Need Help in switching values in a list in python

Question

I have a list of numbers, such as:

[1, 2, 3, 4, 5, 6]

I'm having trouble figuring out how to switch the first 2 items of the list with the last 2, or the first 3 with the last 3, and so on.

When i assign the values of the 1st two numbers to the last 2 items of the list, i then cannot assign the last 2 values (what used to be the last 2 values of the list) to the first two because the last 2 values have been lost.

If i try using another empty list and appending the last 2 values of the original list to that list, then appending the middle values, and then the first 2 values of the old list, I end up with something like this:

[[[5, 6], [3, 4], [1, 2]]]

I don't want nested lists! What I want is:

[5, 6, 3, 4, 1, 2]

Can someone help me?

you're almost there with the second approach, except that you should extend the target rather than append to it. — georg
– georg, Commented Apr 6, 2013 at 11:34

jamylak · Accepted Answer · 2013-04-06 11:35:13Z

10

>>> nums = [1, 2, 3, 4, 5, 6]
>>> nums[:2], nums[-2:] = nums[-2:], nums[:2]
>>> nums
[5, 6, 3, 4, 1, 2]

This modifies the original list but if you want a separate new list you should use the following:

>>> nums = [1, 2, 3, 4, 5, 6]
>>> swapped_nums = nums[-2:] + nums[2:-2] + nums[:2]
>>> swapped_nums
[5, 6, 3, 4, 1, 2]

Note: This won't work properly if your list has < 4 elements

edited Apr 6, 2013 at 11:35

answered Apr 6, 2013 at 11:19

jamylak

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