Python get intersection of elements from list of lists

Question

I have a list of lists where each list is a string of list of int, as shown below

a = ['[11, -12, -14, 13]',
     '[8, 5, -14, 15, 13]',
     '[13, 24, -14]']

I need to find a list of common integers (intersection of lists), in this case output should be [13, -14], since they are common in all.

Is there a simple way to do this since the individual lists are str type, where I can avoid a first iteration to eval and then find set?

Asked By: S_S

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Source

Answer 1

Not the prettiest way of solving it but it works! This will handle any number of lists in A.

a = ['[11, -12, -14, 13]',
     '[8, 5, -14, 15, 13]',
     '[13, 24, -14]']

b = []

## this block converts the strings into integers, after this b will be a, but integer lists
for ls in a:
    truncated_str = ls.replace("[", "").replace("]", "")
    my_list_of_strs = truncated_str.split(",")
    my_list_of_ints = []
    for num in my_list_of_strs:
        my_list_of_ints.append(int(num))
    
    b.append(my_list_of_ints)
    
## this functions finds commonalites
def in_all_lists(lists, value_to_check):
    
    for my_list in lists:
        if value_to_check not in my_list:
            return False
    return True

common_numbers = []


# checks to see if values are common
for lst in b:

    for value in lst:
        if value not in common_numbers and in_all_lists(b, value):
            common_numbers.append(value)
            
print(common_numbers)  # prints [-14, 13]

Answered By: Mitchnoff

Answer 2

map() the ast.literal_eval() function over list b to safely evaluate your list of strings into a iterator of lists.
Then map() the set() constructor to cast your lists to sets.
Finally, use functools.reduce() with set.intersection() to find the intersection of your sets.

from functools import reduce
from ast import literal_eval

answer = reduce(set.intersection,    # 3
             map(set,                # 2
             map(literal_eval, a)))  # 1

Answered By: Steven Rumbalski

Answer 3

Via the json library:

import json

l = [set(json.loads(x)) for x in a]

list(l.pop().intersection(*l))
> [-14, 13]

Explanation

json.loads() converts the string representation of a list to an actual list. Subsequently, we call intersection() on the list of sets. In particular, it is called first on the last element of that list via pop(). Then, all sets are passed as arguments into the intersection() via unpacking with *l. (The fact that the last set technically entered twice won’t matter for the output. It just streamlined the code).

Answered By: 7shoe

Python get intersection of elements from list of lists

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