Problem 6: Reverse a List (without using reverse())

Hey everyone! 👋

Today, we're tackling a fundamental problem: Reversing a List manually.

The Problem

The goal is to write a function that reverses a list without using the built-in reverse() method or slicing with [::-1]. This is a great exercise to understand how list indexing works.

Example:

• reverse_list([1, 2, 3, 4]) should return [4, 3, 2, 1]

The Solution

Here is the Python implementation using a loop:

def reverse_list(lst):
    """
    Reverses a list by iterating backwards.
    """
    reversed_list = []
    # Loop from the last index to 0
    for i in range(len(lst)-1, -1, -1):
        reversed_list.append(lst[i])

    return reversed_list

# Test case
print(reverse_list([1, 2, 3, 4]))
# Output: [4, 3, 2, 1]

Code Breakdown

Let's walk through the code line by line:

1. def reverse_list(lst):

   • Defines a function named reverse_list that takes one parameter lst (a list).

2. reversed_list = []

   • Creates an empty list called reversed_list to store the reversed elements.

3. for i in range(len(lst)-1, -1, -1):

   • Starts a loop that will iterate from the last index of the list to the first (0).

   • len(lst)-1 is the starting index (last element).

   • -1 (the second argument) is the stopping index (stops before -1, so 0 is included).

   • -1 (The third argument) is the step (counts backwards).

4. reversed_list.append(lst[i])

   • For each iteration, it takes the element at index i from the original list and appends it to reversed_list.

5. return reversed_list

   • Returns the new list with elements in reverse order.

Example Walkthrough with [1, 2, 3, 4]

1. i starts at 3 (value 4), appends 4 to reversed_list.

2. i becomes 2 (value 3), appends 3.

3. i becomes 1 (value 2), appends 2.

4. i becomes 0 (value 1), appends 1.

5. Returns [4, 3, 2, 1].

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Happy coding! 💻