Home / Python / Day 7: Intermediate Python / Comprehensions

Comprehensions

Comprehensions provide concise, readable syntax for creating lists, dicts, and sets.

Python Comprehensions

Comprehensions create new collections from existing ones in a single, readable line. They are more Pythonic and often faster than equivalent loops.

Syntax

<pre><code># List comprehension
squares = [x**2 for x in range(10)]
# Equivalent to:
# squares = []
# for x in range(10):
#     squares.append(x**2)

# With condition (filter)
evens = [x for x in range(20) if x % 2 == 0]

# Nested comprehension
matrix = [[i * j for j in range(1, 4)] for i in range(1, 4)]

# Dict comprehension
word_lengths = {word: len(word) for word in ["hello", "world", "python"]}
# {'hello': 5, 'world': 5, 'python': 6}

# Set comprehension
unique_lengths = {len(word) for word in ["hello", "world", "hi"]}
# {2, 5}

# Generator expression (lazy — no list in memory)
gen = (x**2 for x in range(1000000))  # Does not create list
next(gen)  # 0
next(gen)  # 1
</code></pre>

Revision Notes

• [expr for x in iter] — list comprehension
• [expr for x in iter if cond] — with filter
• {k: v for k, v in iter} — dict comprehension
• {expr for x in iter} — set comprehension
• (expr for x in iter) — generator (lazy)
• Prefer comprehensions over map/filter for readability

Build a Multiplication Table

Medium

Write a function multiplication_table(n) that returns an n x n multiplication table as a list of lists, where element [i][j] = (i+1)*(j+1), using a nested list comprehension.

Input:

multiplication_table(3)

Output:

[[1, 2, 3], [2, 4, 6], [3, 6, 9]]

Show Hint

Use [[ (i+1)*(j+1) for j in range(n) ] for i in range(n) ].

Solve this Challenge

Show Solution

def multiplication_table(n):
    return [[(i + 1) * (j + 1) for j in range(n)] for i in range(n)]

print(multiplication_table(3))

Decorators