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 chainfrom itertools import chainini_list = [[1, 2, 3], [3, 6, 7], [7, 5, 4]] # printing initial listprint ("initial list ", str(ini_list))# converting 2d list into 1d# using chain.from_iterablesflatten_list = list(chain.from_iterable(ini_list))# printing flatten_listprint ("final_result", str(flatten_list)) |
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 chainfrom itertools import chainini_list = [[1, 2, 3], [3, 6, 7], [7, 5, 4]] # printing initial listprint ("initial list ", str(ini_list))# converting 2d list into 1d# using list comprehensionflatten_list = [j for sub in ini_list for j in sub]# printing flatten_listprint ("final_result", str(flatten_list)) |
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 functoolsfrom functools import reduceini_list = [[1, 2, 3], [3, 6, 7], [7, 5, 4]] # printing initial listprint ("initial list ", str(ini_list))# converting 2d list into 1d# using functools.reduceflatten_list = reduce(lambda z, y :z + y, ini_list)# printing flatten_listprint ("final_result", str(flatten_list)) |
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 sumini_list = [[1, 2, 3], [3, 6, 7], [7, 5, 4]]# printing initial listprint ("initial list ", str(ini_list))# converting 2d list into 1dflatten_list = sum(ini_list, [])# printing flatten_listprint ("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 - kaapalxini_list=[[1, 2, 3], [3, 6, 7], [7, 5, 4]]#Using lambdaflatten_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 listprint("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 - kaapalximport numpyini_list=[[1, 2, 3], [3, 6, 7], [7 ,5, 4]]print("Initial list ",ini_list) #Printing Initial listprint("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 listini_list=[[1, 2, 3],[3, 6, 7],[7 ,5, 4]]print("Initial list ",ini_list) #Printing Initial listres=[]for i in ini_list: res.extend(i)print("Flattened List ",res) |
Initial list [[1, 2, 3], [3, 6, 7], [7, 5, 4]] Flattened List [1, 2, 3, 3, 6, 7, 7, 5, 4]
