Python Nested Loops (Complete Guide for Beginners)

Nested loops in Python mean a loop inside another loop. In simple words, it is when:
👉 One loop runs inside another loop.

Nested loops are very powerful when working with:

• Patterns (stars, numbers)
• Tables
• Multi-dimensional data (like matrices)
• Repeated combinations

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🔹 What is a Nested Loop in Python?

A nested loop is a loop inside another loop.

👉 Basic Idea:

• Outer loop controls the number of times inner loop runs
• Inner loop runs completely for each iteration of outer loop

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🔹 Syntax of Nested Loops

for outer in range(n):
    for inner in range(m):
        # code block

You can also use:

• for inside for
• while inside while
• for inside while (and vice versa)

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🔹 How Nested Loop Works

Example flow:

Outer loop → runs 1 time
    Inner loop → runs full cycle
Outer loop → runs next time
    Inner loop → runs again fully

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🔹 Example 1: Basic Nested Loop

for i in range(1, 4):
    for j in range(1, 4):
        print(i, j)

🔸 Output:

1 1
1 2
1 3
2 1
2 2
2 3
3 1
3 2
3 3

🔍 Explanation:

• Outer loop controls i
• Inner loop controls j
• Inner loop runs completely for each i

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🔹 Example 2: Multiplication Table

for i in range(1, 6):
    for j in range(1, 11):
        print(i * j, end=" ")
    print()

🔸 Output:

1 2 3 4 5 6 7 8 9 10
2 4 6 8 10 12 14 16 18 20
3 6 9 12 15 18 21 24 27 30
4 8 12 16 20 24 28 32 36 40
5 10 15 20 25 30 35 40 45 50

🔍 Explanation:

• Outer loop selects number
• Inner loop prints multiplication table

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🔹 Example 3: Star Pattern (Rectangle)

for i in range(1, 5):
    for j in range(1, 6):
        print("*", end=" ")
    print()

🔸 Output:

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

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🔹 Example 4: Triangle Pattern

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

🔸 Output:

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

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🔹 Example 5: Nested While Loop

i = 1

while i <= 3:
    j = 1
    while j <= 3:
        print(i, j)
        j += 1
    i += 1

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🔹 Real-Life Example of Nested Loops

👉 Example: Seating Arrangement

rows = 3
cols = 4

for i in range(rows):
    for j in range(cols):
        print(f"Seat({i},{j})", end=" ")
    print()

🔍 Explanation:

• Rows = outer loop
• Columns = inner loop
• Used in real systems like:
  • Movie theaters
  • Bus seating
  • Game grids

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🔹 Nested Loop with Lists (2D Data)

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

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

🔸 Output:

1 2 3
4 5 6
7 8 9

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🔹 Time Complexity Note

Nested loops can be slow if data is large:

• Single loop → O(n)
• Nested loop → O(n²)

So use carefully in big programs.

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🔹 Common Mistakes

❌ Forgetting indentation:

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

✔ Correct:

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

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🔹 Key Points to Remember

• Nested loop = loop inside loop
• Inner loop completes fully for each outer loop iteration
• Used in patterns, grids, and tables
• Can increase time complexity

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🚀 Conclusion

Python nested loops are essential for solving real-world programming problems like patterns, matrices, and structured data processing.

Once you understand nested loops, you can easily build:

• Game grids
• Pattern programs
• Data tables
• Complex logic systems