Loop through list elements to get cumulative sums

I’m very new to python. I’m trying to create a loop so that I can get cumulative sums of the elements in a list. For example, given a list [3, 2, 1] I’m hoping to get [3 (first number), 5 (3+2), 6 (3+2+1)], [2 (second number), 3 (2+1)] and [1].

What I have currently is:

data = [5, 4, 3, 2, 1]
for i in data:
    perf = [sum(data[:i+1]) for i in range(len(data))]
    print(perf)

And I’m getting the following as output, which is the sum from the first element. How do I modify to get the cumulative sums starting with 4, 3, … ?

[5, 9, 12, 14, 15]
[5, 9, 12, 14, 15]
[5, 9, 12, 14, 15]
[5, 9, 12, 14, 15]
[5, 9, 12, 14, 15]

My desired output

[5, 9, 12, 14, 15]
[4, 7, 9, 10]
[3, 5, 6]
[2, 3]
[1]

Thank you!

Answers:

Thank you for visiting the Q&A section on Magenaut. Please note that all the answers may not help you solve the issue immediately. So please treat them as advisements. If you found the post helpful (or not), leave a comment & I’ll get back to you as soon as possible.

Method 1

If I understand you correctly, in the 1. iteration you want to get sums from first element, in the 2. iteration sums from second element and so on:

data = [5, 4, 3, 2, 1]
for i in range(len(data)):
    s = 0
    l = [s := s + d for d in data[i:]]
    print(l)

Prints:

[5, 9, 12, 14, 15]
[4, 7, 9, 10]
[3, 5, 6]
[2, 3]
[1]

Or: using itertools.accumulate

from itertools import accumulate

data = [5, 4, 3, 2, 1]
for i in range(len(data)):
    print(list(accumulate(data[i:])))

Method 2

You’re not far from the answer. You just need to begin your range from the next num in the list. Try this:

data = [5, 4, 3, 2, 1]
for ind, num in enumerate(data):
    perf = [sum(data[ind:i+1]) for i in range(ind, len(data))]
    print(perf)

Output:

[5, 9, 12, 14, 15]
[4, 7, 9, 10]
[3, 5, 6]
[2, 3]
[1]

Method 3

Your statement to compute a commulative sum within the loop is correct.
You had just used i incorrectly in the outer loop statement.

All you need to do is slice your original data and generate sums for each slice.

test_data = [5, 4, 3, 2, 1]

def commulative_sum(a_list: list[int]):
    # The same thing you wrote
    return [sum(a_list[:i+1]) for i in range(len(a_list))]

def commulative_sums(data: list[int]):
    # Slice the input data and generate commulative sums
    return [commulative_sum(data[j:]) for j in range(len(data))]

result = commulative_sums(test_data)
print(result)

[[5, 9, 12, 14, 15], [4, 7, 9, 10], [3, 5, 6], [2, 3], [1]]

Method 4

Here are 2 ways you can solve this problem.

data = [5, 4, 3, 2, 1]

k = 0;
for i in data:
    k = k + i
print(k)

And this is the second method

data = [5, 4, 3, 2, 1]

k = 0;
for i in range(len(data)):
    k = k + data[i]
print(k)

All methods was sourced from stackoverflow.com or stackexchange.com, is licensed under cc by-sa 2.5, cc by-sa 3.0 and cc by-sa 4.0