[Tut] How to Swap List Elements in Python?

[Tut] How to Swap List Elements in Python? - Printable Version

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[Tut] How to Swap List Elements in Python? - xSicKxBot - 03-31-2022

How to Swap List Elements in Python?

Problem Formulation

Given a list of size

Code:
n

and two indices

Code:
i,j < n

.

Swap the element at index

Code:
i

with the element at index

Code:
j

, so that the element

Code:
list[i]

is now at position

Code:
j

and the original element

Code:
list[j]

is now at position

Code:
i

.

Examples:

Swapping indices and
Code:
2
in list
Code:
[1, 2, 3]
modifies the list to
Code:
[3, 2, 1]
.
Swapping indices
Code:
1
and
Code:
2
in list
Code:
[1, 2, 3]
modifies the list to
Code:
[1, 3, 2]
.
Swapping indices
Code:
1
and
Code:
3
in list
Code:
['alice', 'bob', 'carl', 'denis']
modifies the list to
Code:
['alice', 'denis', 'carl', 'bob']
.

Method 1: Multiple Assignment

To swap two list elements by index

Code:
i

and

Code:
j

, use the multiple assignment expression

Code:
lst[i], lst[j] = lst[j], lst[i]

that assigns the element at index

Code:
i

to index

Code:
j

and vice versa.

Code:
lst = ['alice', 'bob', 'carl']
i, j = 0, 2 # Swap index i=0 with index j=2
lst[i], lst[j] = lst[j], lst[i] print(lst)
# ['carl', 'bob', 'alice']

The highlighted line works as follows:

First, it obtains the elements at positions
Code:
j
and
Code:
i
by running the right-hand side of the assignment operation.
Second, it assigns the obtained elements in one go to the inverse indices
Code:
i
and
Code:
j
(see left-hand side of the assignment operation).

To help you better understand this code snippet, I’ve recorded a quick video that shows you how the generalization of multiple assignment, i.e., slice assignment, works as a Python One-Liner:

Method 2: Swap Two Elements by Value Using indexof()

Let’s quickly discuss a variant of this problem whereby you want to swap two elements but you don’t know their indices yet.

To swap two list elements

Code:
x

and

Code:
y

by value, get the index of their first occurrences using the

Code:
list.index(x)

and

Code:
list.index(y)

methods and assign the result to variables

Code:
i

and

Code:
j

, respectively. Then apply the multiple assignment expression

Code:
lst[i], lst[j] = lst[j], lst[i]

to swap the elements.

The latter part, i.e., swapping the list elements, remains the same. The main difference is highlighted in the following code snippet:

Code:
lst = ['alice', 'bob', 'carl']
x, y = 'alice', 'carl' # Get indices i and j associated with elements x and y
i, j = lst.index(x), lst.index(y) # Swap element at index i with element at index j
lst[i], lst[j] = lst[j], lst[i] print(lst)
# ['carl', 'bob', 'alice']

Do you need a quick refresher on the

Code:
list.index()

method?

Background: The

Code:
list.index(value)

method returns the index of the

Code:
value

argument in the

Code:
list

. You can use optional

Code:
start

and

Code:
stop

arguments to limit the index range where to search for the value in the list. If the value is not in the list, the method throws a

Code:
ValueError

.

Feel free to also watch the following quick explainer video:

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