CODEWITHSUNDEEP
pythonlistlist-comprehension
John Smith
John Smith

Reputation: 283

List Comprehension Based On 2 Lists

Given A and B are lists of integers, how would you add the individual elements and create a new list using their sums? What is the issue with this code and how would you do this? The errors are respectively:

ValueError: too many values to unpack

and

NameError: name 'b' is not defined

C = [a+b for (a,b) in (A,B)]

C = [a+b for a in A, b in B]

Upvotes: 0

Views: 256

Answers (2)

dawg
dawg

Reputation: 104102

You can use map, sum, and zip:

>>> A=[1,2,3]
>>> B=[10,20,30]
>>> map(sum, zip(A, B))
[11, 22, 33]

If your lists are of different lengths, you can use izip_longest with a value of 0 for a fillvalue:

>>> from itertools import izip_longest
>>> A=[1,2,3]
>>> B=[10,20,30,40]
>>> map(sum, izip_longest(A, B, fillvalue=0))
[11, 22, 33, 40]
>>> map(sum, izip_longest(A, B, [100], fillvalue=0))
[111, 22, 33, 40]

Upvotes: 0

Óscar López
Óscar López

Reputation: 236150

Try this, assuming that both lists are of equal size:

C = [a+b for (a,b) in zip(A,B)]

The trick here is using the zip() built-in function for joining both lists pair-wise.

Upvotes: 6

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