# 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]
```

## Answers:

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]
```

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]
```

This extends itself to any number of lists:

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

In your case, `myListOfLists`

would be `[first, second]`

From docs

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

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
```

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])
```

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]
```

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])
```

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))]
```

Try the following code:

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

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]
```

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))
```

```
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]
```

```
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]
```

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("")
```

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

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)
```

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]
```

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]
```

one-liner solution

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

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]
```

```
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]
```