Python Nested Loops

Writing a Nested Loop

The inner loop runs completely for every single iteration of the outer loop. So if the outer loop runs 3 times and the inner loop runs 4 times, the inner block executes a total of 12 times.

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

Building Star Patterns

Nested loops are the classic way to print shapes and patterns, since the outer loop typically controls the row and the inner loop controls what gets printed in that row.

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

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

Nested Loops with a 2D List

Nested loops are also the standard way to visit every cell of a 2D structure, like a list of lists (a common way to represent a grid or matrix).

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

Breaking Out of Nested Loops

A plain break only exits the innermost loop it is written in — the outer loop keeps going. To stop everything, use a flag variable checked in the outer loop, or restructure the logic into a function and return early.

found = False
for i in range(3):
    for j in range(3):
        if i == 1 and j == 1:
            found = True
            break
    if found:
        break
print("Stopped at", i, j)

A Note on Time Complexity

Each additional level of nesting multiplies the work: a single loop over n items is O(n), but two nested loops over n items become O(n²), since the inner loop repeats fully for every outer iteration. Watch out for deeply nested loops over large lists, as performance can drop sharply.