Counting the number of unique sets in a list

Question

If you have a list of sets like this:

a_list = [{'a'}, {'a'}, {'a', 'b'}, {'a', 'b'}, {'a', 'c', 'b'}, {'a', 'c', 'b'}]

How could one get the number of unique sets in the list?

I have tried:

len(set(a_list ))

I am getting the error:

TypeError: unhashable type: 'set'

Desired output in this case is: 3 as there are three unique sets in the list.

Ajax1234 · Accepted Answer · 2019-04-15T14:56:44.480

1

You can use a tuple:

a_list = [{'a'}, {'a'}, {'a', 'b'}, {'a', 'b'}, {'a', 'c', 'b'}, {'a', 'c', 'b'}]
result = list(map(set, set(map(tuple, a_list))))
print(len(result))

Output:

[{'a', 'b'}, {'a'}, {'c', 'a', 'b'}]
3

A less functional approach, perhaps a bit more readable:

result = [set(c) for c in set([tuple(i) for i in a_list])]

edited Apr 15 '19 at 14:56

answered Apr 15 '19 at 14:52

Ajax1234

Wohooo! that was fast – DirtyBit Apr 15 '19 at 14:53
1

OP might also want to consider frozensets in case `{a, b}` should be considered the same as `{b, a}`.... eg: `list({frozenset(el) for el in a_list}) ` – Jon Clements Apr 15 '19 at 14:55
I agree with Jon here. This doesn't seem right to me - are `tuple` of `{'a', 'b'}` and `{'b','a'}` guaranteed to be the same? – Zev Chonoles Apr 15 '19 at 14:56
3

However, seems what the OP's likely after is `len({frozenset(el) for el in a_list})`... – Jon Clements Apr 15 '19 at 14:56

score 1 · Answer 2 · answered Apr 15 '19 at 14:55

1

How about Converting to a tuple:

a_list = [{'a'}, {'a'}, {'a', 'b'}, {'a', 'b'}, {'a', 'c', 'b'}, {'a', 'c', 'b'}]

print(len(set(map(tuple, a_list))))

OUTPUT:

answered Apr 15 '19 at 14:55

DirtyBit

2 Answers2