Swap bits in a given number

Given a number x and two positions (from the right side) in the binary representation of x, write a function that swaps n bits at given two positions and returns the result. It is also given that the two sets of bits do not overlap.

Recommended: Please solve it on “PRACTICE” first, before moving on to the solution.

Let p1 and p2 be the two given positions.

Example 1
Input:
x = 47 (00101111)
p1 = 1 (Start from the second bit from the right side)
p2 = 5 (Start from the 6th bit from the right side)
n = 3 (No of bits to be swapped)
Output:
227 (11100011)
The 3 bits starting from the second bit (from the right side) are
swapped with 3 bits starting from 6th position (from the right side)

Example 2
Input:
x = 28 (11100)
p1 = 0 (Start from first bit from right side)
p2 = 3 (Start from 4th bit from right side)
n = 2 (No of bits to be swapped)
Output:
7 (00111)
The 2 bits starting from 0th position (from right side) are
swapped with 2 bits starting from 4th position (from right side)

Solution
We need to swap two sets of bits. XOR can be used in a similar way as it is used to swap 2 numbers. Following is the algorithm.

1) Move all bits of the first set to the rightmost side
   set1 =  (x >> p1) & ((1U << n) - 1)
Here the expression (1U << n) - 1 gives a number that 
contains last n bits set and other bits as 0. We do & 
with this expression so that bits other than the last 
n bits become 0.
2) Move all bits of second set to rightmost side
   set2 =  (x >> p2) & ((1U << n) - 1)
3) XOR the two sets of bits
   xor = (set1 ^ set2) 
4) Put the xor bits back to their original positions. 
   xor = (xor << p1) | (xor << p2)
5) Finally, XOR the xor with original number so 
   that the two sets are swapped.
   result = x ^ xor

Implementation:

C++

// C++ Program to swap bits  
// in a given number  
#include <bits/stdc++.h> 
using namespace std; 
  
int swapBits(unsigned int x, unsigned int p1,  
            unsigned int p2, unsigned int n)  
{  
    /* Move all bits of first set to rightmost side */
    unsigned int set1 = (x >> p1) & ((1U << n) - 1);  
  
    /* Move all bits of second set to rightmost side */
    unsigned int set2 = (x >> p2) & ((1U << n) - 1);  
  
    /* Xor the two sets */
    unsigned int Xor = (set1 ^ set2);  
  
    /* Put the Xor bits back to their original positions */
    Xor = (Xor << p1) | (Xor << p2);  
  
    /* Xor the 'Xor' with the original number so that the  
    two sets are swapped */
    unsigned int result = x ^ Xor;  
  
    return result;  
}  
  
/* Driver code*/
int main()  
{  
    int res = swapBits(28, 0, 3, 2);  
    cout << "Result = " << res;  
    return 0;  
}  
  
// This code is contributed by rathbhupendra 

C

// C Program to swap bits  
// in a given number 
#include<stdio.h> 
  
int swapBits(unsigned int x, unsigned int p1, unsigned int p2, unsigned int n) 
{ 
    /* Move all bits of first set to rightmost side */
    unsigned int set1 =  (x >> p1) & ((1U << n) - 1); 
  
    /* Move all bits of second set to rightmost side */
    unsigned int set2 =  (x >> p2) & ((1U << n) - 1); 
  
    /* XOR the two sets */
    unsigned int xor = (set1 ^ set2); 
  
    /* Put the xor bits back to their original positions */
    xor = (xor << p1) | (xor << p2); 
  
    /* XOR the 'xor' with the original number so that the  
       two sets are swapped */
    unsigned int result = x ^ xor; 
  
    return result; 
} 
  
/* Driver program to test above function*/
int main() 
{ 
    int res =  swapBits(28, 0, 3, 2); 
    printf("\nResult = %d ", res); 
    return 0; 
} 

Java

//Java Program to swap bits  
// in a given number 
  
class GFG { 
      
    static int swapBits(int x, int p1, int p2, int n) 
    { 
        // Move all bits of first set 
        // to rightmost side  
        int set1 = (x >> p1) & ((1 << n) - 1); 
      
        // Move all bits of second set  
        //to rightmost side  
        int set2 = (x >> p2) & ((1 << n) - 1); 
      
        // XOR the two sets  
        int xor = (set1 ^ set2); 
      
        // Put the xor bits back to  
        // their original positions  
        xor = (xor << p1) | (xor << p2); 
      
        // XOR the 'xor' with the original number  
        // so that the  two sets are swapped  
        int result = x ^ xor; 
      
        return result; 
    } 
      
    // Driver program  
    public static void main(String[] args) 
    { 
        int res = swapBits(28, 0, 3, 2); 
        System.out.println("Result = " + res); 
    } 
} 
  
// This code is contributed by prerna saini. 

Python3

# Python program to 
# swap bits in a given number 
  
def swapBits(x,p1,p2,n): 
  
    # Move all bits of first 
    # set to rightmost side  
    set1 =  (x >> p1) & ((1<< n) - 1) 
   
    # Moce all bits of second 
    # set to rightmost side  
    set2 =  (x >> p2) & ((1 << n) - 1) 
   
    # XOR the two sets  
    xor = (set1 ^ set2) 
   
    # Put the xor bits back 
    # to their original positions  
    xor = (xor << p1) | (xor << p2) 
   
      # XOR the 'xor' with the 
      # original number so that the  
      # two sets are swapped 
    result = x ^ xor 
   
    return result 
      
# Driver code 
  
res =swapBits(28, 0, 3, 2) 
print("Result =",res) 
  
# This code is contributed 
# by Anant Agarwal. 

C#

// C# Program to swap bits  
// in a given number 
using System; 
  
class GFG { 
      
    static int swapBits(int x, int p1, int p2, int n) 
    { 
        // Move all bits of first  
        //set to rightmost side  
        int set1 = (x >> p1) & ((1 << n) - 1); 
      
        // Move all bits of second set  
        // set to rightmost side  
        int set2 = (x >> p2) & ((1 << n) - 1); 
      
        // XOR the two sets  
        int xor = (set1 ^ set2); 
      
        // Put the xor bits back to  
        // their original positions  
        xor = (xor << p1) | (xor << p2); 
      
        // XOR the 'xor' with the original number  
        // so that the two sets are swapped  
        int result = x ^ xor; 
      
        return result; 
    } 
      
    // Driver program  
    public static void Main() 
    { 
        int res = swapBits(28, 0, 3, 2); 
        Console.WriteLine("Result = " + res); 
    } 
} 
  
// This code is contributed by vt_m. 

PHP

<?php 
// PHP Program to swap bits  
// in a given number 
  
// function returns 
// the swapped bits 
function swapBits($x, $p1, $p2, $n) 
{ 
      
    // Move all bits of first  
    // set to rightmost side  
    $set1 = ($x >> $p1) &  
            ((1 << $n) - 1); 
  
    // Move all bits of second 
    // set to rightmost side  
    $set2 = ($x >> $p2) &  
            ((1 << $n) - 1); 
  
    // XOR the two sets  
    $xor = ($set1 ^ $set2); 
  
    // Put the xor bits back to  
    // their original positions  
    $xor = ($xor << $p1) |  
           ($xor << $p2); 
  
    // XOR the 'xor' with the  
    // original number so that 
    // the two sets are swapped  
    $result = $x ^ $xor; 
  
    return $result; 
} 
  
    // Driver Code 
    $res = swapBits(28, 0, 3, 2); 
    echo "\nResult = ", $res; 
      
// This code is contributed by anuj_67. 
?> 

Output:

 Result = 7

Following is a shorter implementation of the same logic

int swapBits(unsigned int x, unsigned int p1, unsigned int p2, unsigned int n) 
{ 
    /* xor contains xor of two sets */
    unsigned int xor = ((x >> p1) ^ (x >> p2)) & ((1U << n) - 1); 
  
    /* To swap two sets, we need to again XOR the xor with original sets */
    return x ^ ((xor << p1) | (xor << p2)); 
} 

References:
Swapping individual bits with XOR

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

My Personal Notes

Recommended: Please solve it on “PRACTICE” first, before moving on to the solution.

C++

C

Java

Python3

C#

PHP

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