Add two lists element-wise in Python

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Borislav Hadzhiev

Last updated: Jun 29, 2022

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Add two lists element-wise in Python #

To add two lists element-wise:

  1. Use the zip function to get an iterable of tuples with the corresponding items.
  2. Use a list comprehension to iterate over the iterable.
  3. On each iteration, pass the tuple to the sum() function.
main.py
list_1 = [1, 2, 3] list_2 = [4, 5, 6] # 👇️ [(1, 4), (2, 5), (3, 6)] print(list(zip(list_1, list_2))) result = [sum(tup) for tup in zip(list_1, list_2)] print(result) # 👉️ [5, 7, 9]

The zip function iterates over several iterables in parallel and produces tuples with an item from each iterable.

main.py
list_1 = [1, 2, 3] list_2 = [4, 5, 6] # 👇️ [(1, 4), (2, 5), (3, 6)] print(list(zip(list_1, list_2)))

You can imagine that the zip() function iterates over the lists, taking 1 item from each.

The first tuple in the list consists of the elements in each list with an index of 0, the second tuple consists of the elements in each list that have an index of 1, etc.

The last step is to use a list comprehension to iterate over the zip object and sum each tuple.

main.py
list_1 = [1, 2, 3] list_2 = [4, 5, 6] # 👇️ [(1, 4), (2, 5), (3, 6)] print(list(zip(list_1, list_2))) result = [sum(tup) for tup in zip(list_1, list_2)] print(result) # 👉️ [5, 7, 9]
List comprehensions are used to perform some operation for every element, or select a subset of elements that meet a condition.

The sum function takes an iterable, sums its items from left to right and returns the total.

On each iteration, we pass the current tuple to the sum() function and get the total.

You can also use this approach to add more than two lists element-wise.

main.py
list_1 = [1, 2, 3] list_2 = [4, 5, 6] list_3 = [7, 8, 9] # 👇️ [(1, 4, 7), (2, 5, 8), (3, 6, 9)] print(list(zip(list_1, list_2, list_3))) result = [sum(tup) for tup in zip(list_1, list_2, list_3)] print(result) # 👉️ [12, 15, 18]

Alternatively, you can use the map() function.

To add two lists element-wise:

  1. Use the map() function to call the add() function with the two lists.
  2. Use the list() class to convert the map object to a list.
main.py
from operator import add list_1 = [1, 2, 3] list_2 = [4, 5, 6] result = list(map(add, list_1, list_2)) print(result) # 👉️ [5, 7, 9]

The map() function takes a function and an iterable as arguments and calls the function with each item of the iterable.

The add function from the operator module is the same as a + b.

You can imagine that map calls the add function with each item of the two iterables (e.g. items at index 0, then 1, etc).

The map function returns a map object, so we had to use the list() class to convert the result to a list.

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