Getting the lengths of a list of lists of lists of lists

Question

I have a little bit weird looking lists like this:

listlist = [[], [[[[], []], [[]], []]], [[]]]

and I am trying to find the sum of all the lengths of the lists.

If it was a simple multidimensional array(n x n x n x ...), finding the total sum of the lists' lengths would be very easy, but in this case, I am not sure where I should start.

I tried to use 'for loop', but I believe that is not a clear answer to this kind of problem, and there should be a really simple approach to solve this kind of problem.

I would appreciate any advice. Thanks in advance! =]

See https://stackoverflow.com/questions/2158395/flatten-an-irregular-list-of-lists for how to flatten it. — Barmar, Apr 28 '20 at 06:51
@Barmar: he wants to sum the length of the lists. Since there are commas, the result should not be 0 — Thomas Weller, Apr 28 '20 at 06:52
I am sorry for the confusion! I have to find the sum of (lengths of lists + 1). len(listlist) gives us 3, and within 3 items, the first list's length would be 0. (len([]) =0 ), but len(listlist[1]) gives you 1. In this listlist, the output should be 17. and you should ignore 0 length list like [ ]. — Kevin Choi, Apr 28 '20 at 06:52
@KevinChoi Will you tell us the number you expect for your sample input? — user2390182, Apr 28 '20 at 06:54
Go with the for loop and you'll get a result. Then worry about performance in the next step — Thomas Weller, Apr 28 '20 at 06:54
@KevinChoi All the lists' lengths in there sum up to 12. Is that your expectd result? — user2390182, Apr 28 '20 at 06:56
@Barmar Thank you Barmar! I will try recursion for this problem — Kevin Choi, Apr 28 '20 at 06:57
@schwobaseggl exactly! if I ignore +1 part of this problem, the sum of total lengths should be 12 for this problem — Kevin Choi, Apr 28 '20 at 06:58

score 0 · Accepted Answer · answered Apr 28 '20 at 07:01

This might nudge you in the right direction:

def lenall(lst):
    if isinstance(lst, list):
        return 1 + sum(map(lenall, lst))
    return 0

>>> lenall(listlist)
12

This actually counts the total number of lists in the data structure. If you want to add all their lengthes, you would have to change it to:

def lenall(lst):
    if isinstance(lst, list):
        return len(lst) + sum(map(lenall, lst))
    return 0

>>> lenall(listlist)
11

score 0 · Answer 2 · answered Apr 28 '20 at 07:02

0

Here's a possible solution:

listlist = [[], [[[[], []], [[]], []]], [[]]]

def length(lst):
    return len(lst) + sum(length(l) for l in lst if isinstance(l, list))

print(length(listlist))

Output

answered Apr 28 '20 at 07:02

Nick

138,499
22
57
95

score 0 · Answer 3 · answered Apr 28 '20 at 07:11

Here is code that should do what you desire.

I included two versions of the function, one for clarity, and one that is smaller but functionally identical.

####################
# Create a list of lists / elements
####################
#x = [[1,2,3],[3,4],[3,4,[3,4,5],[4],[5,4,6]],[4,3,4,5,[[[6]]]]]
x = [[], [[[[], []], [[]], []]], [[]]]

############################################################
# First implementation (included for clarity)
############################################################
def get_len_lists(this_list):
    # Set number of elements to 0
    num_elem = 0                              

    # Loop through each element in the list...
    for elem in this_list:                          

        # .. if it's a list...
        if type(elem) == list:                      

            # ... if the list is empty, count that as an element
            if elem == list():                      
                # ... so add one
                num_elem += 1                       
            else:
                # ... get the number of elements...
                num_elem += get_len_lists(elem) + 1 

        # ... otherwise...
        else:                                       
            # ... just add one to the length of the list
            num_elem += 1                           

    # Return the number of elements in the list
    return num_elem                                 


############################################################
# Smaller implementation
############################################################
def get_len_lists_2(this_list):
    # Set number of elements to 0
    num_elem = 0                                    

    # Loop through each element in the list...
    for elem in this_list:                          

        # We add one for each level, regardless of whether it is an element or a list
        num_elem += 1                              

        # If it's a list...
        if type(elem) == list:                      

            # .. get the number of elements in the list
            num_elem += get_len_lists(elem)         

    # Return the number of elements in the list
    return num_elem                                 


result1 = get_len_lists(x) + 1
result2 = get_len_lists_2(x) + 1

print(result1)
print(result2)

The output is:

12
12

Getting the lengths of a list of lists of lists of lists

3 Answers3