Generalizing adding nested lists

Question

Given 3 nested vectors:

 >>> a
[[1, 2, 3], [4, 5, 6], [7, 8, 9]]
>>> b
[[10, 20, 30], [40, 50, 60], [70, 80, 90]]
>>> c
[[100, 200, 300], [400, 500, 600], [700, 800, 900]]

I can add these vectors together with a map/sum/zip comprehension like so:

>>> [map(sum,zip(i,j,k)) for i,j,k in zip(a,b,c)]
[[111, 222, 333], [444, 555, 666], [777, 888, 999]]

I've manually expanded this from adding two lists together, but is there a pythonic way to generalize this to handle an arbitrary number of lists?

(Python 2.7 without using external libraries preferred)

Without external libraries you mean? itertools may be a useful import — OneCricketeer
– OneCricketeer, Commented Nov 9, 2016 at 17:17
Additionally - can it be generalized to arbitrary levels of nesting? (not my actual problem but just occurred to me as related) — paulw1128
– paulw1128, Commented Nov 9, 2016 at 17:18
@cricket_007 Thanks - that's more what I meant. I'll edit the question. — paulw1128
– paulw1128, Commented Nov 9, 2016 at 17:18
@paulw1128 you are sliding towards recursion with your thoughts. Be careful son. xD — Ma0
– Ma0, Commented Nov 9, 2016 at 17:20

Kasravnd · Accepted Answer · 2016-11-09 17:44:03Z

4

Here is a general approach:

from itertools import izip
def multiple_sum(*args):
    return [map(sum, izip(*items)) for items in izip(*args)]

Demo:

In [13]: multiple_sum(a, b, c)
Out[13]: [[111, 222, 333], [444, 555, 666], [777, 888, 999]]

Note that since in Python 2.7 zip returns a list it's better off using it when you only want to iterate over the results, and instead use itertools.izip that returns an iterator.

Here is another way using itertools.starmap() which is faster than the previouse approach:

def multiple_sum(*args):
    return [map(sum, lst) for lst in starmap(zip, zip(*args))]

Benchmark:

In [32]: %timeit [map(sum, izip(*items)) for items in izip(a, b, c)]
100000 loops, best of 3: 3.93 µs per loop

In [33]: %timeit [map(sum, lst) for lst in starmap(zip, zip(a, b , c))]
100000 loops, best of 3: 3.01 µs per loop

edited Nov 9, 2016 at 17:44

answered Nov 9, 2016 at 17:18

Kasravnd

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lucasnadalutti · Accepted Answer · 2016-11-09 17:18:32Z

3

This should work:

lists = [a,b,c]
[map(sum,zip(*zipped_lists)) for zipped_lists in zip(*lists)]

answered Nov 9, 2016 at 17:18

lucasnadalutti

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Francisco · Accepted Answer · 2016-11-09 17:19:45Z

3

If you have a list of lists ([a,b,c]), you can do the following:

>>> lists = [a,b,c]
>>> [map(sum, zip(*l)) for l in zip(*lists)]
[[111, 222, 333], [444, 555, 666], [777, 888, 999]]

answered Nov 9, 2016 at 17:19

Francisco

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