Swap two bits at a given position in an integer

Given an integer, swap two bits at given positions in a binary representation of it.

For example,

Input:

n = 31 (31 in binary is 00011111)
p = 2, q = 6 (3rd and 7th bit from the right)

Output: 91

Explanation: 91 in binary is 01011011

Practice this problem

We can solve this problem by checking if the two bits at given positions are the same or not. If they are the same, nothing needs to be done; otherwise, if they are not the same (i.e., one is 0 and the other is 1), then we can XOR them with 1 << position. This logic will work because:

XOR with 1 will toggle the bits
0 ^ 1 = 1
1 ^ 1 = 0

XOR with 0 will have no impact
0 ^ 0 = 0
1 ^ 0 = 1

Following is the C++, Java, and Python implementation of the idea:

C++

#include <iostream>

#include <bitset>

using namespace std;

// Function to swap bits at position `p` and `q` in integer `n`

int swap(int n, int p, int q)

{

// if bits are different at position `p` and `q`

if (((n & (1 << p)) >> p) ^ ((n & (1 << q)) >> q))

{

n ^= (1 << p);

n ^= (1 << q);

}

return n;

}

int main()

{

int n = 31;

int p = 2, q = 6; // swap 3rd and 7th bit from the right

cout << n << " in binary is " << bitset<8>(n) << endl;

n = swap (n, p, q);

cout << n << " in binary is " << bitset<8>(n) << endl;

return 0;

}

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Output:

31 in binary is 00011111
91 in binary is 01011011

Java

class Main

{

// Function to swap bits at position `p` and `q` in integer `n`

public static int swap(int n, int p, int q)

{

// if bits are different at position `p` and `q`

if ((((n & (1 << p)) >> p) ^ ((n & (1 << q)) >> q)) == 1)

{

n ^= (1 << p);

n ^= (1 << q);

}

return n;

}

public static void main (String[] args)

{

int n = 31;

int p = 2, q = 6; // swap 3rd and 7th bit from the right

System.out.println(n + " in binary is " +

String.format("%08d", Integer.parseInt(Integer.toBinaryString(n))));

n = swap (n, p, q);

System.out.println(n + " in binary is " +

String.format("%08d", Integer.parseInt(Integer.toBinaryString(n))));

}

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Output:

31 in binary is 00011111
91 in binary is 01011011

Python

# Function to swap bits at position `p` and `q` in integer `n`

def swap(n, p, q):

# if bits are different at position `p` and `q`

if (((n & (1 << p)) >> p) ^ ((n & (1 << q)) >> q)) == 1:

n ^= (1 << p)

n ^= (1 << q)

return n

if __name__ == '__main__':

n = 31

# swap 3rd and 7th bit from the right

p = 2

q = 6

print(f'{n} in binary is', bin(n)[2:].zfill(8))

n = swap(n, p, q)

print(f'{n} in binary is', bin(n)[2:].zfill(8))

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Output:

31 in binary is 00011111
91 in binary is 01011011