Program to reverse an array up to a given position in Python

In this tutorial, we will learn how to reverse an array up to a given position. This means reversing elements from index 0 to index (n-1), while keeping the remaining elements in their original positions.

Problem Statement

Given an array of integers and a number n, reverse the elements from the 0th index to (n-1)th index ?

Input:
array = [1, 2, 3, 4, 5, 6, 7, 8, 9], n = 5

Output:
[5, 4, 3, 2, 1, 6, 7, 8, 9]

Method 1: Using Swapping

This approach swaps elements from both ends of the subarray until we reach the middle ?

# initializing array and a number
arr = [1, 2, 3, 4, 5, 6, 7, 8, 9]
n = 5

# checking whether the n value is less than length of the array or not
if n > len(arr):
    print(f"{n} value is not valid")
else:
    # loop until n // 2
    for i in range(n // 2):
        arr[i], arr[n - i - 1] = arr[n - i - 1], arr[i]
    
    # printing the array
    print(arr)

[5, 4, 3, 2, 1, 6, 7, 8, 9]

Method 2: Using Slicing

Python slicing provides a more concise way to reverse the subarray ?

# initializing array and a number
arr = [1, 2, 3, 4, 5, 6, 7, 8, 9]
n = 5

# checking whether the n value is less than length of the array or not
if n > len(arr):
    print(f"{n} value is not valid")
else:
    # reversing the arr up to n
    # [n-1::-1] reverses from index (n-1) to 0
    # [n:] keeps elements from index n to end unchanged
    arr = arr[n-1::-1] + arr[n:]
    
    # printing the array
    print(arr)

[5, 4, 3, 2, 1, 6, 7, 8, 9]

How It Works

For the input array [1, 2, 3, 4, 5, 6, 7, 8, 9] with n = 5:

• Method 1: Swaps elements at positions (0,4), (1,3) while position 2 stays the same
• Method 2: arr[4::-1] gives [5, 4, 3, 2, 1] and arr[5:] gives [6, 7, 8, 9]

Comparison

Conclusion

Use the swapping method for in-place reversal with minimal memory usage. Use slicing for cleaner, more readable code when memory isn't a constraint.