Swapping first and last items in a list

Question

How can I go about swapping numbers in a given list?

For example:

list = [5,6,7,10,11,12]

I would like to swap 12 with 5.

Is there an built-in Python function that can allow me to do that?

Solution 1

>>> lis = [5,6,7,10,11,12]
>>> lis[0], lis[-1] = lis[-1], lis[0]
>>> lis
[12, 6, 7, 10, 11, 5]

Order of evaluation of the above expression:

expr3, expr4 = expr1, expr2

First items on RHS are collected in a tuple, and then that tuple is unpacked and assigned to the items on the LHS.

>>> lis = [5,6,7,10,11,12]
>>> tup = lis[-1], lis[0]
>>> tup
(12, 5)
>>> lis[0], lis[-1] = tup
>>> lis
[12, 6, 7, 10, 11, 5]

Solution 2

You can use "*" operator.

my_list = [1,2,3,4,5,6,7,8,9]
a, *middle, b = my_list
my_new_list = [b, *middle, a]
my_list
[1, 2, 3, 4, 5, 6, 7, 8, 9]
my_new_list
[9, 2, 3, 4, 5, 6, 7, 8, 1]

Read here for more information.

Solution 3

you can swap using this code,

list[0],list[-1] = list[-1],list[0]

Comments

[DSM]

What specifies which elements you want to swap? The values (replace every 12 with a 5 and every 5 with a 12), the positions (switch the values in the first and last place), or some other rule? For example, what do you want to happen for [5,5,12,12]?

[user2891763]

I would like the last number in the list to always be swapped with the first number in a list.... in any given list.

[user2891763]

hello ! Thanks for the prompt reply ! Could you please explain what is happening in this piece of Code... I am fairly new at programming.

[Ashwini Chaudhary]

@user2891763 I've added some explanation.

[truthadjustr]

This swapping technique also works for numpy.ndarray

[truthadjustr]

Yes. Also works the same for numpy.ndarray