Python - Diagonal element addition among lists

Sometimes, while working with Python lists, we can have a problem in which we need to perform addition of lists in diagonal manner that i.e. adding ith element of 1 list to i + 1 element of other list. This kind of problem can have application in day-day programming. Lets discuss certain ways in which this task can be performed. 

Method #1 : Using loop This is brute force way in which this task can be performed. In this, we iterate one list and test add the i + 1th element of other list and construct the resultant list. 

# Python3 code to demonstrate  # Diagonal element addition among lists # using loop # Initializing lists test_list1 = [1, 6, 8, 5, 3] test_list2 = [8, 10, 3, 4, 5] # printing original lists print("The original list 1 is : " + str(test_list1)) print("The original list 2 is : " + str(test_list2)) # Diagonal element addition among lists # using loop res = [] for idx in range(0, len(test_list1) - 1): res.append(test_list1[idx] + test_list2[idx + 1]) # printing result  print ("List after diagonal addition : " + str(res)) 

The original list 1 is : [1, 6, 8, 5, 3] The original list 2 is : [8, 10, 3, 4, 5] List after diagonal addition : [11, 9, 12, 10]

Time Complexity: O(n*n), where n is the length of the input list. This is because we’re using the loop which has a time complexity of O(n) in the worst case.
Auxiliary Space: O(n), as we’re using additional space res other than the input list itself with the same size of input list. 

  Method #2 : Using zip() + list comprehension This is yet another way in which this task can be performed. In this, we group ith with i+1th element of other list using zip(). The task of performing addition is done in list comprehension. 

# Python3 code to demonstrate  # Diagonal element addition among lists # using zip() + list comprehension # Initializing lists test_list1 = [1, 6, 8, 5, 3] test_list2 = [8, 10, 3, 4, 5] # printing original lists print("The original list 1 is : " + str(test_list1)) print("The original list 2 is : " + str(test_list2)) # Diagonal element addition among lists # using zip() + list comprehension res = [i + j for i, j in zip(test_list1, test_list2[1:])] # printing result  print ("List after diagonal addition : " + str(res)) 

The original list 1 is : [1, 6, 8, 5, 3] The original list 2 is : [8, 10, 3, 4, 5] List after diagonal addition : [11, 9, 12, 10]

Time Complexity: O(n), where n is length of input list.
Auxiliary Space: O(n), where n is length of res list. 

Method #3 : Using Numpy
This method makes use of the functionality provided by numpy library in python. The library provides the numpy.add function which performs element wise addition of two arrays.

#Python3 code to demonstrate #Diagonal element addition among lists #importing numpy library import numpy as np #Initializing lists test_list1 = [1, 6, 8, 5, 3] test_list2 = [8, 10, 3, 4, 5] #printing original lists print("The original list 1 is : " + str(test_list1)) print("The original list 2 is : " + str(test_list2)) #Diagonal element addition among lists #using Numpy res = np.add(test_list1[:-1], test_list2[1:]).tolist() #printing result print ("List after diagonal addition : " + str(res)) #This code is contributed by Edula Vinay Kumar Reddy 

Output :
The original list 1 is : [1, 6, 8, 5, 3]
The original list 2 is : [8, 10, 3, 4, 5]
List after diagonal addition : [11, 9, 12, 10]

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

Explanation:
Here, the add function in numpy performs element wise addition of two arrays. The tlist function is used to convert the result array to a list. The time and space complexity is O(n) as we are iterating the two arrays and constructing a new resultant array.