How to make a set of lists

Question

I have a list of lists like this:

i = [[1, 2, 3], [2, 4, 5], [1, 2, 3], [2, 4, 5]]

I would like to get a list containing "unique" lists (based on their elements) like:

o = [[1, 2, 3], [2, 4, 5]]

I cannot use set() as there are non-hashable elements in the list. Instead, I am doing this:

o = []
for e in i:
  if e not in o:
    o.append(e)

Is there an easier way to do this?

An unrelated question, but why is this possible in other languages like Java? — gchandra, Mar 30 '21 at 06:47
I guess it is allowed because every object in Java implements its hashcode and equals method (via Java base Object class or overridden methods) and thus can be a key in a map. But even in Java, having mutable structures like Lists or Sets as keys would be a bad idea. See https://stackoverflow.com/a/9973694/5065946 — powersource97, Aug 04 '21 at 01:50

score 111 · Accepted Answer · answered Nov 06 '14 at 15:37

111

You can create a set of tuples, a set of lists will not be possible because of non hashable elements as you mentioned.

>>> l = [[1, 2, 3], [2, 4, 5], [1, 2, 3], [2, 4, 5]]
>>> set(tuple(i) for i in l)
{(1, 2, 3), (2, 4, 5)}

answered Nov 06 '14 at 15:37

Anshul Goyal

22

or `{tuple(i) for i in l}` – wim Nov 06 '14 at 15:38
4

@wim +1, though I find the `set` more intuitive; most of the times I see a `{`, I'm thinking a dictionary :) – Anshul Goyal Nov 06 '14 at 15:39
Just to clarify: set of tuples of unhashable types, like `set(([], {}))` won't work either. – bereal Nov 06 '14 at 15:39
5

or `set(map(tuple, l))` – warvariuc Nov 06 '14 at 15:40
@wim what is that syntax? Is it a set comprehension? – Phani Nov 06 '14 at 15:47
1

@wim first time I see this syntax, where does it come from, it returns a set, but how come? – DevLounge Nov 06 '14 at 15:58
2

@Phani correct, that's a set comprehension. a little bit more efficient than the solution in the answer, which has overhead of a function call and a generator expression. – wim Nov 06 '14 at 17:54

score 7 · Answer 2 · answered Nov 06 '14 at 15:49

7

i = [[1, 2, 3], [2, 4, 5], [1, 2, 3], [2, 4, 5]]

print([ele for ind, ele in enumerate(i) if ele not in i[:ind]])
[[1, 2, 3], [2, 4, 5]]

If you consider [2, 4, 5] to be equal to [2, 5, 4] then you will need to do further checks

answered Nov 06 '14 at 15:49

sets are almost always the way to go but sometimes things cannot be hashed – Padraic Cunningham Nov 06 '14 at 15:56
yeah, we can we use this approach, when we have a list of list of lists in which case we can't use tuples. – Phani Nov 06 '14 at 16:07
@Apero, I don't think it should matter – Padraic Cunningham Nov 06 '14 at 16:09

score 6 · Answer 3 · answered Nov 06 '14 at 15:47

6

You can convert each element to a tuple and then insert it in a set.

Here's some code with your example:

tmp = set()
a = [[1, 2, 3], [2, 4, 5], [1, 2, 3], [2, 4, 5]]
for i in a:
    tmp.add(tuple(i))

tmp will be like this:

{(1, 2, 3), (2, 4, 5)}

answered Nov 06 '14 at 15:47

Alberto Coletta

score 1 · Answer 4 · edited May 18 '19 at 15:30

1

Here's another way to do it:

I = [[1, 2, 3], [2, 4, 5], [1, 2, 3], [2, 4, 5]]
mySet = set()

for j in range(len(I)):
  mySet = mySet | set([tuple(I[j])])

print(mySet)

edited May 18 '19 at 15:30

Malekai

answered May 18 '19 at 12:09

sohrab haghayegh

4 Answers4