In this blog post, I will teach you how to write a C/C++ program to toggle all the bits of a number except k^th bit. After reading this detailed guide post I believe you can write an efficient program to toggle all the bits of a number except the k^th bit.

Consider the examples below for a better understanding,

Example-1:

Input : n = 5


Here's a simple ASCII representation of 5 (an 8-bit integer):
  7   6   5   4   3   2   1   0    (Bit position)
+---+---+---+---+---+---+---+---+
| 0 | 0 | 0 | 0 | 0 | 1 | 0 | 1 |  (Example bit values)
+---+---+---+---+---+---+---+---+
                  ^
                  |
                 Want to toggle all bits except this Bit (3rd bit)


(After toggling bit all bits except 3rd bit) 


Output: 242

  7   6   5   4   3   2   1   0    (Bit position)
+---+---+---+---+---+---+---+---+
| 1 | 1 | 1 | 1 | 0 | 0 | 1 | 0 |  (Example bit values)
+---+---+---+---+---+---+---+---+
                  ^
                  |
                 You can see bit has not been toggled.

Example-2:

Input : n = 5


Here's a simple ASCII representation of 13 (an 8-bit integer):

  7   6   5   4   3   2   1   0    (Bit position)
+---+---+---+---+---+---+---+---+
| 0 | 0 | 0 | 0 | 1 | 1 | 0 | 1 |  (Example bit values)
+---+---+---+---+---+---+---+---+
                      ^
                      |
                      Want to toggle all bits except this Bit (2nd bit)



(After toggling bit all bits except 2nd bit) 



Output: 246

  7   6   5   4   3   2   1   0    (Bit position)
+---+---+---+---+---+---+---+---+
| 1 | 1 | 1 | 1 | 0 | 1 | 1 | 0 |  (Example bit values)
+---+---+---+---+---+---+---+---+
                      ^
                      |
                      You can see bit has not been toggled.

Let’s see the problem statements for this question.

Problem Statement: Given a positive integer n, write a function to toggle all the bits except k^th bit.  Note that k = 0 means the rightmost bit of ‘n’.

Disclaimer: Try to solve the problem yourself first otherwise it would be not worth solving this problem.

Using ^ and ~ operators with a BitMask:

Using Bitwise EX-OR operator (^) and complement operator (~) With the help of a Mask you can easily toggle all the bits except the k^th bit. In this approach first, we will create a MASK using the expression (1 << d).

After creating the mask, you need to perform the bitwise EX_OR between the given number ‘n’ and the MASK it toggles the k^th bit.

Now in the last toggling all bits of the number that gets by the above operation using the complement operator, i.e., transforming the 0 bit to 1 and the 1 bit to 0.

Let’s take an example to understand the above steps, suppose n = 5.

Now if you want to set its 3rd bit (bit-position start from 0), the mask would be (1<<3).

Mask: (1 << 3)


  7   6   5   4   3   2   1   0    (Bit position)
+---+---+---+---+---+---+---+---+
| 0 | 0 | 0 | 0 | 1 | 0 | 0 | 0 |  (Example bit values)
+---+---+---+---+---+---+---+---+

Now Ex-Oring this mask with the original number ‘n’.

Original number n: 5
                   00000101 (5 in binary)
                    ^
Mask:              00001000

Result: =          00001101

Now toggle all bits of the results.

//Toggle all bits of the get result 

 ~ (00001101)


Output: 11110010

C program to toggle all the bits of a number except k-th bit:

/*
C program to Toggle all the bits
of a number except k-th bit.
*/

#include <stdio.h>

/*If k valid, then returns a number
 with all bit toggled in n except k-th bit
*/
unsigned int toggleAllBitsExceptK(unsigned int n, int k)
{
    unsigned int tmpN = n;
    unsigned int MASK = 0;
    //d must be greater than 0
    if(k >= 0)
    {
        //Create MASK
        MASK = (1 << k);

        //Toggle k-th bit by doing n ^ MASK

        tmpN ^= MASK;


        //Toggle all bits of the modified number
        tmpN = ~tmpN;

    }
    else
    {
        printf("Enter valid value of position\n");
    }
    return tmpN;
}



int main()
{
    unsigned int n = 4294967295; //0xffffffff
    
    //toggle all bits except 0th bit
    int k = 0;

    const unsigned int v = toggleAllBitsExceptK(n, k);

    printf("v = %u", v);

    return 0;
}

Output: v = 1

C++ program to toggle all the bits of a number except k-th bit:

/*
C++ program to Toggle all the bits
of a number except k-th bit.
*/
#include <iostream>


/*If k valid, then returns a number
 with all bit toggled in n except k-th bit
*/
unsigned int toggleAllBitsExceptK(unsigned int n, int k)
{
    unsigned int tmpN = n;
    unsigned int MASK = 0;
    //d must be greater than 0
    if(k >= 0)
    {
        //Create MASK
        MASK = (1 << k);
        //Toggle k-th bit by doing n ^ MASK
        tmpN ^= MASK;
        //Toggle all bits of the modified number
        tmpN = ~tmpN;
    }
    else
    {
        std::cout<<"Enter valid value of position\n"<<std::endl;
    }
    return tmpN;
}


int main()
{
    unsigned int n = 4294967295; //0xffffffff

    //toggle all bits except 0th bit
    int k = 0;

    const unsigned int v = toggleAllBitsExceptK(n, k);

    std::cout<<"Toggle All Bits Except "<<k<<std::endl;

    std::cout<<"Value "<<v<<std::endl;

    return 0;
}

Output:

Toggle All Bits Except 0
Value 1

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