Nested Loops

What You’ll Learn

A nested loop is a loop inside a loop. You’ll learn how to use them to iterate over multi-dimensional data.

How it Works

The “inner loop” will be executed one time for each iteration of the “outer loop”.

colors = ["red", "green"]
fruits = ["apple", "banana"]

for color in colors:
    for fruit in fruits:
        print(color, fruit)

Expected Output:

red apple
red banana
green apple
green banana

Breaking down the execution

1. The outer loop starts with color = "red".
2. The inner loop starts and goes through all items: “red apple”, then “red banana”.
3. The inner loop finishes.
4. The outer loop moves to the next item: color = "green".
5. The inner loop starts again from the beginning: “green apple”, then “green banana”.

Common Mistakes

• Performance: Be careful with nested loops. If the outer loop runs 1,000 times, and the inner loop runs 1,000 times, the code inside the inner loop runs 1,000,000 times!