Nested list comprehensions

Great! At this point, you have a good grasp of the basic syntax of list comprehensions. Let's push your code-writing skills a little further. In this exercise, you will be writing a list comprehension within another list comprehension, or nested list comprehensions. It sounds a little tricky, but you can do it!

Let's step aside for a while from strings. One of the ways in which lists can be used are in representing multi-dimension objects such as matrices. Matrices can be represented as a list of lists in Python. For example a 5 x 5 matrix with values 0 to 4 in each row can be written as:

matrix = [[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]]

Your task is to recreate this matrix by using nested listed comprehensions. Recall that you can create one of the rows of the matrix with a single list comprehension. To create the list of lists, you simply have to supply the list comprehension as the output expression of the overall list comprehension:

[[output expression] for iterator variable in iterable]

Note that here, the output expression is itself a list comprehension.

This exercise is part of the course

Python Toolbox

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Exercise instructions

In the inner list comprehension - that is, the output expression of the nested list comprehension - create a list of values from 0 to 4 using range(). Use col as the iterator variable.
In the iterable part of your nested list comprehension, use range() to count 5 rows - that is, create a list of values from 0 to 4. Use row as the iterator variable; note that you won't be needing this variable to create values in the list of lists.

Hands-on interactive exercise

Have a go at this exercise by completing this sample code.

# Create a 5 x 5 matrix using a list of lists: matrix
matrix = [[____] ____]

# Print the matrix
for row in matrix:
    print(row)

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