Nested for loop execution in Python

Introduction

Nested for loops help you repeat actions inside other repeated actions, like checking every seat in a theater row by row.

When you want to go through a grid or table, like rows and columns.

When you need to compare every item in one list with every item in another list.

When you want to create combinations of items from two or more groups.

When you want to print patterns, like stars in rows and columns.

When you want to process multi-level data, like a list of lists.

Syntax

Python

for item1 in collection1:
    for item2 in collection2:
        # do something with item1 and item2

The inner loop runs completely for each single run of the outer loop.

Indentation shows which loop is inside the other.

Examples

Prints pairs of numbers where i goes 0 to 1 and j goes 0 to 2 for each i.

Python

for i in range(2):
    for j in range(3):
        print(i, j)

Prints seat labels combining rows and seat numbers.

Python

for row in ['A', 'B']:
    for seat in [1, 2]:
        print(f"Row {row} Seat {seat}")

Sample Program

This program shows how the inner loop runs fully for each step of the outer loop.

Python

for i in range(3):
    for j in range(2):
        print(f"Outer loop i={i}, Inner loop j={j}")

Important Notes

Remember that the inner loop finishes all its steps before the outer loop moves to the next step.

Too many nested loops can make your program slow, so use them wisely.

Indentation is very important in Python to show which code belongs to which loop.

Summary

Nested for loops let you repeat actions inside other repeated actions.

The inner loop runs completely for each step of the outer loop.

Use nested loops to work with grids, combinations, or multi-level data.