How do I add specific elements of a list of lists in Python?

Question

So I want to add the 3rd value of the elements in a list of lists if the first and second values are equal. And if not, I want the non-equal ones to be added to my sum list.

first=[[1,1,5],[2,3,7],[3,5,2],[4,4,6]]
second=[[1,1,3],[4,2,4],[2,3,2]]
sum=[]

for i in ((first)):
    for j in ((second)):
        if i[0]==j[0] and i[1]==j[1]:
            sum.append([i[0],j[1],i[2]+j[2]])
        

print(sum)

so this gives me [[1, 1, 8], [2, 3, 9]] but I want [3,5,2],[4,4,6],[4,2,4] in my sum list too. How do I do this in python?

Alex Hall · Accepted Answer · 2018-05-06 22:51:36Z

1

One solution is to use collections.defaultdict from the standard library.

The idea is to set your dictionary keys as a tuple of your first 2 elements and increment by the third. Then aggregate keys and values via a dictionary comprehension.

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

from collections import defaultdict
from itertools import chain

d = defaultdict(int)

for i, j, k in chain(first, second):
    d[(i, j)] += k

res = [[*k, v] for k, v in d.items()]

print(res)

[[1, 1, 8], [2, 3, 9], [3, 5, 2], [4, 4, 6], [4, 2, 4]]

Here is the equivalent solution without using any libraries, utilising dict.setdefault:

d = {}
for i, j, k in first+second:
    d.setdefault((i, j), 0)
    d[(i, j)] += k

res = [[*k, v] for k, v in d.items()]

edited May 6, 2018 at 22:51

Alex Hall

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answered May 6, 2018 at 22:37

jpp

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4 Comments

Alex Hall Over a year ago

Since this is Python 3, you could do [*k, v]. But maybe that's not appropriate at this level.

Birkan Ak Over a year ago

Well, this works perfectly well! But I don't want to use any libraries since we haven't seen those in my class yet. But thanks anyways!

Alex Hall Over a year ago

chain.from_iterable((first, second)) can just be chain(first, second) or first + second.

jpp Over a year ago

@AlexHall, Thanks for your suggestions, I have updated. Using chain here to avoid building a new list.

rodriper · Accepted Answer · 2018-05-06 23:36:00Z

0

I tried to do it without dictionaries or libraries:

first = [[1,1,5],[2,3,7],[3,5,2],[4,4,6]]
second = [[1,1,3],[4,2,4],[2,3,2]]
checked = []
sum = []

for n_i, i in enumerate(first):
    for n_j, j in enumerate(second):
        if i[0]==j[0] and i[1]==j[1]:
            sum.append([i[0],j[1],i[2]+j[2]])
            checked.append([n_i,n_j]) # Save the used index

# Delete used index
[first.pop(i[0]) and second.pop(i[1]) for i in reversed(checked)]

# Append non-used index
[sum.append(first.pop(0)) for x in range(0,len(first))]
[sum.append(second.pop(0)) for x in range(0,len(second))]

print(sum)

Returns:

[[1, 1, 8], [2, 3, 9], [3, 5, 2], [4, 4, 6], [4, 2, 4]]

edited May 6, 2018 at 23:36

answered May 6, 2018 at 23:29

rodriper

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1 Comment

Birkan Ak Over a year ago

This is what I was looking for in the first place, because it is easier to do any mathematical operation like subtraction just by changing i[2]+j[2] with i[2]-j[2]

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