Python | Ways to sum list of lists and return sum list

The list is an important container and used almost in every code of day-day programming as well as web-development, more it is used, more is the requirement to master it and hence knowledge of its operations is necessary. Given a list of lists, the program to suppose to return the sum as the final list.

Let’s see some of the methods to sum a list of list and return list.

Method # 1: Using Naive method

# Python code to demonstrate 
# sum of list of list 
# using naive method 
  
# Declaring initial list of list 
L = [[1, 2, 3], 
     [4, 5, 6], 
     [7, 8, 9]] 
          
# Printing list of list 
print("Initial List - ", str(L)) 
  
# Using naive method 
res = list() 
for j in range(0, len(L[0])): 
    tmp = 0
    for i in range(0, len(L)): 
        tmp = tmp + L[i][j] 
    res.append(tmp) 
      
# printing result 
print("final list - ", str(res)) 

Output:

Initial List -  [[1, 2, 3], [4, 5, 6], [7, 8, 9]]
final list -  [12, 15, 18]

Method #2: Using numpy array
A numpy is a general-purpose array-processing package. It provides a high-performance multidimensional array object, and tools for working with these arrays.

# Python code to demonstrate 
# sum of list of list 
# using numpy array functions 
import numpy as np 
  
# Declaring initial list of list 
List = np.array([[1, 2, 3], 
                 [4, 5, 6], 
                 [7, 8, 9]]) 
          
# Printing list of list 
print("Initial List - ", str(List)) 
  
# Using numpy sum 
res = np.sum(List, 0) 
      
# printing result 
print("final list - ", str(res)) 

Output:

Initial List -  [[1 2 3]
                 [4 5 6]
                 [7 8 9]]
final list -  [12 15 18]

Method #3: Using zip() and list comprehension

# Python code to demonstrate 
# sum of list of list using  
# zip and list comprehension 
  
# Declaring initial list of list 
List = [[1, 2, 3], 
        [4, 5, 6], 
        [7, 8, 9]] 
          
# Printing list of list 
print("Initial List - ", str(List)) 
  
# Using list comprehension 
res = [sum(i) for i in zip(*List)] 
      
# printing result 
print("final list - ", str(res)) 

Output:

Initial List -  [[1, 2, 3], [4, 5, 6], [7, 8, 9]]
final list -  [12, 15, 18]

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