Count duplicate lists inside a list

lis = [ [12,34,56],[45,78,334],[56,90,78],[12,34,56] ]

I want the result to be 2 since number of duplicate lists are 2 in total. How do I do that?

I have done something like this

count=0
for i in range(0, len(lis)-1):
    for j in range(i+1, len(lis)):
        if lis[i] == lis[j]:
            count+=1

But the count value is 1 as it returns matched lists. How do I get the total number of duplicate lists?

Answer

Solution

You can use collections.Counter if your sub-lists only contain numbers and therefore are hashable:

>>> from collections import Counter
>>> lis = [[12, 34, 56], [45, 78, 334], [56, 90, 78], [12, 34, 56]]
>>> sum(y for y in Counter(tuple(x) for x in lis).values() if y > 1)
2
>>> lis = [[12, 34, 56], [45, 78, 334], [56, 90, 78], [12, 34, 56], [56, 90, 78], [12, 34, 56]]
>>> sum(y for y in Counter(tuple(x) for x in lis).values() if y > 1)
5

In Steps

Convert your sub-list into tuples:

tuple(x) for x in lis

Count them:

>>> Counter(tuple(x) for x in lis)
Counter({(12, 34, 56): 3, (45, 78, 334): 1, (56, 90, 78): 2})

take only the values:

>>> Counter(tuple(x) for x in lis).values()
dict_values([3, 1, 2])

Finally, sum only the ones that have a count greater than 1:

> sum(y for y in Counter(tuple(x) for x in lis).values() if y > 1)
5

Make it Re-usable

Put it into a function, add a docstring, and a doc test:

"""Count duplicates of sub-lists.
"""


from collections import Counter


def count_duplicates(lis):
    """Count duplicates of sub-lists.

    Assumption: Sub-list contain only hashable elements.
    Result: If a sub-list appreas twice the result is 2.
    If a sub-list aprears three time and a other twice the result is 5.

    >>> count_duplicates([[12, 34, 56], [45, 78, 334], [56, 90, 78],
    ...                   [12, 34, 56]])
    2
    >>> count_duplicates([[12, 34, 56], [45, 78, 334], [56, 90, 78],
    ...                   [12, 34, 56], [56, 90, 78], [12, 34, 56]])
    ...
    5
    """
    # Make it a bit more verbose than necessary for readability and
    # educational purposes.
    tuples = (tuple(elem) for elem in lis)
    counts = Counter(tuples).values()
    return sum(elem for elem in counts if elem > 1)


if __name__ == '__main__':

    import doctest

    doctest.testmod(verbose=True)

Run the test:

python count_dupes.py 
Trying:
    count_duplicates([[12, 34, 56], [45, 78, 334], [56, 90, 78],
                      [12, 34, 56]])
Expecting:
    2
ok
Trying:
    count_duplicates([[12, 34, 56], [45, 78, 334], [56, 90, 78],
                      [12, 34, 56], [56, 90, 78], [12, 34, 56]])
Expecting:
    5
ok
1 items had no tests:
    __main__
1 items passed all tests:
   2 tests in __main__.count_duplicates
2 tests in 2 items.
2 passed and 0 failed.
Test passed.