Nested loops are loops inside another loop. The inner loop completes all its iterations for each iteration of the outer loop.

They are commonly used to work with multi-dimensional data structures like matrices, tables, or grids.

Syntax

for outer_var in outer_sequence:
   for inner_var in inner_sequence:
       # inner loop block
   # outer loop block

You can nest any kind of loop — for inside for, while inside for, etc.

Example 1: Nested for Loop

for i in range(1, 4):       # Outer loop
   for j in range(1, 3):   # Inner loop
       print(f"i={i}, j={j}")

Output:

i=1, j=1
i=1, j=2
i=2, j=1
i=2, j=2
i=3, j=1
i=3, j=2

Example 2: Printing a Pattern

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

Output:

*
**
***
****

Example 3: Nested while Loop

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

Output:

i=1, j=1
i=1, j=2
i=2, j=1
i=2, j=2
i=3, j=1
i=3, j=2

Example 4: Looping Through a Matrix

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 

Things to Keep in Mind

• Nested loops are slower for large datasets.
• Complexity increases with each nested level:
  O(n) → O(n²) → O(n³)…

Common Use Case: Multiplication Table

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

Output:

1    2    3    4    5    
2    4    6    8    10    
3    6    9    12    15    
4    8    12    16    20    
5    10    15    20    25    

Summary Table

Exercise

Task: Print a triangle pattern using nested for loops.

1
12
123
1234

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