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35. Nested Loops

1. Introduction​

A nested loop is a loop inside another loop.
They are useful when working with multi-dimensional data (like matrices), comparing items from two collections, or performing repeated tasks across multiple dimensions.

2. Basic Nested 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

3. Nested while Loops​

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

4. Mixed Loops (for + while)​

for char in ["A", "B"]:
    i = 0
    while i < 2:
        print(char, i)
        i += 1

5. Nested Loops with Lists (Matrices)​

matrix = [[1, 2], [3, 4], [5, 6]]
for row in matrix:
    for value in row:
        print(value)

Output:

1
2
3
4
5
6

6. Using break and continue in Nested Loops​

break exits the innermost loop; continue skips to the next iteration of the innermost loop.

for i in range(3):
    for j in range(3):
        if j == 1:
            break  # stops inner loop only
        print(i, j)

7. Practical Example — Multiplication Table​

for i in range(1, 4):
    for j in range(1, 4):
        print(i * j, end=" ")
    print()

Output:

1 2 3 
2 4 6 
3 6 9

8. Performance Considerations​

Nested loops increase time complexity multiplicatively (e.g., O(n*m) or O(n^2) for two nested loops of size n). For large inputs, consider algorithmic improvements or using libraries (NumPy, itertools).

9. Common Mistakes and Tips​

  • Forgetting to reset inner-loop counters in while loops.
  • Excessive nesting → harder to read and maintain. Break logic into functions when nesting gets deep.
  • Remember to maintain correct indentation.

10. Next Steps​

✅ You now understand nested loops and how to use them effectively.
Next chapter: Comprehensions, a concise way to create collections in Python.