Python | Summation of first N matching condition

Summation of elements in list can be performed using many inbuilt function. Normal summation functions have many applications in various domains. This article discusses to sum just the first N occurrences of elements matching particular condition. 

Method #1 : Naive Method We can sum the elements that are matching condition, after N occurrences of elements have been done and we can stop the operation. Code below demonstrates the same. 

Python3

# Python 3 code to demonstrate
# Summation of first N matching condition
# using Naive Method

# initializing list
test_list = [3, 5, 1, 6, 7, 9, 8, 5]

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

# using Naive Method
# Summation of first N matching condition
# sums first 3 odd occurrences
counter = 1
res = 0
for i in test_list:
    if counter <= 3 and (i % 2 != 0):
        res = res + i
        counter = counter + 1

# printing result
print ("The filtered list is : " + str(res))

Output : 

The original list is : [3, 5, 1, 6, 7, 9, 8, 5]
The filtered list is : 9

Time Complexity: O(n), where n is the length of the input list. 
Auxiliary Space: O(1), as we’re using constant additional space.

Method #2 : Using sum() + list comprehension This is different and elegant way to perform this particular task. It filters out all the numbers that are less than equal N and sums according to condition. This is one liner and preferred method to achieve this task. 

Python3

# Python 3 code to demonstrate
# Summation of first N matching condition
# using sum() + list comprehension

# initializing list
test_list = [3, 5, 1, 6, 7, 9, 8, 5]

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

# using sum() + list comprehension
# to sum first N elements matching condition
# sum first 3 odd occurrences
res = sum([i for i in test_list if i % 2 != 0][:3])

# printing result
print ("The filtered list is : " + str(res))

Output : 

The original list is : [3, 5, 1, 6, 7, 9, 8, 5]
The filtered list is : 9

Time Complexity: O(n) where n is the number of elements in the string list. The sum() + list comprehension is used to perform the task and it takes O(n) time.
Auxiliary Space: O(1) constant additional space is required.