CODEWITHSUNDEEP
pythonlist
djlovesupr3me
djlovesupr3me

Reputation: 79

Sum a nested list of a nested list of a nested list of integers

Given a Python list whose elements are either integers or lists of integers (only we don't know how deep the nesting goes), how can we find the sum of each individual integer within the list?

It's fairly straightforward to find the sum of a list whose nesting only goes one level deep, e.g.

[1, [1, 2, 3]]
# sum is 7

But what if the nesting goes two, three, or more levels deep?

[1, [1, [2, 3]]]
# two levels deep

[1, [1, [2, [3]]]]
# three levels deep

The sum in each of the above cases is the same (i.e. 7). I think the best approach is using recursion where the base case is a list with a single integer element, but beyond that I'm stuck.

Upvotes: 0

Views: 3915

Answers (4)

2rs2ts
2rs2ts

Reputation: 11026

One approach: flatten the list, then use sum().

from collections import Iterable
def flatten(lst):
    for i in lst:
        if isinstance(i, Iterable) and not isinstance(i, basestring):
            for sublst in flatten(i):
                yield sublst
        else:
            yield i

sum(flatten([1, [1, [2, [3]]]]))

If you are dealing only with lists, change isinstance(i, Iterable) to isinstance(i, list) for a massive performance boost.

Note that the basestring check is not necessary when using sum() as Ashwini pointed out.

Upvotes: 2

Blair
Blair

Reputation: 6693

Assuming that you are only using lists, this should do the trick:

def sum_nested(l):
    s = 0
    for item in l:
        if type(item) is list:
            s += sum_nested(item)
        else:
            s += item
    return s

Upvotes: 4

Captain Skyhawk
Captain Skyhawk

Reputation: 3500

Easiest way I can think of:

from compiler.ast import flatten
sum(flatten(numbs))

Upvotes: 5

Ashwini Chaudhary
Ashwini Chaudhary

Reputation: 250881

You can use this recursive solution:

from collections import Iterable
def flatten(collection):
  for element in collection:
    if isinstance(element, Iterable):
      for x in flatten(element):
        yield x
    else:
      yield element

Demo:

>>> lis = [1, [1, [2, [3]]]]
>>> sum(flatten(lis))
7
>>> lis = [1, [1, 2, 3]]
>>> sum(flatten(lis))
7
>>> lis = [1, [1, [2, 3]]]
>>> sum(flatten(lis))
7

Upvotes: 7

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