Python – Index Value Summation List

To access the elements of lists, there are various methods. But sometimes we may require to access the element along with the index on which it is found and compute its summation, and for that, we may need to employ different strategies. This article discusses some of those strategies.

Method 1: Naive method This is the most generic method that can be possibly employed to perform this task of accessing the index along with the value of the list elements. This is done using a loop. The task of performing sum is performed using external variable to add. 

Python3

# Python 3 code to demonstrate
# Index Value Summation List
# using naive method

# initializing list
test_list = [1, 4, 5, 6, 7]

# Printing list
print ("The original list is : " + str(test_list))

# using naive method to
# Index Value Summation List
res = []
for i in range(len(test_list)):
    res.append(i + test_list[i])

print ("The list index-value summation is : " + str(res))

Output : 

The original list is : [1, 4, 5, 6, 7]
The list index-value summation is : [1, 5, 7, 9, 11]

Time Complexity: O(n), where n is the number of elements in the list “test_list”.
Auxiliary Space: O(n), where n is the number of elements in the list “test_list”.

Method 2: Using list comprehension + sum() This method works in similar way as the above method but uses the list comprehension technique for the same, this reduces the possible lines of code to be written and hence saves time. The task of performing summation is done by sum(). 

Python3

# Python 3 code to demonstrate
# Index Value Summation List
# using list comprehension

# initializing list
test_list = [1, 4, 5, 6, 7]

# Printing list
print ("The original list is : " + str(test_list))

# using list comprehension to
# Index Value Summation List
res = [sum(list((i, test_list[i]))) for i in range(len(test_list))]

print ("The list index-value summation is : " + str(res))

Output : 

The original list is : [1, 4, 5, 6, 7]
The list index-value summation is : [1, 5, 7, 9, 11]

Time Complexity: O(n), where n is the number of elements in the list “test_list”.
Auxiliary Space: O(n), where n is the number of elements in the list “test_list”.

Method 3:  Using Enumerate
This method uses the built-in enumerate() method which is used to loop over a list along with the index of the elements present in it.

Python3

# Python 3 code to demonstrate
# Index Value Summation List
# using enumerate

# initializing list
test_list = [1, 4, 5, 6, 7]

# Printing list
print ("The original list is : " + str(test_list))

# using enumerate to
# Index Value Summation List
res = [index + value for index, value in enumerate(test_list)]

print ("The list index-value summation is : " + str(res))
#This code is contributed by Edula Vinay Kumar Reddy

Output

The original list is : [1, 4, 5, 6, 7]
The list index-value summation is : [1, 5, 7, 9, 11]

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

Method 4 : Using map() and lambda function:

• The enumerate() function is used to generate a sequence of tuples (index, value) for each element in test_list.
• The map() function is used to apply a lambda function to each tuple in the sequence.
• The lambda function adds the index and value of each tuple together, which creates a new sequence of integers.
• The list() function is used to convert the sequence of integers to a new list res.
• The print() function is used to display the resulting list res to the user.

Python3

test_list = [1, 4, 5, 6, 7]

# Using map() and lambda function to sum the index
# and value for each element in test_list

# The enumerate() function is used to get the index
# and value of each element in the list
res = list(map(lambda x: x[0]+x[1], enumerate(test_list)))

print("The list index-value summation is : " + str(res))

Output

The list index-value summation is : [1, 5, 7, 9, 11]

Time complexity: O(n), where n is the length of the input list test_list.
Auxiliary space: O(n), as we create a new list res that contains the same number of elements as the input list.