Swap bits in a given number

Try it on GfG Practice

Given a number x and two positions (0 based from the right side) in the binary representation of x, write a function that swaps n bits at the given two positions and returns the result. It is also given that the two sets of bits do not overlap.

Examples:

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)
Explanation:
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)

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)
Explanation:
The 2 bits starting from 0th position (from right side) are
swapped with 2 bits starting from 4th position (from right side)

Using XOR - O(1) time and O(1) space

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

The idea is to swap n bits at positions p1 and p2 in a number x by isolating these bit sets, XORing them to find the differences, and then selectively flipping only the bits that need to change, using the property that XOR marks differing bits and applying a second XOR operation restores the correct values in their swapped positions.

Step by step approach:

1. Extract the first set of n bits from position p1 by right-shifting and masking with an n-bit mask
2. Extract the second set of n bits from position p2 using the same technique
3. XOR these two sets to identify which corresponding bits differ between them
4. Position these XOR differences back at their original locations (p1 and p2)
5. XOR the original number with these positioned differences to selectively flip only the bits that need to change

// C++ Program to swap bits
// in a given number
#include <bits/stdc++.h>
using namespace std;

// Function to swap bits in a given number
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 xorVal = (set1 ^ set2);

    // Put the Xor bits back to their original positions 
    xorVal = (xorVal << p1) | (xorVal << p2);

    // Xor the 'Xor' with the original number so that the 
    // two sets are swapped 
    int result = x ^ xorVal;

    return result;
}

int main() {
    int res = swapBits(28, 0, 3, 2);
    cout << res;
    return 0;
}

// Java Program to swap bits
// in a given number

class GfG {

    // Function to swap bits in a given number
    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 xorVal = (set1 ^ set2);

        // Put the Xor bits back to their original positions 
        xorVal = (xorVal << p1) | (xorVal << p2);

        // Xor the 'Xor' with the original number so that the 
        // two sets are swapped 
        int result = x ^ xorVal;

        return result;
    }

    public static void main(String[] args) {
        int res = swapBits(28, 0, 3, 2);
        System.out.println(res);
    }
}

# Python Program to swap bits
# in a given number

# Function 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)

    # Move all bits of second set to rightmost side 
    set2 = (x >> p2) & ((1 << n) - 1)

    # Xor the two sets 
    xorVal = (set1 ^ set2)

    # Put the Xor bits back to their original positions 
    xorVal = (xorVal << p1) | (xorVal << p2)

    # Xor the 'Xor' with the original number so that the 
    # two sets are swapped 
    result = x ^ xorVal

    return result

if __name__ == "__main__":
    res = swapBits(28, 0, 3, 2)
    print(res)

// C# Program to swap bits
// in a given number
using System;

class GfG {

    // Function to swap bits in a given number
    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 xorVal = (set1 ^ set2);

        // Put the Xor bits back to their original positions 
        xorVal = (xorVal << p1) | (xorVal << p2);

        // Xor the 'Xor' with the original number so that the 
        // two sets are swapped 
        int result = x ^ xorVal;

        return result;
    }

    static void Main(string[] args) {
        int res = swapBits(28, 0, 3, 2);
        Console.WriteLine(res);
    }
}

// JavaScript Program to swap bits
// in a given number

// Function to swap bits in a given number
function swapBits(x, p1, p2, n) {

    // Move all bits of first set to rightmost side 
    let set1 = (x >> p1) & ((1 << n) - 1);

    // Move all bits of second set to rightmost side 
    let set2 = (x >> p2) & ((1 << n) - 1);

    // Xor the two sets 
    let xorVal = (set1 ^ set2);

    // Put the Xor bits back to their original positions 
    xorVal = (xorVal << p1) | (xorVal << p2);

    // Xor the 'Xor' with the original number so that the 
    // two sets are swapped 
    let result = x ^ xorVal;

    return result;
}

let res = swapBits(28, 0, 3, 2);
console.log(res);

7

Checking Each bit - O(1) time and O(1) space

The idea is to perform a bit-by-bit swap by iterating through each of the n positions, checking if the corresponding bits at the two positions differ, and if they do, flipping both bits to effectively swap their values - this approach handles the swap one bit at a time rather than operating on the entire bit sequences in a single operation.

Step by step approach:

1. For each of the n positions, create masks to isolate individual bits at both locations.
2. Check if the bits at the two positions are different (one is 0 and one is 1).
3. If they differ, flip both bits by clearing the set bit and setting the unset bit.
4. If the bits are identical (both 0 or both 1), no action is needed.
5. Increment both position pointers and continue until all n bits are processed.

// C++ Program to swap bits
// in a given number
#include <bits/stdc++.h>
using namespace std;

int swapBits(int x, int p1, int p2, int n) {
    int shift1, shift2, value1, value2;
    while (n--) {
        
        // Setting bit at p1 position to 1
        shift1 = 1 << p1;
        // Setting bit at p2 position to 1
        shift2 = 1 << p2;

        // value1 and value2 will have 0 if num
        // at the respective positions - p1 and p2 is 0.
        value1 = ((x & shift1));
        value2 = ((x & shift2));

        // check if value1 and value2 are different
        // i.e. at one position bit is set and other it is not
        if ((!value1 && value2) || (!value2 && value1)) {
            
            // if bit at p1 position is set
            if (value1) {
                
                // unset bit at p1 position
                x = x & (~shift1);
                
                // set bit at p2 position
                x = x | shift2;
            }
            // if bit at p2 position is set
            else {
                
                // set bit at p2 position
                x = x & (~shift2);
                
                // unset bit at p2 position
                x = x | shift1;
            }
        }
        p1++;
        p2++;
    }
    
    // return final result
    return x;
}

int main() {
    int res = swapBits(28, 0, 3, 2);
    cout << res;
    return 0;
}

// Java Program to swap bits
// in a given number

class GfG {

    static int swapBits(int x, int p1, int p2, int n) {
        int shift1, shift2, value1, value2;
        while (n > 0) {
            
            // Setting bit at p1 position to 1
            shift1 = 1 << p1;
            // Setting bit at p2 position to 1
            shift2 = 1 << p2;

            // value1 and value2 will have 0 if num
            // at the respective positions - p1 and p2 is 0.
            value1 = (x & shift1);
            value2 = (x & shift2);

            // check if value1 and value2 are different
            // i.e. at one position bit is set and other it is not
            if ((value1 == 0 && value2 != 0) || 
            (value2 == 0 && value1 != 0)) {
                
                // if bit at p1 position is set
                if (value1 != 0) {
                    
                    // unset bit at p1 position
                    x = x & (~shift1);
                    
                    // set bit at p2 position
                    x = x | shift2;
                }
                // if bit at p2 position is set
                else {
                    
                    // set bit at p2 position
                    x = x & (~shift2);
                    
                    // unset bit at p2 position
                    x = x | shift1;
                }
            }
            p1++;
            p2++;
            n--;
        }
        
        // return final result
        return x;
    }

    public static void main(String[] args) {
        int res = swapBits(28, 0, 3, 2);
        System.out.println(res);
    }
}

# Python Program to swap bits
# in a given number

def swapBits(x, p1, p2, n):
    shift1 = shift2 = value1 = value2 = 0
    while n > 0:
        
        # Setting bit at p1 position to 1
        shift1 = 1 << p1
        # Setting bit at p2 position to 1
        shift2 = 1 << p2

        # value1 and value2 will have 0 if num
        # at the respective positions - p1 and p2 is 0.
        value1 = x & shift1
        value2 = x & shift2

        # check if value1 and value2 are different
        # i.e. at one position bit is set and other it is not
        if (not value1 and value2) or (not value2 and value1):

            # if bit at p1 position is set
            if value1:

                # unset bit at p1 position
                x = x & (~shift1)

                # set bit at p2 position
                x = x | shift2

            # if bit at p2 position is set
            else:

                # set bit at p2 position
                x = x & (~shift2)

                # unset bit at p2 position
                x = x | shift1

        p1 += 1
        p2 += 1
        n -= 1

    # return final result
    return x

if __name__ == "__main__":
    res = swapBits(28, 0, 3, 2)
    print(res)

// C# Program to swap bits
// in a given number
using System;

class GfG {

    static int swapBits(int x, int p1, int p2, int n) {
        int shift1, shift2, value1, value2;
        while (n-- > 0) {
            
            // Setting bit at p1 position to 1
            shift1 = 1 << p1;
            // Setting bit at p2 position to 1
            shift2 = 1 << p2;

            // value1 and value2 will have 0 if num
            // at the respective positions - p1 and p2 is 0.
            value1 = (x & shift1);
            value2 = (x & shift2);

            // check if value1 and value2 are different
            // i.e. at one position bit is set and other it is not
            if ((!Convert.ToBoolean(value1) && value2 != 0) || 
            (!Convert.ToBoolean(value2) && value1 != 0)) {
                
                // if bit at p1 position is set
                if (value1 != 0) {
                    
                    // unset bit at p1 position
                    x = x & (~shift1);
                    
                    // set bit at p2 position
                    x = x | shift2;
                }
                // if bit at p2 position is set
                else {
                    
                    // set bit at p2 position
                    x = x & (~shift2);
                    
                    // unset bit at p2 position
                    x = x | shift1;
                }
            }
            p1++;
            p2++;
        }

        // return final result
        return x;
    }

    static void Main(string[] args) {
        int res = swapBits(28, 0, 3, 2);
        Console.WriteLine(res);
    }
}

// JavaScript Program to swap bits
// in a given number

function swapBits(x, p1, p2, n) {
    let shift1, shift2, value1, value2;
    while (n--) {

        // Setting bit at p1 position to 1
        shift1 = 1 << p1;
        // Setting bit at p2 position to 1
        shift2 = 1 << p2;

        // value1 and value2 will have 0 if num
        // at the respective positions - p1 and p2 is 0.
        value1 = x & shift1;
        value2 = x & shift2;

        // check if value1 and value2 are different
        // i.e. at one position bit is set and other it is not
        if ((!value1 && value2) || (!value2 && value1)) {

            // if bit at p1 position is set
            if (value1) {

                // unset bit at p1 position
                x = x & (~shift1);

                // set bit at p2 position
                x = x | shift2;
            }
            // if bit at p2 position is set
            else {

                // set bit at p2 position
                x = x & (~shift2);

                // unset bit at p2 position
                x = x | shift1;
            }
        }
        p1++;
        p2++;
    }

    // return final result
    return x;
}

let res = swapBits(28, 0, 3, 2);
console.log(res);

7

Related Article:

• Swapping individual bits with XOR 
   

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Article Tags :

• Bit Magic
• DSA
• Bitwise-XOR

Practice Tags :

• Bit Magic