Python | Adding two list elements

There can be many situations in which one requires to find the index-wise summation of two different lists. This can have a possible application in day-day programming. Let’s discuss various ways in which this task can be performed in Python. 

Input: input_list 1 : [1, 3, 4, 6, 8]
            input _list 2 : [4, 5, 6, 2, 10]
Output: output_list[5, 8, 10, 8, 18]
Explanation: Every element of the input_list1 is added to its respective element 
of input_list2 and got the output_list.

Adding Two list Elements in Python

Below are the methods that we will cover in this article:

• Using Naive Method
• Using List Comprehension
• Using map() with add() function
• Using zip() with sum()
• Adding two list elements using numpy.sum()

Adding two list elements Naive Method

In this method, we simply run a loop and append to the new list the summation of both list elements at a similar index till we reach the end of the smaller list. This is the basic method to achieve this task. 

Python3

# initializing lists
test_list1 = [1, 3, 4, 6, 8]
test_list2 = [4, 5, 6, 2, 10]

# printing original lists
print ("Original list 1 : " + str(test_list1))
print ("Original list 2 : " + str(test_list2))

# using naive method to
# add two list
res_list = []
for i in range(0, len(test_list1)):
    res_list.append(test_list1[i] + test_list2[i])

# printing resultant list
print ("Resultant list is :" + str(res_list))

Output :

Original list 1 : [1, 3, 4, 6, 8]
Original list 2 : [4, 5, 6, 2, 10]
Resultant list is : [5, 8, 10, 8, 18]

Time Complexity: O(n)
Auxiliary Space: O(n)

Adding two list elements using List Comprehension

The shorthand for the above-explained technique, list comprehensions are usually quicker to type and hence must be preferred to perform this kind of programming task. 

Python3

test_list1 = [1, 3, 4, 6, 8]
test_list2 = [4, 5, 6, 2, 10]

# printing original lists
print ("Original list 1 :" + str(test_list1))
print ("Original list 2 : " + str(test_list2))

# using list comprehension to
# add two list
res_list = [test_list1[i] + test_list2[i] for i in range(len(test_list1))]

# printing resultant list
print ("Resultant list is : " + str(res_list))

Output :

Original list 1 : [1, 3, 4, 6, 8]
Original list 2 : [4, 5, 6, 2, 10]
Resultant list is : [5, 8, 10, 8, 18]

Time Complexity: O(n)
Auxiliary Space: O(n)

Adding two list elements using map() with add() Function

The map() can also be used, as we can input the add operation to the map() along with the two lists, and map() can perform the addition of both lists techniques. This can be extended to any mathematical operation possible. 

Python3

from operator import add

# initializing lists
test_list1 = [1, 3, 4, 6, 8]
test_list2 = [4, 5, 6, 2, 10]

# printing original lists
print ("Original list 1 : "+ str(test_list1))
print ("Original list 2 : " + str(test_list2))

# using map() + add() to
# add two list
res_list = list(map(add, test_list1, test_list2))

# printing resultant list
print ("Resultant list is : " + str(res_list))

Output :

Original list 1 : [1, 3, 4, 6, 8]
Original list 2 : [4, 5, 6, 2, 10]
Resultant list is : [5, 8, 10, 8, 18]

Time Complexity: O(n)
Auxiliary Space: O(n)

Adding two list elements using zip() with sum()

The sum() can perform the index-wise addition of the list that can be “zipped” together using the zip(). This is quite an elegant way to perform this particular task. 

Python3

# initializing lists
test_list1 = [1, 3, 4, 6, 8]
test_list2 = [4, 5, 6, 2, 10]

# printing original lists
print ("Original list 1 :" + str(test_list1))
print ("Original list 2 :" + str(test_list2))

# using zip() + sum() to
# add two list
res_list = [sum(i) for i in zip(test_list1, test_list2)]

# printing resultant list
print ("Resultant list is : " + str(res_list))

Output :

Original list 1 : [1, 3, 4, 6, 8]
Original list 2 : [4, 5, 6, 2, 10]
Resultant list is : [5, 8, 10, 8, 18]

Time Complexity: O(n)
Auxiliary Space: O(n)

Adding two list elements using numpy.sum()

Import the Numpy library then Initialize the two lists and convert the lists to numpy arrays using the numpy.array() method.Use the numpy.sum() method with axis=0 to sum the two arrays element-wise.Convert the result back to a list using the tolist() method.

Python3

import numpy as np

# initializing lists
test_list1 = [1, 3, 4, 6, 8]
test_list2 = [4, 5, 6, 2, 10]

# printing original lists
print("Original list 1 : " + str(test_list1))
print("Original list 2 : " + str(test_list2))

# using numpy.sum() to add two lists
res_array = np.array(test_list1) + np.array(test_list2)
res_list = res_array.tolist()

# printing resultant list
print("Resultant list is : " + str(res_list))

Output

Original list 1 : [1, 3, 4, 6, 8]
Original list 2 : [4, 5, 6, 2, 10]
Resultant list is : [5, 8, 10, 8, 18]

Time complexity: O(n), where n is the length of the lists.
Auxiliary space: O(n), where n is the length of the lists, for the numpy arrays.