Question

I need to write a program that compute cumulative sums from a list of numbers with def but ONLY with recursion.
I did it, but now I need to write the same program without using the method sum, but no success so far.
Any idea?

my code:

def rec_cumsum(numbers):
        ''' Input: numbers - a list of numbers,
                Output: a list of cumulative sums of the numbers'''
        if len(numbers)==0: return numbers

        return rec_cumsum(numbers[:-1])+ [sum(numbers)]

input:

1 [1,2,3]

2 [2, 2, 2, 3]

output:

1 [1,3,6]

2 [2, 4, 6, 9]

---

my code without sum:

def rec_cumsum(numbers):
        ''' Input: numbers - a list of numbers,
                Output: a list of cumulative sums of the numbers'''
        if len(numbers) == 0: return numbers
        my_list=[]
        rec_cumsum(my_list + numbers)
        my_list[0]=numbers[0]
        rec_cumsum(my_list)
        temp_sum=my_list[0]+numbers[-1]
        my_list[0]=temp_sum
        return my_list

Answer 1

What you can do is: –

• Create a temp list(an empty one).
• Pass your original list and the empty list in your method.
• Now, when you pass your list for the first time, just add the first element from your original list to it. And call the same method with the rest of the list. Starting from 1st element.
• When your method is invoked after wards, you need to take a sum of last element of your temp list and first element of the original list that is now modified. And add the sum to your temp list as new element.
• Finally, when the length of your original list becomes 0. Return your temp.

**Here’s the code for the above steps. You can compare it with the one you have implemented, and see where you went wrong: –

def rec_cumsum(numbers):
    if len(numbers) == 0 : return temp

    # You need to check, if `temp` is empty, that means method is called first time.
    if not temp:   
        temp.extend([numbers[0]])   // Just add the first element to it.

    else:
        # Else, get the last element from `temp`, 
        # add it to `first elemt` in `numbers` and add it to `temp`.
        temp.extend([temp[-1] + numbers[0]])

    return rec_cumsum(numbers[1:])

my_list = [2, 2, 2, 3]
temp = []
print rec_cumsum(my_list)