Swapping even and odd index pairs internally in JavaScript

We are required to write a JavaScript function that takes in an array of literals as the first and the only argument.

Our function should swap each consecutive even index with each other, and swap each consecutive odd indexes with each other. The function should do these swappings in place.

For example, if the input array is:

const arr = [0, 1, 2, 3, 4, 5, 6, 7, 8];

Then the array should become:

const output = [2, 3, 0, 1, 6, 7, 4, 5, 8];

Because 0 and 2 get swapped, 1 and 3 get swapped, 4 and 6 get swapped, 5 and 7 get swapped, and finally 8 remains the same since it has no pair.

How It Works

The algorithm processes the array in groups of 4 elements. For each group, it swaps:

• Even indices: positions 0 ? 2, then 4 ? 6, etc.
• Odd indices: positions 1 ? 3, then 5 ? 7, etc.

Example

Following is the implementation:

const arr = [0, 1, 2, 3, 4, 5, 6, 7, 8];

const swapPairs = (arr = []) => {
    const swap = (array, ind1, ind2) => {
        const temp = array[ind1];
        array[ind1] = array[ind2];
        array[ind2] = temp;
    };
    
    let i = 0;
    // Process groups of 4 elements
    for(; i + 3 < arr.length; i += 4){
        swap(arr, i, i + 2);      // Swap even indices
        swap(arr, i + 1, i + 3);  // Swap odd indices
    };
    
    // Handle remaining elements if array length is not divisible by 4
    if (i + 2 < arr.length){
        swap(arr, i, i + 2);
    };
};

swapPairs(arr);
console.log(arr);

Output

[
  2, 3, 0, 1, 6,
  7, 4, 5, 8
]

Alternative Approach

Here's a cleaner version using array destructuring for swapping:

const arr2 = [0, 1, 2, 3, 4, 5, 6, 7, 8];

const swapPairsClean = (arr) => {
    for (let i = 0; i + 3 < arr.length; i += 4) {
        // Swap even indices using destructuring
        [arr[i], arr[i + 2]] = [arr[i + 2], arr[i]];
        // Swap odd indices using destructuring
        [arr[i + 1], arr[i + 3]] = [arr[i + 3], arr[i + 1]];
    }
    
    // Handle remaining even pair
    const remaining = arr.length % 4;
    if (remaining >= 3) {
        const lastGroupStart = arr.length - remaining;
        [arr[lastGroupStart], arr[lastGroupStart + 2]] = [arr[lastGroupStart + 2], arr[lastGroupStart]];
    }
};

swapPairsClean(arr2);
console.log(arr2);

Output

[
  2, 3, 0, 1, 6,
  7, 4, 5, 8
]

Conclusion

This algorithm efficiently swaps even and odd index pairs by processing the array in groups of 4 elements. The swap operation is done in-place, modifying the original array without requiring additional memory space.