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Python | Ways to flatten a 2D list

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Given a 2D list, Write a Python program to convert the given list into a flattened list. 

Method #1: Using chain.iterable() 

Python3




# Python code to demonstrate
# converting 2d list into 1d list
# using chain.from_iterables
 
# import chain
from itertools import chain
 
ini_list = [[1, 2, 3],
            [3, 6, 7],
            [7, 5, 4]]
             
# printing initial list
print ("initial list ", str(ini_list))
 
# converting 2d list into 1d
# using chain.from_iterables
flatten_list = list(chain.from_iterable(ini_list))
 
# printing flatten_list
print ("final_result", str(flatten_list))


Output: 

initial list  [[1, 2, 3], [3, 6, 7], [7, 5, 4]]
final_result [1, 2, 3, 3, 6, 7, 7, 5, 4]

 

The time complexity of the code is O(m*n) where m is the number of lists and n is the maximum length of the lists. 

The auxiliary space complexity of the code is O(mn) because a new list of size mn is created to store the flattened list. 

Python3




# Python code to demonstrate
# converting 2d list into 1d list
# using list comprehension
 
# import chain
from itertools import chain
 
ini_list = [[1, 2, 3],
            [3, 6, 7],
            [7, 5, 4]]
             
# printing initial list
print ("initial list ", str(ini_list))
 
# converting 2d list into 1d
# using list comprehension
flatten_list = [j for sub in ini_list for j in sub]
 
# printing flatten_list
print ("final_result", str(flatten_list))


Output: 

initial list  [[1, 2, 3], [3, 6, 7], [7, 5, 4]]
final_result [1, 2, 3, 3, 6, 7, 7, 5, 4]

 

The time complexity of this program is O(mn), where m is the number of rows and n is the number of columns in the input 2D list. 

The space complexity of this program is O(mn), as the size of the output list is proportional to the size of the input list. 

Method #3: Using functools.reduce  

Python3




# Python code to demonstrate
# converting 2d list into 1d list
# using functools.reduce
 
# import functools
from functools import reduce
 
ini_list = [[1, 2, 3],
            [3, 6, 7],
            [7, 5, 4]]
             
# printing initial list
print ("initial list ", str(ini_list))
 
# converting 2d list into 1d
# using functools.reduce
flatten_list = reduce(lambda z, y :z + y, ini_list)
 
# printing flatten_list
print ("final_result", str(flatten_list))


Output: 

initial list  [[1, 2, 3], [3, 6, 7], [7, 5, 4]]
final_result [1, 2, 3, 3, 6, 7, 7, 5, 4]

 

The time complexity of the above code is O(N^2) as it has to traverse all the elements in a 2D list once.
The space complexity is also O(N^2), as the entire input list needs to be stored in memory and a new list is created to store the flattened version of the list. 

Method #4: Using sum
sum has an optional argument: sum(iterable [, start])

Python3




# Python code to demonstrate
# converting 2d list into 1d list
# using sum
 
ini_list = [[1, 2, 3],
            [3, 6, 7],
            [7, 5, 4]]
 
# printing initial list
print ("initial list ", str(ini_list))
 
# converting 2d list into 1d
flatten_list = sum(ini_list, [])
 
# printing flatten_list
print ("final_result", str(flatten_list))
 
# This code is contributed by
# Mayank Chaudhary - chaudhary_19


Output: 

initial list  [[1, 2, 3], [3, 6, 7], [7, 5, 4]]
final_result [1, 2, 3, 3, 6, 7, 7, 5, 4]

Time Complexity: The time complexity of this program is O(mn), where m is the number of rows and n is the number of columns in the input 2D list.
Auxiliary Space: The auxiliary space of this program is O(mn), as the size of the output list is proportional to the size of the input list. 
Method #5: Using lambda  

Python3




#Python 3 code to flatten nested list
#contributed by S Lakshman Rao - kaapalx
ini_list=[[1, 2, 3],
          [3, 6, 7],
          [7, 5, 4]]
 
#Using lambda
 
flatten_list = lambda y:[x for a in y for x in flatten_list(a)] if type(y) is list else [y]
 
print("Initial list ",ini_list) #printing initial list
 
print("Flattened List ",flatten_list(ini_list)) # printing flattened list


Output: 

Initial list  [[1, 2, 3], [3, 6, 7], [7, 5, 4]]
Flattened List  [1, 2, 3, 3, 6, 7, 7, 5, 4]

Method #6: Using numpy  

Python3




#Python 3 code to flatten nested list
#Contributed by S Lakshman Rao - kaapalx
import numpy
 
ini_list=[[1, 2, 3],
          [3, 6, 7],
          [7 ,5, 4]]
 
print("Initial list ",ini_list) #Printing Initial list
 
print("Flattened List ",list(numpy.concatenate(ini_list).flat))
#Using numpy to flatten list and printing the result


Output: 

Initial list  [[1, 2, 3], [3, 6, 7], [7, 5, 4]]
Flattened List  [1, 2, 3, 3, 6, 7, 7, 5, 4]

The time complexity of this program is O(N^2) where N is the number of elements in the nested list.

The auxiliary space complexity is O(N), as the flattened list is stored in memory as a new list.

 Method #7: Using extend() method

Python3




#Python 3 code to flatten nested list
 
ini_list=[[1, 2, 3],[3, 6, 7],[7 ,5, 4]]
print("Initial list ",ini_list) #Printing Initial list
res=[]
for i in ini_list:
    res.extend(i)
print("Flattened List ",res)


Output

Initial list  [[1, 2, 3], [3, 6, 7], [7, 5, 4]]
Flattened List  [1, 2, 3, 3, 6, 7, 7, 5, 4]

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Last Updated : 18 Apr, 2023
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