[Tut] How to Swap List Elements in Python?

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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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