# Python Program to Flatten a Nested List using Recursion

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Recursion in Python:

Python also supports function recursion, which means that a specified function can call itself.

Recursion is a mathematical and programming concept that is widely used. It signifies that a function calls itself. This has the advantage of allowing you to loop through data to obtain a result.

The developer must exercise extreme caution when using recursion since it is quite easy to write a function that never terminates or consumes excessive amounts of memory or computing power. However, when performed correctly, recursion may be a tremendously efficient and mathematically elegant way to programming.

Nested List:

A list can include any form of the object, including another list (sublist), which can contain sublists, and so on. This is referred to as a nested list.

Given a nested list, the task is to write a python program that converts the given list to flatted list.

Examples:

I)Integer Nested List

Example1:

Input:

given Nested List = [ [7, 8, 2], [11, 19, 29, 63, 91], [12, 52, 98], [11, 19, 18],
[11, 28, 34, 39, 35, 56, 87, 46, 1, 2], [5], [9, 23],[3, 23, 124] ]

Output:

Printing the given nested list :
[[7, 8, 2], [11, 19, 29, 63, 91], [12, 52, 98], [11, 19, 18], [11, 28, 34, 39, 35, 56, 87, 46, 1, 2], [5], [9, 23], [3, 23, 124]]
Printing the flattened list :
[7, 8, 2, 11, 19, 29, 63, 91, 12, 52, 98, 11, 19, 18, 11, 28, 34, 39, 35, 56, 87, 46, 1, 2, 5, 9, 23, 3, 23, 124]

ii)String Nested List

Example2:

Input:

 given Nested List = [ ['hello', 'this'], ['btechgeeks', 'online', 'python'], [ 'programming', 'and'], ['learning', 'platform'],
['for'], ['coding', 'students']]

Output:

Printing the given nested list :
[['hello', 'this'], ['btechgeeks', 'online', 'python'], ['programming', 'and'], ['learning', 'platform'], ['for'], ['coding', 'students']]
Printing the flattened list :
['hello', 'this', 'btechgeeks', 'online', 'python', 'programming', 'and', 'learning', 'platform', 'for', 'coding', 'students']

iii)Random Nested List(Elements can be of any data type)

Example3:

Input:

given nested list = [[928.4, 1], ['btechgeeks', 'online', True],
['programming', 'and'], ['learning', 'platform'], ['for'], [False, 'students'], [4, 18]]

Output:

Printing the given nested list :
[[928.4, 1], ['btechgeeks', 'online', True], ['programming', 'and'], ['learning', 'platform'], ['for'], [False, 'students'], [4, 18]]
Printing the flattened list :
[928.4, 1, 'btechgeeks', 'online', True, 'programming', 'and', 'learning', 'platform', 'for', False, 'students', 4, 18]

## Program to Flatten a Nested List using Recursion in Python

Below are the ways to Flatten a Nested List using recursion in Python.

Approach:

• Give the nested list as static input and store it in a variable.
• To flatten the list, pass it as an argument to a recursive function.
• If the list is empty, the function returns the list.
• Otherwise, use the function with the sublists as parameters recursively until the entire list is flattened.
• Print the list that has been flattened.
• The exit of the program.

### 1)Integer Nested List

Below is the implementation:

# function which takes the nested list as argument and
# returns the flattened list using recursion

def flattenListRecursion(given_NestedList):
# If the list is empty, the function returns the list.
if given_NestedList == []:
return given_NestedList
# Otherwise, use the function with the sublists as parameters
# recursively until the entire list is flattened.
if isinstance(given_NestedList[0], list):
return flattenListRecursion(given_NestedList[0]) + flattenListRecursion(given_NestedList[1:])
return given_NestedList[:1] + flattenListRecursion(given_NestedList[1:])

# Give the nested list as static input and store it in a variable.
givenNestedList = [[7, 8, 2], [11, 19, 29, 63, 91], [12, 52, 98], [11, 19, 18],
[11, 28, 34, 39, 35, 56, 87, 46, 1, 2], [5], [9, 23], [3, 23, 124]]
# printing the given nested list
print('Printing the given nested list : \n', givenNestedList)
# passing the given nested list as an argument to the recursive function flattenListRecursion
# which returns the flattened lsit
print("Printing the flattened list : \n",
flattenListRecursion(givenNestedList))


Output:

Printing the given nested list :
[[7, 8, 2], [11, 19, 29, 63, 91], [12, 52, 98], [11, 19, 18], [11, 28, 34, 39, 35, 56, 87, 46, 1, 2], [5], [9, 23], [3, 23, 124]]
Printing the flattened list :
[7, 8, 2, 11, 19, 29, 63, 91, 12, 52, 98, 11, 19, 18, 11, 28, 34, 39, 35, 56, 87, 46, 1, 2, 5, 9, 23, 3, 23, 124]

Explanation:

• A nested list is initialized in a variable.
• To flatten the list, the list is given as an argument to a recursive function.
• If the list is empty, the function returns the list.
• Otherwise, the function is repeated recursively with the sublists as arguments until the entire list is flattened.
• The list has been flattened and printed.

### 2)String Nested List

Below is the implementation:

# function which takes the nested list as argument and
# returns the flattened list using recursion

def flattenListRecursion(given_NestedList):
# If the list is empty, the function returns the list.
if given_NestedList == []:
return given_NestedList
# Otherwise, use the function with the sublists as parameters
# recursively until the entire list is flattened.
if isinstance(given_NestedList[0], list):
return flattenListRecursion(given_NestedList[0]) + flattenListRecursion(given_NestedList[1:])
return given_NestedList[:1] + flattenListRecursion(given_NestedList[1:])

# Give the nested list as static input and store it in a variable.
givenNestedList = [['hello', 'this'], ['btechgeeks', 'online', 'python'], [
'programming', 'and'], ['learning', 'platform'], ['for'], ['coding', 'students']]
# printing the given nested list
print('Printing the given nested list : \n', givenNestedList)
# passing the given nested list as an argument to the recursive function flattenListRecursion
# which returns the flattened lsit
print("Printing the flattened list : \n",
flattenListRecursion(givenNestedList))


Output:

Printing the given nested list :
[['hello', 'this'], ['btechgeeks', 'online', 'python'], ['programming', 'and'], ['learning', 'platform'], ['for'], ['coding', 'students']]
Printing the flattened list :
['hello', 'this', 'btechgeeks', 'online', 'python', 'programming', 'and', 'learning', 'platform', 'for', 'coding', 'students']

### 3)Random Nested List(Elements can be of any data type)

Below is the implementation:

# function which takes the nested list as argument and
# returns the flattened list using recursion

def flattenListRecursion(given_NestedList):
# If the list is empty, the function returns the list.
if given_NestedList == []:
return given_NestedList
# Otherwise, use the function with the sublists as parameters
# recursively until the entire list is flattened.
if isinstance(given_NestedList[0], list):
return flattenListRecursion(given_NestedList[0]) + flattenListRecursion(given_NestedList[1:])
return given_NestedList[:1] + flattenListRecursion(given_NestedList[1:])

# Give the nested list as static input and store it in a variable.
givenNestedList = [[928.4, 1], ['btechgeeks', 'online', True], [
'programming', 'and'], ['learning', 'platform'], ['for'], [False, 'students'], [4, 18]]
# printing the given nested list
print('Printing the given nested list : \n', givenNestedList)
# passing the given nested list as an argument to the recursive function flattenListRecursion
# which returns the flattened lsit
print("Printing the flattened list : \n",
flattenListRecursion(givenNestedList))


Output:

Printing the given nested list :
[[928.4, 1], ['btechgeeks', 'online', True], ['programming', 'and'], ['learning', 'platform'], ['for'], [False, 'students'], [4, 18]]
Printing the flattened list :
[928.4, 1, 'btechgeeks', 'online', True, 'programming', 'and', 'learning', 'platform', 'for', False, 'students', 4, 18]

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