Python modify list in for loop

Question

Can somebody help me understand why I'm getting this result?

I'm trying to reverse this list using indexing.

def reverse_list(nums):
    for i in range(len(nums)):
        nums[i] = nums[-i-1]  # I think the problem is here in the assigning part

numbers = [1, 2, 3, 4]
reverse_list(numbers)

Output:

[4, 3, 3, 4]

Desired output:

[4, 3, 2, 1]

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You might also want to read [How to ask and answer homework questions](https://meta.stackoverflow.com/q/334822/4518341). I assume this is homework because otherwise you'd just use `numbers.reverse()`. — wjandrea, May 04 '23 at 23:46

score 3 · Answer 1 · answered May 04 '23 at 23:29

After you do nums[i] = nums[-i-1] you no longer have the original value of nums[i] in the list. So you won't be able to assign that to the corresponding element at the other end.

You need to swap the elements, not just assign one. And you should only iterate halfway through the list, otherwise you'll just end up swapping them back when you get to the second half.

def reverse_list(nums):
    for i in range(len(nums) // 2):
        nums[i], nums[-i-1] = nums[-i-1], nums[i]

Python modify list in for loop

1 Answers1