Python | Chuncked summation every K value

Last Updated : 26 Nov, 2024

The prefix array is quite famous in the programming world. This article would discuss a variation of this scheme. To perform a chunked summation where the sum resets at every occurrence of the value K , we need to modify the summation logic. The idea is:

Traverse the list.
Keep adding to the cumulative sum (sm).
If the element is equal to K, reset the sum and continue the accumulation from the next element.

Method #1 : Using Naive Method In the naive method, we just construct the new list comprising of the sum of prev. value of list until K and restarts the procedure once a non K value is encountered.

Python

# Python3 code to demonstrate
# Chunked summation every K value

# Initializing the list
lst = [1, 2, 0, 3, 4, 0, 5]
K = 0  # The value where sum resets

# Initialize empty list for cumulative sum
ls = []
sm = 0

# Chunked summation using cumulative approach
for i in range(len(lst)):
    if lst[i] == K:
        sm = 0  # Reset sum when we encounter K
    sm += lst[i]  # Add current element to sum
    ls.append(sm)  # Append cumulative sum to the list

# Printing result
print("The computed modified new list:", ls)

Output

The computed modified new list: [1, 3, 0, 3, 7, 0, 5]

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
In List Comprehenson, it creates a new list from an original list by adding each element to the sum of the K-1 previous elements. This is done using a list comprehension and the sum() function. The slice of the original list used to compute the sum starts from the maximum of 0 and i-K+1 to handle the first few elements, and ends at i+1 to include the current element. The new list is then printed.

Python

# Python3 code to demonstrate 
# Chuncked summation every K value
# using list comprehension

# initializing list of lists
test_list = [1, 3, 4, 10, 4, 5, 10, 7, 8]

# initializing K 
K = 10

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

# Chuncked summation every K value using list comprehension
new_list = [sum(test_list[max(0, i-K+1):i+1]) for i in range(len(test_list))]

# printing result
print("The computed modified new list : " + str(new_list))

Output

The original list is : [1, 3, 4, 10, 4, 5, 10, 7, 8]
The computed modified new list : [1, 4, 8, 18, 22, 27, 37, 44, 52]

Time Complexity: O(n*m), where n is the length of the input list and m is the chunk size.
Auxiliary Space: O(n), as we need to create a new list of the same size as the input list