CODEWITHSUNDEEP
pythonarrayspython-3.xlistarraylist
Azer
Azer

Reputation: 21

What is wrong with this cumulative sum function, and is there a better way to write the function?

Function:

def comulative_sum(arr):
    arr1 = []
    for number in range(1, len(arr)):
        sum1 = 0
        for number1 in range(0, arr[number]):
            sum1 += number1
        arr1.append(sum1)
    return arr1
arr = [1, 2, 3, 4]
print(comulative_sum(arr))

Output:

[3, 6, 10]

Expected output:

[1, 3, 6, 10]

I have tried slicing ([1:], [0:number) instead of the range function. Still no results.

Upvotes: 0

Views: 118

Answers (5)

Azer
Azer

Reputation: 21

The for loop will only deal with numbers from index 1 to the end of the array. Thus index 0 will be ignored.

Working function:

def comulative_sum(arr):
arr1 = []
for number in arr[:len(arr)]:
    sum1 = 0
    for number1 in arr[0:number]:
        sum1 += number1
    arr1.append(sum1)
return arr1
arr = [1, 2, 3, 4]
print(comulative_sum(arr))

All that was changed was the indexing in the first and second for loop.

Upvotes: 0

rpontual
rpontual

Reputation: 95

I built in your solution. This version relies on Python's loop through indexes and also addresses a possible number '0'.

def cumulative_sum(arr):
    arr1 = []
    # Python does the hard work for you, it loops through each value on the list
    for value in arr:
        sum1 = 0
        # Range is going to have a list as follows [0, ... , value-1]
        for number1 in range(0, value):
            sum1 += number1
        # Adding 'value' to sum1 because the loop did not add the last value
        arr1.append(sum1+value)
    return arr1

arr = [1, 3, 6, 10]
# Calling the function with 'print' to make sure the results are displayed
print(cumulative_sum(arr))

Upvotes: 0

Bharel
Bharel

Reputation: 26994

Multiple issues:

  • On the first loop you need to range until len() + 1 as the last number is len() and then the last number in second loop will be len() - 1 which is also the index.
  • On the second loop, instead of ranging until number, you range until the original number in the array.

Fixed code:

def comulative_sum(arr):
    arr1 = []
    for number in range(1, len(arr)+1):
        sum1 = 0
        for number1 in range(0, number):
            sum1 += arr[number1]
        arr1.append(sum1)
    return arr1
arr = [1, 2, 3, 4]
print(comulative_sum(arr))

If you wish to improve upon this code and not use a built-in accumulate, you can do so:

def comulative_sum(input_list):
    output = []
    sum_ = 0  # The underscore is to avoid renaming the built-in sum()
    for i in input_list:
        sum_ += i
        output.append(sum_)
    return output

input_list = [1, 2, 3, 4]
print(comulative_sum(input_list))

Advantages:

  1. Better variable naming.
  2. Less code nesting.
  3. Easier overall code readability.
  4. Faster code (no need to recalculate everything on each iteration)

Upvotes: 0

Joran Beasley
Joran Beasley

Reputation: 114038

you dont need to loop it twice

def cum_sum(x):
    result = []
    current_sum = 0
    for value in x:
        current_sum += value
        result.append(current_sum)
    return result

Upvotes: 0

Bharel
Bharel

Reputation: 26994

The better way to write this function is simply using itertools.accumulate() which does exactly that:

>>> import itertools
>>> print(list(itertools.accumulate([1,2,3,4]))
[1, 3, 6, 10]

Upvotes: 3

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