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Nested List Comprehensions in Python

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List Comprehension are one of the most amazing features of Python. It is a smart and concise way of creating lists by iterating over an iterable object. Nested List Comprehensions are nothing but a list comprehension within another list comprehension which is quite similar to nested for loops.

Nested List Comprehension in Python Syntax

Below is the syntax of nested list comprehension:

Syntax: new_list = [[expression for item in list] for item in list]

Parameters:

  • Expression: Expression that is used to modify each item in the statement
  • Item: The element in the iterable
  • List: An iterable object

Python Nested List Comprehensions Examples

Below are some examples of nested list comprehension:

Example 1: Creating a Matrix

In this example, we will compare how we can create a matrix when we are creating it with

Without List Comprehension

In this example, a 5×5 matrix is created using a nested loop structure. An outer loop iterates five times, appending empty sublists to the matrix, while an inner loop populates each sublist with values ranging from 0 to 4, resulting in a matrix with consecutive integer values.

Python3

matrix = []
for i in range(5):
    # Append an empty sublist inside the list
    matrix.append([])
    for j in range(5):
        matrix[i].append(j)
print(matrix)

                    

Output
[[0, 1, 2, 3, 4], [0, 1, 2, 3, 4], [0, 1, 2, 3, 4], [0, 1, 2, 3, 4], [0, 1, 2, 3, 4]]

Using List Comprehension

The same output can be achieved using nested list comprehension in just one line. In this example, a 5×5 matrix is generated using a nested list comprehension. The outer comprehension iterates five times, representing the rows, while the inner comprehension populates each row with values ranging from 0 to 4, resulting in a matrix with consecutive integer values.

Python3

# Nested list comprehension
matrix = [[j for j in range(5)] for i in range(5)]
 
print(matrix)

                    

Output
[[0, 1, 2, 3, 4], [0, 1, 2, 3, 4], [0, 1, 2, 3, 4], [0, 1, 2, 3, 4], [0, 1, 2, 3, 4]]

Example 2: Filtering a Nested List Using List Comprehension

Here, we will see how we can filter a list with and without using list comprehension.

Without Using List Comprehension

In this example, a nested loop traverses a 2D matrix, extracting odd numbers from Python list within list and appending them to the list odd_numbers. The resulting list contains all odd elements from the matrix.

Python3

matrix = [[1, 2, 3], [4, 5, 6], [7, 8, 9]]
 
odd_numbers = []
for row in matrix:
    for element in row:
        if element % 2 != 0:
            odd_numbers.append(element)
 
print(odd_numbers)

                    

Output
[1, 3, 5, 7, 9]

Using List Comprehension

In this example, a list comprehension is used to succinctly generate the list odd_numbers by iterating through the elements of a 2D matrix. Only odd elements are included in the resulting list, providing a concise and readable alternative to the equivalent nested loop structure.

Python3

matrix = [[1, 2, 3], [4, 5, 6], [7, 8, 9]]
 
odd_numbers = [
    element for row in matrix for element in row if element % 2 != 0]
 
print(odd_numbers)

                    

Output
[1, 3, 5, 7, 9]

Example 3: Flattening Nested Sub-Lists

Without List Comprehension

In this example, a 2D list named matrix with varying sublist lengths is flattened using nested loops. The elements from each sublist are sequentially appended to the list flatten_matrix, resulting in a flattened representation of the original matrix.

Python3

# 2-D List
matrix = [[1, 2, 3], [4, 5], [6, 7, 8, 9]]
 
flatten_matrix = []
 
for sublist in matrix:
    for val in sublist:
        flatten_matrix.append(val)
 
print(flatten_matrix)

                    

Output
[1, 2, 3, 4, 5, 6, 7, 8, 9]

With List Comprehension

Again this can be done using nested list comprehension which has been shown below. In this example, a 2D list named matrix with varying sublist lengths is flattened using nested list comprehension. The expression [val for sublist in matrix for val in sublist] succinctly generates a flattened list by sequentially including each element from the sublists.

Python3

# 2-D List
matrix = [[1, 2, 3], [4, 5], [6, 7, 8, 9]]
 
# Nested List Comprehension to flatten a given 2-D matrix
flatten_matrix = [val for sublist in matrix for val in sublist]
 
print(flatten_matrix)

                    

Output
[1, 2, 3, 4, 5, 6, 7, 8, 9]

Example 4: Manipulate String Using List Comprehension

Without List Comprehension

In this example, a 2D list named matrix containing strings is modified using nested loops. The inner loop capitalizes the first letter of each fruit, and the outer loop constructs a new 2D list, modified_matrix, with the capitalized fruits, resulting in a matrix of strings with initial capital letters.

Python3

matrix = [["apple", "banana", "cherry"],
          ["date", "fig", "grape"],
          ["kiwi", "lemon", "mango"]]
 
modified_matrix = []
for row in matrix:
    modified_row = []
    for fruit in row:
        modified_row.append(fruit.capitalize())
    modified_matrix.append(modified_row)
 
print(modified_matrix)

                    

Output
[['Apple', 'Banana', 'Cherry'], ['Date', 'Fig', 'Grape'], ['Kiwi', 'Lemon', 'Mango']]

With List Comprehension

In this example, a 2D list named matrix containing strings is transformed using nested list comprehension. The expression [[fruit.capitalize() for fruit in row] for row in matrix] efficiently generates a modified matrix where the first letter of each fruit is capitalized, resulting in a new matrix of strings with initial capital letters.

Python3

matrix = [["apple", "banana", "cherry"],
          ["date", "fig", "grape"],
          ["kiwi", "lemon", "mango"]]
 
modified_matrix = [[fruit.capitalize() for fruit in row] for row in matrix]
 
print(modified_matrix)

                    

Output
[['Apple', 'Banana', 'Cherry'], ['Date', 'Fig', 'Grape'], ['Kiwi', 'Lemon', 'Mango']]


Last Updated : 13 Dec, 2023
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