# Find intersection of two nested lists?

You don't need to define intersection. It's already a first-class part of set.

`>>> b1 = [1,2,3,4,5,9,11,15]>>> b2 = [4,5,6,7,8]>>> set(b1).intersection(b2)set([4, 5])`

**If you want:**

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

**Then here is your solution for Python 2:**

`c3 = [filter(lambda x: x in c1, sublist) for sublist in c2]`

**In Python 3 filter returns an iterable instead of list, so you need to wrap filter calls with list():**

`c3 = [list(filter(lambda x: x in c1, sublist)) for sublist in c2]`

**Explanation:**

The filter part takes each sublist's item and checks to see if it is in the source list c1. The list comprehension is executed for each sublist in c2.

For people just looking to find the intersection of two lists, the Asker provided two methods:

`b1 = [1,2,3,4,5,9,11,15]b2 = [4,5,6,7,8]b3 = [val for val in b1 if val in b2]`

and

`def intersect(a, b): return list(set(a) & set(b))print intersect(b1, b2)`

But there is a hybrid method that is more efficient, because you only have to do one conversion between list/set, as opposed to three:

`b1 = [1,2,3,4,5]b2 = [3,4,5,6]s2 = set(b2)b3 = [val for val in b1 if val in s2]`

This will run in O(n), whereas his original method involving list comprehension will run in O(n^2)