Nested Loops in Python

Looping is a foundational concept in programming that allows you to execute a block of code repeatedly. But what if you need to perform a loop inside another loop? That’s where nested loops come in. If you’re just starting out in Python, understanding nested loops can open the door to solving more complex problems like matrix manipulation, pattern printing, or building simple games.

In this guide, we’ll take a deep dive into nested loops in Python, exploring what they are, why they matter, and how to use them with real-world, easy-to-understand examples.

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📌 Why Are Nested Loops Important?

Nested loops give you multi-layered control over repetitive operations. They are crucial for:

• Working with multi-dimensional data like matrices or tables
• Pattern generation, such as pyramid or star shapes
• Comparing pairs of data
• Iterating through nested structures like lists of lists

Without nested loops, many tasks would require unnecessarily complex or repetitive code.

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✅ Prerequisites

Before tackling nested loops, ensure you understand:

• Basic for and while loops
• Indentation in Python (crucial for nesting)
• Data structures like lists and strings
• Boolean conditions and loop control statements

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📖 What This Guide Covers

1. What are nested loops?
2. Syntax of nested loops
3. Types of nested loops (for in for, for in while, etc.)
4. Real-world examples and use cases
5. Common mistakes to avoid
6. Best practices for beginners

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🔍 What Are Nested Loops?

A nested loop is a loop inside another loop. The inner loop completes all its iterations for each iteration of the outer loop.

📌 General Syntax:

for outer in range(x):
    for inner in range(y):
        # Code block executed y times for each outer iteration

This means if x = 3 and y = 2, the inner block runs 3 × 2 = 6 times.

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🔄 Types of Nested Loops

Python allows nesting any combination of loops:

• for loop inside a for loop
• for loop inside a while loop
• while loop inside a for loop
• while loop inside another while loop

Let’s go over each.

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✅ 1. For Loop Inside a For Loop

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

Output:

i=0, j=0
i=0, j=1
i=1, j=0
i=1, j=1
i=2, j=0
i=2, j=1

This loop runs 2 inner iterations for every 1 outer iteration.

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✅ 2. While Loop Inside a For Loop

for i in range(3):
    j = 0
    while j < 2:
        print(f"i={i}, j={j}")
        j += 1

Same output as above. It just mixes loop types.

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✅ 3. For Loop Inside a While Loop

i = 0
while i < 2:
    for j in range(3):
        print(f"i={i}, j={j}")
    i += 1

Output:

i=0, j=0
i=0, j=1
i=0, j=2
i=1, j=0
i=1, j=1
i=1, j=2

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✅ 4. While Loop Inside Another While Loop

i = 0
while i < 2:
    j = 0
    while j < 3:
        print(f"i={i}, j={j}")
        j += 1
    i += 1

Same output—loop nesting can be flexible in Python.

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📊 Real-World Examples of Nested Loops

📌 Example 1: Printing a Rectangle of Stars

for row in range(4):
    for col in range(5):
        print("*", end=" ")
    print()

Output:

* * * * * 
* * * * * 
* * * * * 
* * * * * 

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📌 Example 2: Multiplication Table

for i in range(1, 6):
    for j in range(1, 6):
        print(i * j, end="\t")
    print()

Prints a 5×5 multiplication table.

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📌 Example 3: Working with 2D Lists

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

for row in matrix:
    for item in row:
        print(item, end=" ")
    print()

Output:

1 2 3
4 5 6
7 8 9

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📌 Example 4: Comparing All Pairs in a List

nums = [1, 2, 3]
for i in nums:
    for j in nums:
        print(f"({i}, {j})")

Useful in algorithms and data comparison tasks.

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📌 Example 5: Nested Loops with Conditions

for i in range(5):
    for j in range(i + 1):
        print("*", end=" ")
    print()

Output:

* 
* * 
* * * 
* * * * 
* * * * * 

This prints a right-angled triangle of stars.

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⚠️ Common Mistakes in Nested Loops

1. Indentation Errors – Proper indentation is critical in Python.
2. Infinite Loops – Especially when using nested while loops.
3. Unnecessary Complexity – Sometimes nesting can be avoided.
4. Mixing Variables – Be careful not to confuse loop variables.
5. Overusing Loops – Nested loops can be inefficient for large datasets.

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💡 Best Practices

• Use descriptive variable names (e.g., row, col, i, j)
• Keep loop depth to 2 or 3 levels max
• Consider breaking complex logic into functions
• Use break and continue wisely
• Profile and optimize loops in performance-sensitive code

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✅ Summary

Nested loops are a powerful way to handle repetitive, multi-dimensional, or structured tasks in Python. Whether you’re printing patterns, processing tables, or building more complex applications, understanding how and when to use nested loops is essential.

Although powerful, nested loops must be used carefully to avoid unnecessary complexity and performance issues. Practice with real examples, and soon you’ll be using nested loops like a pro.

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Would you like exercises with solutions or visual diagrams to help understand nested loops better? I’d be happy to assist!