Add SUM of values of two LISTS into new LIST

Question

I have the following two lists:

first = [1,2,3,4,5]
second = [6,7,8,9,10]

Now I want to add the items from both of these lists into a new list.

output should be

third = [7,9,11,13,15]

Raggletaggle

Does this answer your question? Element-wise addition of 2 lists? – Crook

Answer 1

The zip function is useful here, used with a list comprehension.

[x + y for x, y in zip(first, second)]

If you have a list of lists (instead of just two lists):

lists_of_lists = [[1, 2, 3], [4, 5, 6]]
[sum(x) for x in zip(*lists_of_lists)]
# -> [5, 7, 9]

Joscelin

just curious how would zip() handles if array lengths for different? i.e what does zip returns for different array lengths and how would that affect the operation for x + y – Hodge

@ealeon: The "zipping" stops when the shortest iterable is exhausted. So if first is length 10 and second is length 6, the result of zipping the iterables will be length 6. – Joscelin

Is there a way to do this when you don't know the number of lists? – Bendigo

@traggatmot: >>> lists_of_lists = [[1, 2, 3], [4, 5, 6]] >>> [sum(x) for x in zip(*lists_of_lists)] [5, 7, 9] – Joscelin

Answer 2

From docs

import operator
list(map(operator.add, first,second))

Snapback

Answer 3

Default behavior in numpy.add (numpy.subtract, etc) is element-wise:

import numpy as np
np.add(first, second)

which outputs

array([7,9,11,13,15])

Cybele

use np.add(first, second).tolist() to get the result in a list – Stopover

If first and second are np arrays you can just do first + second. And if you want to save the result in first, you can also do: first += second. – Dozen

Answer 4

Assuming both lists a and b have same length, you do not need zip, numpy or anything else.

Python 2.x and 3.x:

[a[i]+b[i] for i in range(len(a))]

Culley

Answer 5

Try the following code:

first = [1, 2, 3, 4]
second = [2, 3, 4, 5]
third = map(sum, zip(first, second))

Sarson

+1 for this compact and self explanatory solution. It's worth noting that this works for more than 2 lists as well: map(sum, zip(first, second, third, fourth, ...)) – Mena

Answer 6

This extends itself to any number of lists:

[sum(sublist) for sublist in itertools.izip(*myListOfLists)]

In your case, myListOfLists would be [first, second]

Forward

Answer 7

The easy way and fast way to do this is:

three = [sum(i) for i in zip(first,second)] # [7,9,11,13,15]

Alternatively, you can use numpy sum:

from numpy import sum
three = sum([first,second], axis=0) # array([7,9,11,13,15])

Daunt

Answer 8

first = [1, 2, 3, 4, 5]
second = [6, 7, 8, 9, 10]
three = list(map(sum, first, second))
print(three)



# Output 
[7, 9, 11, 13, 15]

Drowse

Answer 9

one-liner solution

list(map(lambda x,y: x+y, a,b))

Uriisa

Answer 10

If you have an unknown number of lists of the same length, you can use the below function.

Here the *args accepts a variable number of list arguments (but only sums the same number of elements in each). The * is used again to unpack the elements in each of the lists.

def sum_lists(*args):
    return list(map(sum, zip(*args)))

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

sum_lists(a,b)

Output:

[2, 4, 6]

Or with 3 lists

sum_lists([5,5,5,5,5], [10,10,10,10,10], [4,4,4,4,4])

Output:

[19, 19, 19, 19, 19]

Greenlee

Answer 11

My answer is repeated with Thiru's that answered it in Mar 17 at 9:25.

It was simpler and quicker, here are his solutions:

The easy way and fast way to do this is:

 three = [sum(i) for i in zip(first,second)] # [7,9,11,13,15]

Alternatively, you can use numpy sum:

 from numpy import sum
 three = sum([first,second], axis=0) # array([7,9,11,13,15])

---

You need numpy!

numpy array could do some operation like vectors

import numpy as np
a = [1,2,3,4,5]
b = [6,7,8,9,10]
c = list(np.array(a) + np.array(b))
print c
# [7, 9, 11, 13, 15]

Arris

You don't need numpy to sum pairwise elements in two lists. – Emblematize

Answer 12

What if you have list with different length, then you can try something like this (using zip_longest)

from itertools import zip_longest  # izip_longest for python2.x

l1 = [1, 2, 3]
l2 = [4, 5, 6, 7]

>>> list(map(sum, zip_longest(l1, l2, fillvalue=0)))
[5, 7, 9, 7]

Skittle

Answer 13

You can use zip(), which will "interleave" the two arrays together, and then map(), which will apply a function to each element in an iterable:

>>> a = [1,2,3,4,5]
>>> b = [6,7,8,9,10]
>>> zip(a, b)
[(1, 6), (2, 7), (3, 8), (4, 9), (5, 10)]
>>> map(lambda x: x[0] + x[1], zip(a, b))
[7, 9, 11, 13, 15]

Envisage

Answer 14

Here is another way to do it. We make use of the internal __add__ function of python:

class SumList(object):
    def __init__(self, this_list):
        self.mylist = this_list

    def __add__(self, other):
        new_list = []
        zipped_list = zip(self.mylist, other.mylist)
        for item in zipped_list:
            new_list.append(item[0] + item[1])
        return SumList(new_list)

    def __repr__(self):
        return str(self.mylist)

list1 = SumList([1,2,3,4,5])
list2 = SumList([10,20,30,40,50])
sum_list1_list2 = list1 + list2
print(sum_list1_list2)

Output

[11, 22, 33, 44, 55]

Barney

Answer 15

If you want to add also the rest of the values in the lists you can use this (this is working in Python3.5)

def addVectors(v1, v2):
    sum = [x + y for x, y in zip(v1, v2)]
    if not len(v1) >= len(v2):
        sum += v2[len(v1):]
    else:
        sum += v1[len(v2):]

    return sum


#for testing 
if __name__=='__main__':
    a = [1, 2]
    b = [1, 2, 3, 4]
    print(a)
    print(b)
    print(addVectors(a,b))

Spandau

Answer 16

    first = [1,2,3,4,5]
    second = [6,7,8,9,10]
    #one way
    third = [x + y for x, y in zip(first, second)]
    print("third" , third) 
    #otherway
    fourth = []
    for i,j in zip(first,second):
        global fourth
        fourth.append(i + j)
    print("fourth" , fourth )
#third [7, 9, 11, 13, 15]
#fourth [7, 9, 11, 13, 15]

Pepe

Answer 17

Here is another way to do it.It is working fine for me .

N=int(input())
num1 = list(map(int, input().split()))
num2 = list(map(int, input().split()))
sum=[]

for i in range(0,N):
  sum.append(num1[i]+num2[i])

for element in sum:
  print(element, end=" ")

print("")

Jarvis

Answer 18

j = min(len(l1), len(l2))
l3 = [l1[i]+l2[i] for i in range(j)]

Bitchy

While this code snippet may be the solution, including an explanation really helps to improve the quality of your post. Remember that you are answering the question for readers in the future, and those people might not know the reasons for your code suggestion. – Gerund

Answer 19

If you consider your lists as numpy array, then you need to easily sum them:

import numpy as np

third = np.array(first) + np.array(second)

print third

[7, 9, 11, 13, 15]

Elke

Answer 20

Perhaps the simplest approach:

first = [1,2,3,4,5]
second = [6,7,8,9,10]
three=[]

for i in range(0,5):
    three.append(first[i]+second[i])

print(three)

Daly

Answer 21

first = [1,2,3,4,5]
second = [6,7,8,9,10]
third=[]
for i,j in zip(first,second):
    t=i+j
    third.append(t)
print("Third List=",third)

output -- Third List= [7, 9, 11, 13, 15]

Taegu

Please describe this code and why you think it solves the problem – Lauter

Please don't post code-only answers. The main audience, future readers, will be grateful to see explained why this answers the question instead of having to infer it from the code. Also, since this is an old question, please explain how it complements all other answers. – Chloropicrin

Answer 22

In this question your code gets two lists of molecular weights.

Print:

The second lowest molecular weight for each list (2 numbers) The average molecular weight for the two lists together (single number) Make a new list that includes all the elements from both lists, except for the maximum and minimum values (all values from the two list except 2 values). Then sort this list and print it.

Note: the number of elements in each list is not fixed.

Variables names: mws1, mws2 You may define additional variables in your code if needed. See example of the output.

An example for the output (mws1 = [71, 88, 90], mws2 = [77, 5, 24, 65]): Second lowest of first list: 88 Second lowest of second list: 24 Average of merged lists: 60.0 Sorted values from both lists, excluding extreme values: [24, 65, 71, 77, 88]

Another example for the output (num = mws1 = [26, 34, 14, 66, 78], mws2 = [98, 15, 88]): Second lowest of first list: 26 Second lowest of second list: 88 Average of merged lists: 52.375 Sorted values from both lists, excluding extreme values: [15, 26, 34, 66, 78, 88]

Another example for the output (mws1 = [10, 53, 70, 1], mws2 = [22, 27]): Second lowest of first list: 10 Second lowest of second list: 27 Average of merged lists: 30.5 Sorted values from both lists, excluding extreme values: [10, 22, 27, 53]

Sldney

Answer 23

You can use this method but it will work only if both the list are of the same size:

first = [1, 2, 3, 4, 5]
second = [6, 7, 8, 9, 10]
third = []

a = len(first)
b = int(0)
while True:
    x = first[b]
    y = second[b]
    ans = x + y
    third.append(ans)
    b = b + 1
    if b == a:
        break

print third

Petronius