How can I get unique arrays in Python?

Question

I got a fairly large array of arrays of length 2 (List[List[int, int]]) How can I unique arrays of them? Preferably without using different libraries

I've seen several solutions that use numpy, but I'm unlikely to be able to use this in olympiads

# Example input:
nums = [[2, 9], [3, 6], [9, 2], [6, 3]]

for i in nums:
    # some code here

# Output:
# nums = [[2, 9], [3, 6]]

I tried doing this but I guess it's not a very fast solution

# Example input:
nums = [[2, 9], [3, 6], [9, 2], [6, 3]]

unique = []
for i in nums:
    if sorted(i) not in unique:
        unique.append(sorted(i))

# Output:
print(unique) # [[2, 9], [3, 6]]

It seems that by 'unique' you mean 'order independant'; if so, turn your lists into sets (then back to lists if you want) — Swifty
– Swifty, Commented Jan 28, 2023 at 13:29
Is the order of elements in the outer list important or can the result be reordered (makes use of sets easier)? — Michael Butscher
– Michael Butscher, Commented Jan 28, 2023 at 13:30

Swifty · Accepted Answer · 2023-01-28 13:35:39Z

3

To deal with sets not being hashable, you can create a set of frozensets this way:

unique = {frozenset(i) for i in nums}

Then you can use whichever means to turn the results into the objects you want; for example:

unique = [list(i) for i in unique]

answered Jan 28, 2023 at 13:35

Swifty

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Samwise · Accepted Answer · 2023-01-28 13:33:36Z

1

Turn each pair into a sorted tuple, put them all into a set, then turn that back into a list of lists.

>>> nums = [[2, 9], [3, 6], [9, 2], [6, 3]]
>>> {tuple(sorted(n)) for n in nums}
{(2, 9), (3, 6)}
>>> [list(t) for t in {tuple(sorted(n)) for n in nums}]
[[2, 9], [3, 6]]

The tuple is necessary because a set (which is created via the {} set comprehension expression) needs to contain hashable (immutable) objects.

answered Jan 28, 2023 at 13:33

Samwise

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Fractalism · Accepted Answer · 2023-01-29 10:16:09Z

0

This answer is wrong. The `set` constructor does not sort its elements.

As @Swifty mentioned, you can use set to solve this problem. Send each pair through the set constructor to ~~sort the pair~~, then convert it to tuple to make it hashable and use set again to remove duplicate tuples.

nums = [[2, 9], [3, 6], [9, 2], [6, 3]]

num_tuples = set(tuple(set(pair)) for pair in nums)

print(num_tuples)  # {(9, 2), (3, 6)}

Warning: As pointed out by @Samwise

This is a little dodgy because you're assuming that tuple(set(pair)) will deterministically create the same tuple ordering for any given set. This is probably true in practice (IIRC when you iterate over a set the items always come out in hash order, at least in CPython 3) but I'm not sure it's necessarily guaranteed by the Python spec. –

edited Jan 29, 2023 at 10:16

answered Jan 28, 2023 at 13:32

Fractalism

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3 Comments

Samwise Over a year ago

This is a little dodgy because you're assuming that tuple(set(pair)) will deterministically create the same tuple ordering for any given set. This is probably true in practice (IIRC when you iterate over a set the items always come out in hash order, at least in CPython 3) but I'm not sure it's necessarily guaranteed by the Python spec.

Fractalism Over a year ago

@Samwise Thanks for pointing it out. I'll add this as a warning to my answer.

Kelly Bundy Over a year ago

@Samwise Nah, what they're really assuming is that two equal sets become equal tuples. Or in their words: "Send each pair through the set constructor to sort the pair" (emphasis mine). No idea why they're not using sorted for sorting. And their assumption is wrong.

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