3. Nested Loops and List Comprehension

Quiz: 0/4

In Python, you can use nested loops to loop through multiple sequences, such as lists or ranges. When using list comprehension, you can also use nested loops to handle situations where you need to work with more than one iterable (like two lists or two ranges) at the same time. This allows you to create more complex lists in a compact and efficient way.

Basic syntax:

When using nested loops in a list comprehension, the syntax is as follows:

[expression for item1 in iterable1 for item2 in iterable2]

• item1 and item2 are the elements from iterable1 and iterable2 (the two lists or ranges).
• The expression is what you want to put in your new list. It can use both item1 and item2.

Example 1: Creating coordinate pairs from two lists

Let’s say you have two lists: one with x coordinates and one with y coordinates, and you want to create all possible coordinate pairs:

x_coords = [1, 2, 3]
y_coords = [4, 5, 6]

coordinates = [(x, y) for x in x_coords for y in y_coords]
print(coordinates)

Explanation:

• x_coords contains the values [1, 2, 3] and y_coords contains the values [4, 5, 6].
• The list comprehension will loop through each x in x_coords and pair it with each y in y_coords.
• The result is a list of tuples representing all possible coordinate pairs: [(1, 4), (1, 5), (1, 6), (2, 4), (2, 5), (2, 6), (3, 4), (3, 5), (3, 6)].

Output:

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

Example 2: Flattening a 2D List (Matrix) using nested list comprehension

Imagine you have a 2D list (a matrix) and you want to "flatten" it into a 1D list:

matrix = [
    [1, 2, 3],
    [4, 5, 6],
    [7, 8, 9]
]

flattened = [item for row in matrix for item in row]
print(flattened)

Explanation:

• The 2D list matrix has three rows, each with three numbers.
• The list comprehension loops through each row in the matrix, and then through each item in the row.
• The result is a flattened version of the matrix where all the numbers are in one single list.

Output:

[1, 2, 3, 4, 5, 6, 7, 8, 9]

Why use nested loops in list comprehension?

Using nested loops in list comprehension helps you:

1. Simplify your code: Instead of writing multiple loops, you can combine them in a single line.
2. Improve performance: List comprehensions are faster and more memory efficient compared to traditional loops in many cases.
3. Make your code more readable: List comprehensions are compact, which makes your code cleaner and easier to understand.

This way, you can create more complex lists without needing to write lengthy loops!