Loops · Lesson 13 of 39

Nested Loops and Loop Patterns

4 min read Updated Nov 7, 2026 Saurabh Samir

On this page

How nested loops run
A multiplication table
Pattern programs
Searching a grid
A word on speed
Practice
Summary

A loop inside another loop is called a nested loop. It is how you work with anything two-dimensional – rows and columns, seats in a cinema, pixels in an image – and it is behind the famous "print this pattern" questions in coding tests.

In this lesson

How the inner loop runs completely for every outer pass
Tables and grids
Star and number patterns, step by step
Breaking out of nested loops cleanly

How nested loops run

for row in range(1, 4):
    for col in range(1, 3):
        print(f"(row {row}, col {col})")

Output

(row 1, col 1)
(row 1, col 2)
(row 2, col 1)
(row 2, col 2)
(row 3, col 1)
(row 3, col 2)

The inner loop finishes all its passes before the outer loop moves on – exactly like the hands of a clock: the minute hand goes all the way round for every step of the hour hand. 3 outer passes × 2 inner passes = 6 lines.

Outer and inner

Outer loop: row = 1, 2, 3moves one step…
Inner loop: col = 1, 2…only after the inner loop has completed all its steps
Total work = outer × inner3 × 2 = 6 runs of the inner block

A multiplication table

for i in range(1, 6):
    for j in range(1, 6):
        print(f"{i * j:4}", end="")
    print()          # new line after each row

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

Two tricks make grids work: end="" keeps the inner prints on one line, and the empty print() after the inner loop moves to the next row.

Pattern programs

Pattern questions test whether you can turn a picture into loop rules. The method is always the same: write down what each row contains as a function of the row number.

Right triangle

n = 5
for row in range(1, n + 1):
    print("* " * row)

Output

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

Row r has r stars. Python's string repetition ("* " * row) replaces the inner loop.

Pyramid

Row	Spaces	Stars
1	4	1
2	3	3
3	2	5
r	n − r	2r − 1

n = 5
for r in range(1, n + 1):
    print(" " * (n - r) + "*" * (2 * r - 1))

Output

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

Number triangle (with a real inner loop)

for r in range(1, 5):
    for c in range(1, r + 1):
        print(c, end=" ")
    print()

Output

Floyd's triangle

number = 1
for r in range(1, 5):
    for _ in range(r):
        print(f"{number:3}", end="")
        number += 1
    print()

Output

The name _ is a convention for "I need a loop variable but I won't use it".

Searching a grid

A cinema hall as a list of rows; find the first free seat (0 = free, 1 = booked):

hall = [
    [1, 1, 1, 1],
    [1, 1, 0, 1],
    [0, 0, 0, 0],
]

found = None
for r, row in enumerate(hall):
    for c, seat in enumerate(row):
        if seat == 0:
            found = (r + 1, c + 1)
            break          # leaves the inner loop only
    if found:
        break              # leaves the outer loop too

print("First free seat (row, seat):", found)

Output

First free seat (row, seat): (2, 3)

Common mistake: expecting break to exit every loop

break leaves only the innermost loop. To stop both, check a flag in the outer loop (as above) or, cleaner, put the search in a function and return as soon as you find the answer.

A word on speed

Nested loops multiply work: two loops over 1,000 items each mean a million passes. That is fine for small data, but in interviews you will be asked to replace nested loops with a set or dict lookup where possible. Chapter 8 shows how.

Practice

Try it yourself

Print an inverted right triangle of stars with 4 rows.
Show solution
```
for r in range(4, 0, -1):
    print("*" * r)
```
Output
```
****
***
**
*
```

Print all pairs (a, b) with 1 ≤ a < b ≤ 4.

Show solution

for a in range(1, 5):
    for b in range(a + 1, 5):
        print((a, b), end=" ")

Output

(1, 2) (1, 3) (1, 4) (2, 3) (2, 4) (3, 4)

Print this hollow square of size 4:

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

Show solution

n = 4
for r in range(n):
    if r == 0 or r == n - 1:
        print("*" * n)
    else:
        print("*" + " " * (n - 2) + "*")

Output

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

Quick check

How many times does the inner print run in for i in range(4): for j in range(3): print()?

12 (4 × 3).

Which loop does break exit in nested loops?

Only the innermost loop that contains it.

In a pyramid of height n, how many stars are in row r?

2r − 1, preceded by n − r spaces.

Summary

The inner loop completes fully for every pass of the outer loop.
Use end="" and an empty print() to build rows.
Solve patterns by writing each row's content as a formula of the row number.
break exits one loop; use a flag or return to exit several.

Test yourself with real interview problems: Python coding interview questions (part 1).

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