Nested comprehension

Learn nested comprehensions.

Nested comprehension

A comprehension can contain multiple for clauses. This corresponds to nested loops:

pairs = [(a, b) for a in [1, 2] for b in [3, 4]]
# [(1, 3), (1, 4), (2, 3), (2, 4)]

The first for is the outer loop, the second for is the inner loop. The result lists each (a, b) pair in order.

When a nested comprehension grows complex, readability suffers; then a classic loop may be better.

Flatten a matrix

A nested comprehension is handy for flattening a matrix into a single list:

matrix = [[1, 2], [3, 4]]
flat = [n for row in matrix for n in row]
# [1, 2, 3, 4]

First row is taken from the matrix, then n is taken from that row. This is a very short form.

Exercise Build a multiplication table

Assign to products a nested comprehension that computes a * b for every pair of a in [1, 2] and b in [3, 4]. Then print print(products).

Show example solution

products = [a * b for a in [1, 2] for b in [3, 4]]
print(products)

Test yourself

Match the nested parts

for a in [1, 2]

Outer loop
for b in [3, 4]

Inner loop
(a, b)

Expression
[...]

Result list

Complete the nested comprehension

flat = [n for row in matrix for ____ in row]

Show answer

flat = [n for row in matrix for n in row]

What does [(a, b) for a in [1, 2] for b in [3]] produce?
- [(1, 3)]
- [(2, 3)]
- [(1, 3), (1, 4), (2, 3), (2, 4)]
- [(1, 3), (2, 3)]
Show answer

[(1, 3), (2, 3)] — Pairs (1,3) and (2,3) are produced.

Next lesson: Generator expression

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