sum two lists element-by-element in python recursively

Question

Is it possible to recursively sum two lists element by element and then return the new list?

def sumListElements(listOne, listTwo):
    list = []
    i = 0
    while i < len(listOne):
        list.append(listOne[i] + listTwo[i])
        i += 1
    return list

So,

a = [1, 2, 3]
b = [3, 4, 5]

Results

R = [4, 6, 8]

Why recursive? You have no nested structure here. – Martijn Pieters Apr 06 '14 at 11:18 — Martijn Pieters, Apr 06 '14 at 11:18

thefourtheye · Accepted Answer · 2014-04-06T11:32:20.613

3

Here is a recursive implementation

def recursive_sum(l1, l2, idx = 0):
    if idx < min(len(l1), len(l2)):
        return [l1[idx] + l2[idx]] + recursive_sum(l1, l2, idx + 1)
    else:
        return []

print recursive_sum([1, 2, 3], [4, 5, 6])
# [5, 7, 9]

Or

def recursive_sum(l1, l2, result = None, idx = 0):
    if result is None:
        result = []
    if idx < min(len(l1), len(l2)):
        result.append(l1[idx] + l2[idx])
        return recursive_sum(l1, l2, result, idx + 1)
    else:
        return result

edited Apr 06 '14 at 11:32

answered Apr 06 '14 at 11:25

thefourtheye

233,700
52
457
497

2

@Xerath Make sure to site this answer in your homework... – Inbar Rose Apr 06 '14 at 11:45

score 2 · Answer 2 · answered Apr 06 '14 at 11:18

2

Use zip() and map() here:

R = map(sum, zip(a, b))

Demo:

>>> a = [1, 2, 3]
>>> b = [3, 4, 5]
>>> map(sum, zip(a, b))
[4, 6, 8]

For Python 3 compatibility, replace map() with a list comprehension:

[sum(z) for z in zip(a, b)]

answered Apr 06 '14 at 11:18

Martijn Pieters

1,048,767
296
4,058
3,343

So, that's the only way to do this ? – Xerath Apr 06 '14 at 11:25
1

No, this is the most efficient way to do it. – Martijn Pieters Apr 06 '14 at 11:28
I like the efficient way, but I have to write it recursively, just like @thefourtheye is done. – Xerath Apr 06 '14 at 11:33

score 1 · Answer 3 · answered Apr 06 '14 at 11:23

Function which takes n lists, and adds each element in an i-th index together with the others:

from itertools import izip_longest

def sum_elements(*lists):
    return map(sum, izip_longest(*lists, fillvalue=0))

Showed with your data:

>>> sum_elements([1, 2, 3], [3, 4, 5])
[4, 6, 8]

Lists with uneven lengths still have a nice result:

>>> sum_elements([1, 2, 3], [3, 4, 5, 6])
[4, 6, 8, 6]

And it can take any number of lists:

>>> sum_elements([1, 2, 3], [3, 4, 5, 6], [8,9])
[12, 15, 8, 6]

sum two lists element-by-element in python recursively

3 Answers3