How to add all items in a list of lists in Python

Question

I was trying to make a definition that would add all numbers within each sublist in a list of lists.

def MassAddition(_list):
    output = []
    total = 0
    for i in _list:
        if isinstance(i, list):
            output.append(MassAddition(i))
        else:
            total = total + i
    output.append(total)
    return output

Problem is that it returns an extra item in a list at the end. I think its because I made total = 0 and then appended it to output list outside of for loop. Can someone help me clean this up? Ps. This definition should be able to handle any level of nested lists.

example: input = [[0,1,2], [2,1,5],[2,2,2],2,2,1] desiredoutput = [[3],[8],[6],5]

Thank you,

Why not take out `output.append(total)` and just `return output, total`? — TigerhawkT3, May 12 '15 at 21:07
Is that supposed to be a sublist at the end? `[..., [2, 2, 1]]` — Malik Brahimi, May 12 '15 at 21:10
@MalikBrahimi no, I am assuming that i might get a mix of lists and actual values. I think i can handle it from here though. — konrad, May 12 '15 at 21:12
I don't understand what your problem is. I ran your function on your example input and it returned the desired output. — Paulo Almeida, May 12 '15 at 21:18
Interesting question about flattening [lists](http://stackoverflow.com/questions/406121/flattening-a-shallow-list-in-python) — Tom, May 12 '15 at 21:21
@PauloAlmeida what about an input that is [[2,2,1],[1,2],[2,2]] which will return an extra output with a [0] in it. — konrad, May 12 '15 at 21:26
@konrad - hacky solution, loop through the list and delete any indexes that are 0 — Tom, May 12 '15 at 21:29
@TigerhawkT3 Better would be `if total==0: output.append(total)` — Bhargav Rao, May 12 '15 at 21:30
It sounded like he _didn't_ want an extra `0` if there were no bare `int`s. — TigerhawkT3, May 12 '15 at 21:46

Paulo Almeida · Answer 1 · 2015-05-12T22:11:17.060

0

You can check the additional numbers for type too. If they can only be int:

def mass_addition(lst):
    output = []
    total = 0
    extra_flag = False
    for i in lst:
        if isinstance(i, list):
            output.append(mass_addition(i))
        elif isinstance(i, int):
            extra_flag = True
            total += i
    if extra_flag:
        output.append(total)
    return output

edited May 12 '15 at 22:11

answered May 12 '15 at 21:54

Paulo Almeida

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if the list is nested as so: [[1,2], [[1], [2], [3,4]]]. The output still contains nested lists - [[3], [[1], [2], [7]]]. is this what the OP wants? – Tom May 12 '15 at 21:59
@EpsilonX, I'm not sure, but if it isn't, the problem must be defined more clearly. That would mean the problem with the original function wasn't just returning an extra item in the end of the list, as konrad said. – Paulo Almeida May 12 '15 at 22:06
I think the main problem is that, indeed, we don't know what the OP wants. – TigerhawkT3 May 12 '15 at 22:08
True. @PauloAlmeida Perhaps both are the problem?? – Tom May 12 '15 at 22:10

How to add all items in a list of lists in Python

1 Answers1