How Can I Efficiently Find Intersections of Nested Lists in Python?

Finding Intersections of Nested Lists

Retrieving intersections from nested lists poses a challenge that differs from the straightforward approach for flat lists. This article explores a solution to efficiently determine the intersections of nested lists.

As demonstrated in the problem content, finding intersections of flat lists can be easily achieved using set intersection:

b1 = [1,2,3,4,5,9,11,15]
b2 = [4,5,6,7,8]
print(set(b1) & set(b2))  # Output: {4, 5}

However, when dealing with nested lists like:

c1 = [1, 6, 7, 10, 13, 28, 32, 41, 58, 63]
c2 = [[13, 17, 18, 21, 32], [7, 11, 13, 14, 28], [1, 5, 6, 8, 15, 16]]

the straightforward approach fails to produce the desired result:

print(set(c1) & set(c2))  # Output: set([])

The intersection we aim for is:

c3 = [[13, 32], [7, 13, 28], [1, 6]]

Solution:

The solution lies in converting the nested lists to sets, performing set intersections, and then reconstructing the original nested list structure:

# Convert nested lists to sets 
set_c1 = set(c1) 
set_c2 = [set(sublist) for sublist in c2]

# Compute intersections
intersections = [set_c1.intersection(sublist) for sublist in set_c2]

# Reconstruct nested list structure 
result = [[item for item in intersection] for intersection in intersections] 

# Print the result
print(result)  # Output: [[13, 32], [7, 13, 28], [1, 6]]

By leveraging set intersection and set comprehension, this solution efficiently retrieves the intersections of nested lists, preserving their structure.

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