Legend Legend - 1 month ago 29
Python Question

Python -Intersection of multiple lists?

I am playing with python and am able to get the intersection of two lists:

result = set(a).intersection(b)


Now if
d
is a list containing
a
and
b
and a third element
c
, is there an built-in function for finding the intersection of all the three lists inside
d
? So for instance,

d = [[1,2,3,4], [2,3,4], [3,4,5,6,7]]


then the result should be

[3,4]

Answer

for 2.4, you can just define an intersection function.

def intersect(*d):
    sets = iter(map(set, d))
    result = sets.next()
    for s in sets:
        result = result.intersection(s)
    return result

for newer versions of python:

the intersection method takes an arbitrary amount of arguments

result = set(d[0]).intersection(*d[:1])

alternatively, you can intersect the first set with itself to avoid slicing the list and making a copy:

result = set(d[0]).intersection(*d)

I'm not really sure which would be more efficient and have a feeling that it would depend on the size of the d[0] and the size of the list unless python has an inbuilt check for it like

if s1 is s2:
    return s1

in the intersection method.

>>> d = [[1,2,3,4], [2,3,4], [3,4,5,6,7]]
>>> set(d[0]).intersection(*d)
set([3, 4])
>>> set(d[0]).intersection(*d[1:])
set([3, 4])
>>> 
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