Swap All Odd and Even Bits in C++

We can swap all odd and even bits in a given integer. Swapping of odd and even bits means changing the bits present at odd positions with the bits at even positions in a binary representation of a number. One real-life application is optimizing data storage or modifying data patterns in memory.

In this article, we are going to learn how we can swap all odd and even bits of a number in C++ using different approaches.

Formula for Swapping Odd and Even Bits

• Extract all even-positioned bits using a bit mask and shift them right.
• Extract all odd-positioned bits using a bit mask and shift them left.
• Combine both values using the bitwise OR (|) operator.

Here are two examples based on the above formula:

Example 1

Input:
n = 23

Output:
Swapped number = 43

Explanation:
Binary representation of 23 is 00010111. 
After swapping odd and even places:
It becomes 00101011, which is 43 in decimal.

Example 2

Input:
n = 10

Output:
Swapped number = 5

Explanation:
Binary representation of 10 is 00001010. 
After swapping: 
It becomes 00000101, which is 5 in decimal.

Below are different approaches to swap all odd and even bits of a number:

• Using Bit Manipulation
• Using Loops
• Using XOR and Shift Operations

Using Bit Manipulation

In this approach, we use bitwise operators to extract, shift, and combine bits efficiently.

• Define the integer number.
• Now, extract even bits using 0xAAAAAAAA and shift them right by 1.
• Now, extract odd bits using 0x55555555 and shift them left by 1.
• Combine these using the OR (|) operator.
• Finally, output the swapped number.

Example

Here is an example code implementing above steps to swap odd and even bits using bit manipulation.

#include<bits/stdc++.h>
using namespace std;

unsigned int swapOddEvenBits(unsigned int n) {
    unsigned int even_bits = n & 0xAAAAAAAA;  // Extract even bits
    unsigned int odd_bits = n & 0x55555555;   // Extract odd bits
    even_bits >>= 1;  // Right shift even bits
    odd_bits <<= 1;   // Left shift odd bits
    return (even_bits | odd_bits); // Combine both
}

int main() {
    unsigned int n = 23;
    cout << "Swapped number: " << swapOddEvenBits(n);
    return 0;
}

Using Loops

In this approach, we manually swap bits by iterating through each bit pair.

• First, extract each bit pair using bitwise AND masks.
• Now, swap and shift them accordingly.
• Combine the answer.
• Output the swapped number.

Example

The following example code uses loops to swap odd and even bits.

#include<bits/stdc++.h>
using namespace std;

unsigned int swapBitsLoop(unsigned int n) {
    unsigned int result = 0;
    for (int i = 0; i < 32; i += 2) {
        unsigned int even_bit = (n & (1 << i)) << 1;
        unsigned int odd_bit = (n & (1 << (i + 1))) >> 1;
        result |= even_bit | odd_bit;
    }
    return result;
}

int main() {
    unsigned int n = 10;
    cout << "Swapped number: " << swapBitsLoop(n);
    return 0;
}

Using XOR and Shift Operations

This is a more optimized approach that reduces redundant bitwise operations by using XOR and shifting.

• First, extract even bits using a mask.
• Now, extract odd bits using another mask.
• Swap them using XOR and shift operations.
• Output the answer.

Example

Here is an example code implementing XOR and shift operations to swap odd and even bits.

#include<bits/stdc++.h>
using namespace std;

unsigned int swapBitsOptimized(unsigned int n) {
    return ((n & 0xAAAAAAAA) >> 1) | ((n & 0x55555555) << 1);
}

int main() {
    unsigned int n = 42;
    cout << "Swapped number: " << swapBitsOptimized(n);
    return 0;
}

Complexity Comparison

Here is a comparison of time and space complexity of all the above approaches.

Real-Life Applications

• It is used in data encoding and cryptography.
• It is used for helping in graphics rendering to manipulate pixel data.
• It is used for optimizing hardware memory management in embedded systems.
• It is used in parallel computing for efficient data processing.