Read a list of n elements. Pass this list to a function which reverses this list in-place without creating a new list.

Question

Accepted Answer

> [!TLDR]
> Swap the first element with the last, the second with the second-last, and so on up to the middle — the list is reversed inside the *same* object, with no new list created.

**The idea (in-place reversal).** Because a list is mutable and is passed to a function *by reference*, the function can rearrange the caller's own list. Reversal in place needs only pairwise swaps: for a list of `n` elements, swap index `i` with index `n-1-i` for `i = 0, 1, …, n//2 - 1`. We stop at the middle — going further would swap every pair back again.

**Program**

```python
def reverse_in_place(lst):
    n = len(lst)
    for i in range(n // 2):
        lst[i], lst[n - 1 - i] = lst[n - 1 - i], lst[i]

# ---- main program ----
n = int(input("How many elements? "))
lst = []
for i in range(n):
    lst.append(int(input("Enter element " + str(i + 1) + ": ")))

print("Original list:", lst)
reverse_in_place(lst)
print("Reversed list:", lst)
```

**Dry run** for `lst = [1, 2, 3, 4, 5]` (`n = 5`, so the loop runs for `i = 0, 1`):

| Step | i | Swap performed | List after the swap |
| --- | --- | --- | --- |
| start | — | — | [1, 2, 3, 4, 5] |
| 1 | 0 | lst[0] and lst[4] (1 and 5) | [5, 2, 3, 4, 1] |
| 2 | 1 | lst[1] and lst[3] (2 and 4) | [5, 4, 3, 2, 1] |

The middle element (index 2) stays put for odd `n`.

**Sample run (expected output)**

```
How many elements? 5
Enter element 1: 1
Enter element 2: 2
Enter element 3: 3
Enter element 4: 4
Enter element 5: 5
Original list: [1, 2, 3, 4, 5]
Reversed list: [5, 4, 3, 2, 1]
``` …

Q.Read a list of n elements. Pass this list to a function which reverses this list in-place without creating a new list.

Concept understanding — In Place List Reversal

In-Place List Reversal: The Intuition

The Precise Statement

Why It Works

Complexity …

Step	i	Swap performed	List after the swap
start	—	—	[1, 2, 3, 4, 5]
1	0	lst[0] and lst[4] (1 and 5)	[5, 2, 3, 4, 1]
2	1	lst[1] and lst[3] (2 and 4)	[5, 4, 3, 2, 1]

Q.Read a list of n elements. Pass this list to a function which reverses this list in-place without creating a new list.

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