CODEWITHSUNDEEP
pythonlistsum
George Burrows
George Burrows

Reputation: 3501

Finding the sum of matching components in two lists

I have two lists:

A = [1, 2, 3, 4, 5]
B = [6, 7, 8, 9, 10]

And I need to be able to find the sum of the nth terms from both lists i.e. 1+6, 2+7, 3+8 etc

Could someone please tell me how to refer to items in both lists at the same time?

I read somewhere that I could do Sum = a[i] + b[i] but I'm not convinced on how that would work.

Upvotes: 4

Views: 7942

Answers (5)

Learner27
Learner27

Reputation: 405

Hi You can try this too:

>>>a=[1,2,3,4,5]
>>>b=[6,7,8,9,10]
>>>c=[]
>>>for i in range(0,5):
    c.append(a[i]+b[i])
>>> c
[7, 9, 11, 13, 15]

Upvotes: -1

rob mayoff
rob mayoff

Reputation: 385590

If you know the lists will be the same length, you could do this:

AB = [A[i] + B[i] for i in range(len(A))]

In Python 2, you might want to use xrange instead of range if your lists are quite large. I think that's an explicit, simple, readable, obvious way to do it, but some might differ.

If the lists might be different lengths, you have to decide how you want to handle the extra elements. Let's say you want to ignore the extra elements of whichever list is longer. Here are three ways to do it:

AB = [A[i] + B[i] for i in range(min(len(A), len(B)))]

AB = map(sum, zip(A, B))

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

The downside of using zip is that it will allocate a list of tuples, which can be a lot of memory if your lists are already large. Using for i in xrange with subscripting won't allocate all that memory, or you can use itertools.izip:

import itertools
AB = map(sum, itertools.izip(A, B))

If you instead want to pretend the shorter list is padded with zeros, using itertools.izip_longest is the shortest answer:

import itertools
AB = map(sum, itertools.izip_longest(A, B, fillvalue=0))

or

import itertools
AB = [a + b for a, b in itertools.izip_longest(A, B, fillvalue=0)]

Upvotes: 0

Brian Weidenbaum
Brian Weidenbaum

Reputation: 477

Although Jazz's solution works for 2 lists, what if you have more than 2 lists? Here's a solution:

def apply_elementwise_function(elements_in_iterables, function):
    elementwise_function = lambda x, y: itertools.imap(function, itertools.izip(x, y))
    return reduce(elementwise_function, elements_in_iterables)

a = b = c = [1, 2, 3]
>>> list(apply_elementwise_function([a, b, c], sum))
[3, 6, 9]

Upvotes: 0

jazz
jazz

Reputation: 2379

>>> import operator
>>> map(operator.add, A, B)
[7, 9, 11, 13, 15]

just to demonstrate Pythons elegance :-)

Upvotes: 15

Andrew Jaffe
Andrew Jaffe

Reputation: 27077

Use a list comprehension and zip:

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

Are these questions homework? Or self-study?

Upvotes: 13

Related Questions