When one loop isn't enough

The problem...

You need to loop through something. And for every item — loop through something else.

A table. A grid. Every combination of two sequences. One loop can't do it alone.

The idea!

You can put a for loop inside another for loop. The outer loop runs once per item. For every outer iteration — the inner loop runs completely.

The syntax

for outer_item in outer_sequence:
    for inner_item in inner_sequence:
        # runs for every combination

Two levels of indentation. The inner block is indented 8 spaces from the left — 4 for the outer loop, 4 more for the inner.

Your first nested loop

for i in range(1, 4):
    for j in range(1, 4):
        print(f"{i} x {j} = {i * j}")

# 1 x 1 = 1
# 1 x 2 = 2
# 1 x 3 = 3
# 2 x 1 = 2
# 2 x 2 = 4
# 2 x 3 = 6
# 3 x 1 = 3
# 3 x 2 = 6
# 3 x 3 = 9

The outer loop runs 3 times. Each time — the inner loop runs 3 times. 3 × 3 = 9 lines total.

What's really happening

# i = 1 → inner loop runs: j = 1, 2, 3
# i = 2 → inner loop runs: j = 1, 2, 3
# i = 3 → inner loop runs: j = 1, 2, 3

The inner loop completes fully before the outer loop moves to the next item.

Looping through two strings

vowels = "aeiou"
word = "Bull"

for letter in word:
    for vowel in vowels:
        if letter == vowel:
            print(f"{letter} is a vowel")

# u is a vowel

For every letter in word — check it against every vowel. If it matches — print it.

Heads up!

• The inner loop runs completely for every outer iteration
• Total iterations = outer count × inner count
• Each level adds 4 spaces of indentation
• Nested loops get slow with large sequences — keep them small at this stage

The mindset shift

Stop thinking: "I need one loop per task."

Start thinking: "For every item in A — do something with every item in B."

What you should understand now

• A nested loop is a loop inside a loop
• The inner loop runs completely before the outer loop moves on
• Total iterations = outer × inner
• Each level of nesting adds 4 spaces of indentation