Python Program for array rotation

In this article, we will learn how to rotate an array by a given number of positions. Array rotation is a fundamental operation where elements are shifted left or right, with elements that fall off one end being placed at the other end.

Problem statement ? Given an array and a number of positions, we need to rotate the array elements by the specified number of positions.

Using Slicing (Left Rotation)

The simplest approach uses Python's slicing to rotate an array to the left ?

def rotate_left(arr, positions):
    n = len(arr)
    positions = positions % n  # Handle positions greater than array length
    return arr[positions:] + arr[:positions]

# Example usage
numbers = [1, 2, 3, 4, 5]
rotated = rotate_left(numbers, 2)
print("Original array:", numbers)
print("After left rotation by 2:", rotated)

Original array: [1, 2, 3, 4, 5]
After left rotation by 2: [3, 4, 5, 1, 2]

Using Slicing (Right Rotation)

For right rotation, we move elements in the opposite direction ?

def rotate_right(arr, positions):
    n = len(arr)
    positions = positions % n  # Handle positions greater than array length
    return arr[-positions:] + arr[:-positions]

# Example usage
numbers = [1, 2, 3, 4, 5]
rotated = rotate_right(numbers, 2)
print("Original array:", numbers)
print("After right rotation by 2:", rotated)

Original array: [1, 2, 3, 4, 5]
After right rotation by 2: [4, 5, 1, 2, 3]

In-Place Rotation Using Reverse

This method uses array reversal to achieve rotation without extra space ?

def reverse_array(arr, start, end):
    while start < end:
        arr[start], arr[end] = arr[end], arr[start]
        start += 1
        end -= 1

def rotate_left_inplace(arr, positions):
    n = len(arr)
    positions = positions % n
    
    # Reverse entire array
    reverse_array(arr, 0, n - 1)
    # Reverse first n-positions elements
    reverse_array(arr, 0, n - positions - 1)
    # Reverse last positions elements
    reverse_array(arr, n - positions, n - 1)

# Example usage
numbers = [1, 2, 3, 4, 5]
print("Original array:", numbers)
rotate_left_inplace(numbers, 2)
print("After in-place left rotation by 2:", numbers)

Original array: [1, 2, 3, 4, 5]
After in-place left rotation by 2: [3, 4, 5, 1, 2]

Comparison of Methods

Conclusion

Array rotation can be implemented using slicing for simplicity or in-place reversal for memory efficiency. Use slicing when you want to preserve the original array, and in-place methods when memory is a constraint.