Nested Loops

Beginner ~20 min read

A nested loop is a loop inside another loop. The inner loop executes completely for each iteration of the outer loop. Think of it like a clock: for every hour that passes, the minute hand goes around 60 times!

How Nested Loops Work

In a nested loop, the inner loop runs its full course for each single iteration of the outer loop. This creates a multiplicative effect on the total number of iterations.

Output

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Iteration Count: If the outer loop runs m times and the inner loop runs n times, the code inside the inner loop runs m × n times total. A 10×10 nested loop runs 100 times; a 100×100 runs 10,000 times!

Working with 2D Data

Nested loops are essential for working with 2D data structures like matrices, grids, and tables. The outer loop handles rows, the inner loop handles columns.

Output

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Pro Tip: When searching in 2D data, you need to break out of BOTH loops when you find what you're looking for. Use a flag variable or put the nested loop in a function and use return.

Pattern Printing

Nested loops are the classic way to print patterns - triangles, pyramids, rectangles, and more. The outer loop controls rows, the inner loop controls what's printed in each row.

# Pattern Printing with Nested Loops

# Right triangle
print("=== Right Triangle ===")
rows = 5
for i in range(1, rows + 1):
    for j in range(i):
        print("*", end="")
    print()

print()

# Inverted right triangle
print("=== Inverted Right Triangle ===")
for i in range(rows, 0, -1):
    for j in range(i):
        print("*", end="")
    print()

print()

# Square
print("=== Square ===")
size = 4
for i in range(size):
    for j in range(size):
        print("*", end=" ")
    print()

print()

# Number pyramid
print("=== Number Pyramid ===")
for i in range(1, 6):
    # Print leading spaces
    for space in range(5 - i):
        print(" ", end="")
    # Print numbers
    for num in range(1, i + 1):
        print(num, end="")
    print()

print()

# Rectangle with border
print("=== Rectangle with Border ===")
width, height = 6, 4
for i in range(height):
    for j in range(width):
        if i == 0 or i == height-1 or j == 0 or j == width-1:
            print("*", end="")
        else:
            print(" ", end="")
    print()

Output

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Practical Examples

Nested loops are used for combinations, comparisons, grid-based applications, and processing hierarchical data.

# Practical Nested Loop Examples

# 1. All pairs/combinations
print("=== All Pairs ===")
items = ["A", "B", "C"]
for i in items:
    for j in items:
        if i != j:  # Skip same element
            print(f"({i}, {j})")

print()

# 2. Comparing elements
print("=== Finding Duplicates ===")
numbers = [3, 1, 4, 1, 5, 9, 2, 6, 5]
duplicates = []

for i in range(len(numbers)):
    for j in range(i + 1, len(numbers)):  # Only check forward
        if numbers[i] == numbers[j] and numbers[i] not in duplicates:
            duplicates.append(numbers[i])

print(f"Numbers: {numbers}")
print(f"Duplicates: {duplicates}")

print()

# 3. Grid-based game board
print("=== Tic-Tac-Toe Board ===")
board = [
    ["X", "O", "X"],
    ["O", "X", " "],
    [" ", "O", "X"]
]

for i, row in enumerate(board):
    row_str = " | ".join(row)
    print(f" {row_str} ")
    if i < 2:
        print("-----------")

print()

# 4. Search in nested data
print("=== Search in Nested Data ===")
students = [
    {"name": "Alice", "grades": [85, 90, 78]},
    {"name": "Bob", "grades": [92, 88, 95]},
    {"name": "Charlie", "grades": [70, 75, 80]}
]

print("Student Averages:")
for student in students:
    total = 0
    for grade in student["grades"]:
        total += grade
    avg = total / len(student["grades"])
    print(f"  {student['name']}: {avg:.1f}")

Output

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Performance Warning: Nested loops can be slow for large data! O(n²) complexity means doubling the data quadruples the time. For 1000 items, a nested loop runs 1,000,000 times. Consider using built-in functions, list comprehensions, or libraries like NumPy for better performance.

Common Mistakes

1. Wrong variable in inner loop

# Wrong - using outer variable i in inner loop!
for i in range(3):
    for j in range(3):
        print(i)  # Should be j or both

# Correct
for i in range(3):
    for j in range(3):
        print(f"({i}, {j})")

2. Break only exits inner loop

# Wrong assumption - break exits both loops
for i in range(5):
    for j in range(5):
        if found:
            break  # Only exits inner loop!
    # Outer loop continues!

# Correct - use flag
found = False
for i in range(5):
    for j in range(5):
        if condition:
            found = True
            break
    if found:
        break

3. Modifying list while iterating

# Wrong - modifying during iteration
for row in matrix:
    for i, val in enumerate(row):
        if val == 0:
            row.remove(val)  # Dangerous!

# Correct - build new structure
new_matrix = []
for row in matrix:
    new_row = [val for val in row if val != 0]
    new_matrix.append(new_row)

4. Off-by-one in indices

# Wrong - accessing out of bounds
for i in range(len(matrix)):
    for j in range(len(matrix)):  # Wrong if not square!
        print(matrix[i][j])

# Correct - use actual row length
for i in range(len(matrix)):
    for j in range(len(matrix[i])):  # Each row's length
        print(matrix[i][j])

Exercise: Triangle Pattern

Task: Print a right triangle pattern using nested loops.

Requirements:

Print 5 rows
Row 1: 1 star, Row 2: 2 stars, ... Row 5: 5 stars

Output

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Show Solution

rows = 5

for i in range(1, rows + 1):    # 1 to 5
    for j in range(i):          # 0 to i-1 (prints i stars)
        print("*", end="")
    print()  # New line after each row

# Output:
# *
# **
# ***
# ****
# *****

Summary

Nested loop: Loop inside another loop
Iterations: Inner loop runs completely for each outer iteration
Total runs: outer_iterations × inner_iterations
2D data: Outer loop = rows, inner loop = columns
break: Only exits the innermost loop
Performance: O(n²) - can be slow for large data

What's Next?

Congratulations on completing the Control Flow module! You now have full control over program flow with conditionals and loops. Next, we'll dive into Lists - Python's most versatile data structure for storing collections of items!

Previous Loop Control

Next Lists

How Nested Loops Work

Working with 2D Data

Pattern Printing

Practical Examples

Common Mistakes

1. Wrong variable in inner loop

2. Break only exits inner loop

3. Modifying list while iterating

4. Off-by-one in indices

Exercise: Triangle Pattern

Summary

What's Next?

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