# 5 Best Ways to Pattern after Double Reverse and Swap in Python

π‘ Problem Formulation: Consider you have a list in Python and you want to apply a pattern that involves reversing the list twice and then swapping its elements. Specifically, given an input list, you want to reverse it, reverse the result again (essentially reverting to the original order), and then switch the positions of certain elements according to a specified pattern. For example, given the input list `[1, 2, 3, 4, 5]`, after a double reverse and a swap of the first and last elements, the desired output is `[5, 2, 3, 4, 1]`.

## Method 1: Using List Slicing

This method involves reversing the list by using the slicing technique, which is a concise and efficient way to reverse a list in Python. Then, we apply the pattern that reverses the list back and proceeds with the element swapping.

Here’s an example:

```def double_reverse_and_swap(lst, i, j):
# Double reverse is essentially the original list, so we skip it
lst[i], lst[j] = lst[j], lst[i]
return lst

my_list = [1, 2, 3, 4, 5]
swapped_list = double_reverse_and_swap(my_list, 0, -1)
print(swapped_list)```

The output of this code snippet is:

`[5, 2, 3, 4, 1]`

In this example, we simply perform the swapping of the first and the last elements, as the double reverse brings the list back to its original order. Slicing is skipped entirely for efficiency.

## Method 2: Using reverse() Method

This method relies on Python’s built-in `reverse()` method to reverse the list twice and then swaps the specified elements.

Here’s an example:

```def double_reverse_and_swap(lst, i, j):
lst.reverse()
lst.reverse()
lst[i], lst[j] = lst[j], lst[i]
return lst

my_list = [1, 2, 3, 4, 5]
swapped_list = double_reverse_and_swap(my_list, 0, -1)
print(swapped_list)```

The output of this code snippet is:

`[5, 2, 3, 4, 1]`

This snippet explicitly performs the reversing twice before doing the swap, which makes the logic clear but is redundant as reversing a reversed list results in the original list.

## Method 3: Utilizing a Function

A functional approach encapsulates the logic in a dedicated function, enhancing reusability and readability. The function uses list slicing to swap elements directly, without actually performing the double reverse.

Here’s an example:

```def swap_elements(lst, i, j):
lst[i], lst[j] = lst[j], lst[i]
return lst

my_list = [1, 2, 3, 4, 5]
swapped_list = swap_elements(my_list, 0, -1)
print(swapped_list)```

The output of this code snippet is:

`[5, 2, 3, 4, 1]`

The function `swap_elements()` is clear and concise, focusing solely on the swapping of elements. The double reverse is omitted for simplicity and effectiveness.

## Method 4: Using Intermediate Variables

This method introduces the use of temporary intermediate variables to handle the swapping of the elements, which can make the logic easier to follow for some.

Here’s an example:

```def double_reverse_and_swap(lst, i, j):
temp = lst[i]
lst[i] = lst[j]
lst[j] = temp
return lst

my_list = [1, 2, 3, 4, 5]
swapped_list = double_reverse_and_swap(my_list, 0, -1)
print(swapped_list)```

The output of this code snippet is:

`[5, 2, 3, 4, 1]`

In this code, we directly swap the elements using a temporary variable without performing the double reverse. The use of a temporary variable makes the swaps explicit.

## Bonus One-Liner Method 5: Using Tuple Unpacking

Tuple unpacking allows for a sleek one-liner to swap elements elegantly. It’s Pythonic and straightforward.

Here’s an example:

```my_list = [1, 2, 3, 4, 5]
my_list[0], my_list[-1] = my_list[-1], my_list[0]
print(my_list)```

The output of this code snippet is:

`[5, 2, 3, 4, 1]`

By using tuple unpacking, we achieve the swapping of elements in a single line of code, which is efficient and exceptionally concise.

## Summary/Discussion

• Method 1: List Slicing. Efficient. Skips unnecessary steps. Not explicitly showing the ‘double reverse’.
• Method 2: reverse() Method. Clear logic. Unnecessary operations. More understandable for beginners.
• Method 3: Utilizing a Function. Reusable. Readable. Omits the redundant double reverse step.
• Method 4: Intermediate Variables. Clear variable swaps. Slightly verbose. Good for step-by-step debugging.
• Method 5: Tuple Unpacking. Elegant and Pythonic. Extremely concise. Best when looking for a one-liner solution.