Find sum of nested list using recursion

Question

Find the sum of nested list using recursion

lst = [1,2,[3,4],[5,6]]
def findSum(values,sum,idx):
    if len(values) == idx:
        return sum
    if isinstance(values[idx],list):
       return findSum(values[idx],sum,0)
    else:
        sum = sum + values[idx]
        return findSum(values,sum,idx+1)

print(findSum(lst,0,0))

Output should be 21 but I am getting only 10 last index subarray([5,6]) not getting calculated.

Can anyone suggest me, what I am doing wrong in the logic?

Answer 1

You can recurse through the list as shown below

def recur_sum(lst):
    s = 0
    for x in lst:
        if type(x) == list:
            s += recur_sum(x)
        else:
            s += x
    return s


lst = [1,2,[3,4],[5,6]]
print(recur_sum(lst))

EDIT: The problem with OP's code is, the code is not moving forward after its done processing a sublist and returns from there.

Answer 2

There are just some good pointers earlier about the origin codes problem, so I am not going to repeat here.

This is just a simpler approach for this problem, maybe as a reference.

A = [1, 2, [3, 4], [5,6]]


def sum_lst(A):
    total = 0
    
    for x in A:
        if isinstance(x, list):
            total += sum_lst(x)
        else:
            total += x
            
    return total
    
print(sum_lst(A))      # 21

Answer 3

Just for fun: a recursive approach using reduce .

A (mathematical) recursion is defined by an initial value and a relationship between a certain terms and its previous ones. The initial value can be specified by the initializer parameter of reduce and, in this case, it is set to 0 . The recursive function can be passed as a parameter to reduce .

Notice:

• no for -loop is required
• only two nested level list supported

from functools import reduce


def recursive_relationship(i, j):
    if isinstance(j, list):
        if j:
            return recursive_relationship(i+j[0], j[1:])
        else:
            return i
    return i + j


lst = [1,2,[3,4],[5,6], [8]]
#lst = [[1, 2], [1, 2,3], [1]]

print(reduce(recursive_relationship, lst, 0))