ``while` `(a > 0 || b > 0) {`   `            ``int` `t1 = (a & 1);` `            ``int` `t2 = (b & 1);`   `            ``if` `(t1 != t2) {` `                ``flips++;` `            ``}` `            ``// right shifting a and b` `            ``a >>= 1;` `            ``b >>= 1;` `        ``}`   `        ``return` `flips;` `    ``}`   `    ``// Driver code` `    ``public` `static` `void` `Main(String[] args)` `    ``{` `        ``int` `a = 10;` `        ``int` `b = 20;` `      `  `          ``// Function call` `        ``Console.Write(countFlips(a, b));` `    ``}` `}`   `// This code is contributed by shivanisinghss2110`

## Javascript

 `/*package whatever //do not write package name here */`   `function` `countFlips(a, b){` `          ``// initially flips is equal to 0` `        ``var` `flips = 0;` `      `  `          ``// & each bits of a && b with 1` `          ``// and store them if t1 and t2` `        ``// if t1 != t2 then we will flip that bit` `        `  `          ``while``(a>0 || b>0){` `          `  `            ``var` `t1 = (a&1);` `              ``var` `t2 = (b&1);` `              `  `              ``if``(t1!=t2){` `                ``flips++;` `            ``}` `              ``// right shifting a and b` `              ``a>>>=1;` `              ``b>>>=1;` `        ``}` `      `  `      ``return` `flips;` `    ``}` `      `  `          ``var` `a = 10;` `          ``var` `b = 20;` `          ``document.write(countFlips(a, b));`   `// This code is contributed by shivanisinghss2110`

Output

`4`

Time Complexity: O(K) where K is the number of bits
Auxiliary Space: O(1)

Thanks to Sahil Rajput for providing the above implementation.
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