C++ program to clear Kth bit of a number

Program to clear the Kth bit of a number in C++: Here, we are going to use bitwise operators to clear Kth bit in the binary representation of a given number.
Submitted by Saksham Bhayana, on January 07, 2019

Here we are going to use bitwise operators to clear K^th bit in the binary representation of a given number.

Problem Statement: To write a C++ program to clear K^th bit of a number.

Constraints: 1<=n<=100

Example:

    Input:
    Enter number: 11
    Enter k: 4

    Output:
    original number before clearing: 11
    new number after clearing: 3

Problem Explanation:

Suppose the given number is 11 and the bit to be cleared is 4^th bit that is 3 left to the LSB.

Now the binary representation of 11 is:

    n = 00001011
    mask = 00001000 (left shift 1, three times (4-1) as k=4)
    Bitwise complement of mask= ~mask = 11110111
    n &(~ mask) = 00000011
    Hence decimal representation of new number is 3.

This can be explained as follows:

Performing & with the ~mask clears Kth bit only as Kth bit in (~mask) is 0.

If X represents bits of n then, 0 & X is always 0. For remaining bits of original number, & with 1 gives the bit itself.

Therefore performing bitwise AND of original number with (~mask) clears Kth bit in a number.

Algorithm:

Input the number and K^th bit to be cleared.
Left shift 1 - (K-1) times to create a mask where only K^th bit is set.
Take bitwise complement of the mask.
Perform bitwise AND of original number with this mask to clear the K^th bit.
Output the result after bitwise AND in decimal form.

C++ Implementation:

#include <iostream>
using namespace std;

int clearKthBit(int n,int k)
{
	// left shift 1 , (k-1) times to get a mask 
	// in which only kth bit is set
	int m=1<<(k-1); 
	// bitwise complement of mask
	m=~(m);
	// new number after clearing kth bit 
	return (n&m);   
}

//driver program to check the code

int main()
{
	int num,k;

	cout<<"Enter number: ";
	cin>>num;
	cout<<"Enter k: ";
	cin>>k;

	cout<<"original number before clearing: "<<num<<endl;

	int new_number= clearKthBit(num,k);

	cout<<"new number after clearing: "<<new_number<<endl;

	return 0;
}

Output

Enter number: 11
Enter k: 4
original number before clearing: 11
new number after clearing: 3