How to sort a set in python? [duplicate]

Question

I am trying to sort a set in python, by adding elements to it. I tried using the sorted() method, but it is converting my set into a list. I need to sort my elements and print it as a set itself. How can I do that?

I tried using the sorted() method and obtained a list. Tried to put the list into a set. But, didn't receive the required result

a={}
a=set()
a.add(1)
a.add(-1)
a.add(0)
a.add(4)
a.add(-3)
print(a)

Expected result: {-3, -1, 0, 1, 4} Actual result: {0, 1, 4, -3, -1}

sets don't have order, if you want an ordered collection, a list could be a good way to go — Henry
– Henry, Commented Mar 28, 2019 at 2:10
set's are unordered. They don't have an order. You could use OrderedDict or just dict in Python 3.7+ if you like. — iz_
– iz_, Commented Mar 28, 2019 at 2:10
@Tomothy32 Just noting here that the dict in Python3.6 and above only preserves insertion order in the CPython implementation. — mttpgn
– mttpgn, Commented Mar 28, 2019 at 2:17
@mttpgn In 3.6, it's a CPython implementation detail. In 3.7+, it's part of the standard. — iz_
– iz_, Commented Mar 28, 2019 at 2:20

Ender Look · Accepted Answer · 2019-03-28 02:28:49Z

20

You can't

Just read python documentation:

Python also includes a data type for sets. A set is an unordered collection with no duplicate elements. Basic uses include membership testing and eliminating duplicate entries. Set objects also support mathematical operations like union, intersection, difference, and symmetric difference.

^{Emphasis mine.}

That means you will never be able to sort items inside a set*.

At least, with "normal" sets, you can't. You should better look for third-party libraries (both, default dictionaries and collections.OrderedDict), because I don't think that python has inbuilt ordered sets, at least no in collections.

*Well, not never, I have understood that early versions of python hadn't ordered dictionaries, and now they have. But at least in the current version (python 3.7), there aren't ordered sets.

Anyway, dictionaries work in a similar manner to sets. In a dictionary, you can't have two times the same key, like in a set you can't have two times the same item. So if you work with the keys of a dictionary and just ignore the values of those keys, a collections.OrderedDict may work for you. Even base dictionaries (those you make with dict()) will work since Python 3.7.

>>> a = dict()
>>> a[1] = None
>>> a[-1] = None
>>> a[0] = None
>>> a[4] = None
>>> a[-3] = None
>>> print(a.keys())
dict_keys([1, -1, 0, 4, -3])

edited Mar 28, 2019 at 2:28

answered Mar 28, 2019 at 2:19

Ender Look

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6 Comments

iz_ Over a year ago

There is collections.OrderedDict.

Ender Look Over a year ago

@Tomothy32, as you said collections.OrderedDict, not collections.OrderedSet.

iz_ Over a year ago

However, dictionaries can be used as sets. The implementation is very similar.

Ender Look Over a year ago

@Tomothy32, yes, but OP asked about ordered sets, not something similar to ordered sets. That is why I said it doesn't exist ordered sets.

iz_ Over a year ago

That's fair. BTW, dictionaries in 3.7 are ordered across all implementations.

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