Two For Loops in List Comprehension - Python

List comprehension is a concise and readable way to create lists in Python. Using two for loops in list comprehension is a great way to handle nested iterations and generate complex lists.

Using two for-loops

This is the simplest and most efficient way to write two for loops in a list comprehension. Let's suppose we want to combine two loops inside one. We have two lists, and we want to combine their values in pairs.

x = [1, 2]
y = [3, 4]

# Using two for loops inside a list comprehension
ans = [(a, b) for a in x for b in y]

print(ans)

[(1, 3), (1, 4), (2, 3), (2, 4)]

There are various methods to write two loops in List Comprehension in Python.

Filter Based on a Condition (Adding an if Clause)

We can also add a condition to filter the results while using two for loops. Let’s say we only want pairs where the sum of a and b is greater than 4.

x = [1, 2, 3]
y = [3, 4, 5]

# Using two for loops and an if condition to filter
ans = [(a, b) for a in x for b in y if a + b > 4]

print(ans)

[(1, 4), (1, 5), (2, 3), (2, 4), (2, 5), (3, 3), (3, 4), (3, 5)]

Nested Loops with Different Range

Sometimes, we might want to generate numbers or perform a calculation based on the ranges. In such cases, using two for loops is helpful.

# Generating pairs of numbers from 1 to 3 and 4 to 6
ans = [(a, b) for a in range(1, 4) for b in range(4, 7)]

print(ans)

[(1, 4), (1, 5), (1, 6), (2, 4), (2, 5), (2, 6), (3, 4), (3, 5), (3, 6)]

Using Two Loops with a More Complex Condition

Now let’s try something a bit more complex. We want to generate pairs where a is even and b is odd.

# Generating pairs with specific conditions
ans = [(a, b) for a in range(1, 6) for b in range(1, 6) if a % 2 == 0 and b % 2 != 0]

print(ans)

[(2, 1), (2, 3), (2, 5), (4, 1), (4, 3), (4, 5)]