Delete unique elements from a list

Question

I faced some problem with solving the next problem:

We have a list of elements (integers), and we should return a list consisting of only the non-unique elements in

Answer 1

Would it not be easier to generate a new list?

def unique_list(lst):
    new_list = []
    for value in lst:
        if value not in new_list:
            new_list.append(value)
    return new_list

lst = [1,2,3,1,4,5,1,6,2,3,7,8,9]
print(unique_list(lst))

Prints [1,2,3,4,5,6,7,8,9]

Answer 2

Just I used list Comprehensions.

def checkio(data):
    a=[i for i in data if data.count(i)>1]
    return a
print checkio([1,1,2,2,1,1,1,3,4,5,6,7,8]) 

Answer 3

Your function can be made to work by iterating over the list in reverse:

def checkio(data):
    for index in range(len(data) - 1, -1, -1):
        if data.count(data[index]) == 1:
            del data[index]
    return data

print(checkio([3, 3, 5, 8, 1, 4, 5, 2, 4, 4, 3, 0]))
[3, 3, 5, 4, 5, 4, 4, 3]
print(checkio([1, 2, 3, 4]))
[]

This works, because it only deletes numbers in the section of the list that has already been iterated over.