Problem Formulation
Given a list of size n and two indices i,j < n.
Swap the element at index i with the element at index j, so that the element list[i] is now at position j and the original element list[j] is now at position i.
Examples:
- Swapping indices
0and2in list[1, 2, 3]modifies the list to[3, 2, 1]. - Swapping indices
1and2in list[1, 2, 3]modifies the list to[1, 3, 2]. - Swapping indices
1and3in list['alice', 'bob', 'carl', 'denis']modifies the list to['alice', 'denis', 'carl', 'bob']
Method 1: Multiple Assignment
To swap two list elements by index i and j, use the multiple assignment expression lst[i], lst[j] = lst[j], lst[i] that assigns the element at index i to index j and vice versa.
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
jandiby running the right-hand side of the assignment operation. - Second, it assigns the obtained elements in one go to the inverse indices
iandj(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 x and y by value, get the index of their first occurrences using the list.index(x) and list.index(y) methods and assign the result to variables i and j, respectively. Then apply the multiple assignment expression 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:
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 list.index() method?
💡 Background: The list.index(value) method returns the index of the value argument in the list. You can use optional start and 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 ValueError.
Feel free to also watch the following quick explainer video:
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