In Python, a loop inside a loop is known as a nested loop. In this tutorial, we will learn about nested loops in Python with the help of examples.
Table of contents
- What is a Nested Loop in Python?
- Python Nested for Loop
- Break Nested loop
- Continue Nested loop
- Single Line Nested Loops Using List Comprehension
- Nested while Loop in Python
- When To Use a Nested Loop in Python?
What is a Nested Loop in Python?
A nested loop is an inner loop in the loop body of the outer loop. The inner or outer loop can be any type, such as a while loop or for loop. For example, the outer
for loop can contain a
while loop and vice versa.
The outer loop can contain more than one inner loop. There is no limitation on the chaining of loops.
In the nested loop, the number of iterations will be equal to the number of iterations in the outer loop multiplied by the iterations in the inner loop.
In each iteration of the outer loop inner loop execute all its iteration. For each iteration of an outer loop the inner loop re-start and completes its execution before the outer loop can continue to its next iteration.
Nested loops are typically used for working with multidimensional data structures, such as printing two-dimensional arrays, iterating a list that contains a nested list.
Python Nested for Loop
Syntax of using a nested for loop in Python
# outer for loop for element in sequence # inner for loop for element in sequence: body of inner for loop body of outer for loop
In this example, we are using a for loop inside a
for loop. In this example, we are printing a multiplication table of the first ten numbers.
- The outer
forloop uses the range() function to iterate over the first ten numbers
- The inner
forloop will execute ten times for each outer number
- In the body of the inner loop, we will print the multiplication of the outer number and current number
- The inner loop is nothing but a body of an outer loop.
Example: Write a nested
for loop program to print multiplication table in Python
# outer loop for i in range(1, 11): # nested loop # to iterate from 1 to 10 for j in range(1, 11): # print multiplication print(i * j, end=' ') print()
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 6 12 18 24 30 36 42 48 54 60 7 14 21 28 35 42 49 56 63 70 8 16 24 32 40 48 56 64 72 80 9 18 27 36 45 54 63 72 81 90 10 20 30 40 50 60 70 80 90 100
- In this program, the outer
forloop is iterate numbers from 1 to 10. The
range()return 10 numbers. So total number of iteration of the outer loop is 10.
- In the first iteration of the nested loop, the number is 1. In the next, it 2. and so on till 10.
- Next, the inner loop will also execute ten times because we rea printing multiplication table up to ten. For each iteration of the outer loop, the inner loop will execute ten times.
- In each iteration of an inner loop, we calculated the multiplication of two numbers.
Nested Loop to Print Pattern
Another most common use of nested loop is to print various star and number pattern.
Let’s see how to use nested loop to print the following pattern in Python.
* * * * * * * * * * * * * * *
rows = 5 # outer loop for i in range(1, rows + 1): # inner loop for j in range(1, i + 1): print("*", end=" ") print('')
- In this program, the outer loop is the number of rows print.
- The number of rows is five, so the outer loop will execute five times
- Next, the inner loop is the total number of columns in each row.
- For each iteration of the outer loop, the columns count gets incremented by 1
- In the first iteration of the outer loop, the column count is 1, in the next it 2. and so on.
- The inner loop iteration is equal to the count of columns.
- In each iteration of an inner loop, we print star
While loop inside a for loop
Its is very common and helpful to use one type of loop inside another. we can put a while loop inside the
Assume we wanted to repeat each name from a list five times.
- Here we will iterate the list using an outer for loop
- In each iteration of outer for loop, the inner for loop execute five times to print the current name five times
names = ['Kelly', 'Jessa', 'Emma'] # outer loop for name in names: # inner while loop count = 0 while count < 5: print(name, end=' ') # increment counter count = count + 1 print()
Kelly Kelly Kelly Kelly Kelly Jessa Jessa Jessa Jessa Jessa Emma Emma Emma Emma Emma
Practice: Print a rectangle Pattern with 5 rows and 3 columns of stars
Solve the below Python nested loop exercise.
Print following rectangle of stars
*** *** *** *** ***
Solve more loop exercise.
# 5 rows for name in range(5): # 3 column for j in range(3): print('*', end='') print()
Break Nested loop
In the following example, we have two loops. The outer
for loop iterates the first four numbers using the
range() function, and the inner
for loop also iterates the first four numbers. If the outer number and a current number of the inner loop are the same, then break the inner (nested) loop.
for i in range(4): for j in range(4): if j == i: break print(i, j)
1 0 2 0 2 1 3 0 3 1 3 2
As you can see in the output, no rows contain the same number.
Continue Nested loop
The continue statement skip the current iteration and move to the next iteration. In Python, when the
continue statement is encountered inside the loop, it skips all the statements below it and immediately jumps to the next iteration.
In the following example, we have two loops. The outer for loop iterates the first list, and the inner loop also iterates the second list of numbers.
If the outer number and the inner loop’s current number are the same, then move to the next iteration of an inner loop.
first = [2, 4, 6] second = [2, 4, 6] for i in first: for j in second: if i == j: continue print(i, '*', j, '= ', i * j)
2 * 4 = 8 2 * 6 = 12 4 * 2 = 8 4 * 6 = 24 6 * 2 = 12 6 * 4 = 24
As you can see in the output, no same numbers multiplying to each other.
Single Line Nested Loops Using List Comprehension
For example, if you had two lists and want to get all combinations of them, To achieve this, you need to use two nested loops as mentioned below.
first = [2, 3, 4] second = [20, 30, 40] final =  for i in first: for j in second: final.append(i+j) print(final)
You can write more fast and compact code using the list compression and nested loop like as shown below.
first = [2, 3, 4] second = [20, 30, 40] final = [i+j for i in first for j in second] print(final)
How to write it:
- First, Write an outer
forloop that will iterate the first list like
[for i in first]
- Next, Write an inner loop that will iterate the second list after the outer loop like
[for i in first for j in second]
- Last, calculate the addition of the outer number and inner number like
[i+j for i in first for j in second]
- At last, store result in a new list like
final = [i+j for i in first for j in second]
Let’s see more such examples.
In this example, we will use two
for loops in list Comprehension and the final result would be a list of lists. we will not include the same numbers in each list. we will filter them using an if condition.
final = [[x, y] for x in [10, 20, 30] for y in [30, 10, 50] if x != y] print(final)
[[10, 30], [10, 50], [20, 30], [20, 10], [20, 50], [30, 10], [30, 50]]
Nested while Loop in Python
In Python, The while loop statement repeatedly executes a code block while a particular condition is true. We use w a while loop when number iteration is not fixed.
In this section we will see how to use a while loop inside another while loop.
The syntax to write a nested while loop statement in Python is as follows:
while expression: while expression: statement(s) statement(s)
In this example, we will print the first 10 numbers on each line 5 times.
i = 1 while i <= 5: j = 1 while j <= 10: print(j, end='') j = j + 1 i = i + 1 print()
12345678910 12345678910 12345678910 12345678910 12345678910
for loop inside While loop
Its is helpful idea to use one type of loop inside another. we can put a
for loop inside the
Assume we wanted to print all perfect numbers from 1 to 100
- Here we will iterate the first 100 numbers using a
- In each iteration of the outer
whileloop, the inner
forloop execute from 1 up to the current outer number to check if the current number is a perfect number.
print('Show Perfect number fom 1 to 100') n = 2 # outer while loop while n <= 100: x_sum = 0 # inner for loop for i in range(1, n): if n % i == 0: x_sum += i if x_sum == n: print('Perfect number:', n) n += 1
When To Use a Nested Loop in Python?
- Nested loops are handy when you have nested arrays or lists that need to be looped through the same function.
- When you want to print different star and number patterns using rows can columns
Keep the time complexity in mind. Let’s understand this with examples on how nested for loop work in Python.
We use for loop to iterates on the given elements of a sequence or iterable. like
for i in list. Here time complexity is O(n) because we are iterating all items from a list.
The number of execution steps (iterations) determines the time complexity of a loop.
When you use a nested loop and both outer and inner loop runs without any if condition in it, the time complexity is
O(n^2) because, for all of the n elements, the code is executed n times.
numbers = [[1, 2, 3], [4, 5, 6]] cnt = 0 for i in numbers: for j in i: print('iteration', cnt, end=': ') print(j) cnt = cnt + 1
iteration 0: 1 iteration 1: 2 iteration 2: 3 iteration 3: 4 iteration 4: 5 iteration 5: 6
If you give a condition in the inner loop that will stop executing after some elements and not execute all n iterations of the inner loop or outer loop, it will have less time complexity.
Use nested loop when you don’t have any better alternatives, Remember writing efficient and compact code is far better than writing complex code.