Python program to find the group sum till each K in a list

When you need to find the cumulative sum of elements in groups separated by a specific key value K, you can use a simple iteration approach. This technique sums elements between occurrences of K and includes K itself in the result.

Example

Below is a demonstration of finding group sums separated by key value K ?

my_list = [21, 4, 37, 46, 7, 56, 7, 69, 2, 86, 1]

print("The list is :")
print(my_list)

my_key = 46
print("The key is")
print(my_key)

my_sum = 0
my_result = []

for ele in my_list:
    if ele != my_key:
        my_sum += ele
    else:
        my_result.append(my_sum)
        my_result.append(ele)
        my_sum = 0

my_result.append(my_sum)

print("The resultant list is :")
print(my_result)

The list is :
[21, 4, 37, 46, 7, 56, 7, 69, 2, 86, 1]
The key is
46
The resultant list is :
[62, 46, 228]

How It Works

The algorithm follows these steps:

• Initialize a sum variable to 0 and an empty result list
• Iterate through each element in the input list
• If the element is not equal to the key K, add it to the running sum
• If the element equals K, append the current sum and K to the result, then reset sum to 0
• After the loop, append the final sum to handle remaining elements

Another Example

Let's see how it works with a different key value ?

numbers = [5, 10, 3, 15, 8, 20, 3, 7, 12]
key = 3

print(f"List: {numbers}")
print(f"Key: {key}")

current_sum = 0
result = []

for num in numbers:
    if num != key:
        current_sum += num
    else:
        result.append(current_sum)
        result.append(num)
        current_sum = 0

result.append(current_sum)

print(f"Group sums: {result}")

List: [5, 10, 3, 15, 8, 20, 3, 7, 12]
Key: 3
Group sums: [15, 3, 43, 3, 19]

Key Points

• The key value K acts as a separator and appears in the final result
• Elements before each K are summed together
• The final sum after the last K (or if no K exists) is always included
• Time complexity is O(n) where n is the length of the list

Conclusion

This approach efficiently groups and sums list elements separated by a key value K. The algorithm maintains a running sum and resets it whenever the key is encountered, creating distinct groups with their cumulative sums.