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pythonlistsetcomparisonset-intersection

Compare two nested lists and return common elements

I have two list of lists as follows.

list1 = [["cup", "mug"], ["happy", "joy", "smile"], ["trees", "bushes"]]
list2 = [["cat", "dog"], ["trees", "bushes"], ["cup", "mug"]]

Now, I want to return common elements in the two lists.

Common elements = [["trees", "bushes"], ["cup", "mug"]]

I tried the following code:

print(list(set(list1).intersection(list2)))

However, it does not work. Any suggestions?

Solution

  • You're probably looking for:

    In [773]: [list(x) for x in set(map(tuple, list1)).intersection(map(tuple, list2))]
Out[773]: [['trees', 'bushes'], ['cup', 'mug']]

    You could also use the & operator (the intersection operator for sets):

    In [778]: [list(x) for x in set(map(tuple, list1)) & set(map(tuple, list2))]
Out[778]: [['trees', 'bushes'], ['cup', 'mug']]

    The reason your code does not work is because you're not converting each individual list element into something that a set can hash. For example, tuples are hashable but lists are not, and so your method gives:

    In [774]: set(list1)
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-774-a09f7eaac729> in <module>()
----> 1 set(list1)

TypeError: unhashable type: 'list'

    However, converting each element to a tuple allows them to be hashed:

    In [775]: set(map(tuple, list1))
Out[775]: {('cup', 'mug'), ('happy', 'joy', 'smile'), ('trees', 'bushes')}

    The reason for this is that tuples are immutable containers.