Nested list comprehension in Python - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a nested list comprehension in Python?

A nested list comprehension is a way to create a list inside another list comprehension, allowing you to build multi-dimensional lists or perform operations on nested loops in a single, concise expression.

beginner

How does the order of 'for' loops affect a nested list comprehension?

The order of 'for' loops in a nested list comprehension matches the order of nested loops in regular code. The first 'for' is the outer loop, and the second 'for' is the inner loop, which runs completely for each iteration of the outer loop.

beginner

Example: What does this nested list comprehension do? [[x * y for y in range(3)] for x in range(2)]

It creates a 2D list where each inner list contains the products of x and y. For x=0 and x=1, it multiplies by y=0,1,2, resulting in [[0, 0, 0], [0, 1, 2]].

intermediate

Can nested list comprehensions replace nested for loops?

Yes, nested list comprehensions can replace nested for loops to create lists in a shorter and more readable way, especially when building or transforming multi-dimensional lists.

intermediate

What is a common use case for nested list comprehensions?

They are commonly used to create matrices, flatten nested lists, or apply operations across multiple dimensions in a compact and readable way.

What does the outer 'for' loop represent in a nested list comprehension?

A The first loop that runs once per outer element

B The last loop that runs for each inner element

C A conditional filter

D A function call

The outer 'for' loop runs once per outer element, controlling how many inner lists are created.

What is the output of [[i + j for j in range(2)] for i in range(3)]?

A [[0, 1, 2], [0, 1, 2]]

B [[0, 1], [1, 2], [2, 3]]

C [[0, 1], [0, 1], [0, 1]]

D [[1, 2], [2, 3], [3, 4]]

For i=0, j=0,1 → [0,1]; i=1 → [1,2]; i=2 → [2,3].

Which of these is a valid nested list comprehension?

A [x*y for x in range(2) y in range(3)]

B [x for y in range(3) for x in range(2)]

C [x*y for x in range(2), y in range(3)]

D [[x*y for y in range(3)] for x in range(2)]

Option D correctly nests two 'for' loops inside the list comprehension.

How can nested list comprehensions improve code?

A By removing the need for variables

B By making code run slower

C By making nested loops shorter and more readable

D By avoiding all loops

Nested list comprehensions make nested loops shorter and often easier to read.

What does this nested list comprehension do? [[i for i in range(3)] for _ in range(2)]

A Creates two lists each with [0, 1, 2]

B Creates one list with [0, 1, 2, 0, 1, 2]

C Creates a list of numbers from 0 to 5

D Creates an empty list

It creates two inner lists, each containing [0, 1, 2].

Explain how nested list comprehensions work and how they relate to nested for loops.

Think about how loops inside loops work and how list comprehensions can replace them.

Describe a real-life example where you might use a nested list comprehension.

Imagine working with tables or grids of numbers.