Can you sort a Set?

In Python, I can do:

a = set([2,1,5,3,2])
b = sorted(a)
print(b)  # Returns a sorted List

In Julia, Is it possible to sort a Set?

Edit: There is no particular reason why I require a Set. I was reading the documentation and attempted to sort a simple Set, and was puzzled by the error returned.

No. You can however convert it to a vector and sort that:

julia> a = Set([2,1,5,3,2]); sort!(collect(a))
4-element Array{Int64,1}:
 1
 2
 3
 5
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Any reason why this wasn’t implemented?

The Set type is built on top of the Dict type which has no ordering so there is no way for the set to “order” the items in any particular way.

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sorted(a) isn’t actually sorting the set itself in python either–it’s creating a new list from the elements of the set and sorting it. It should be quite similar to doing sort(collect(a)) in Julia. You could even define a helper function in your own code:

sorted(x) = sort(collect(x))

Alternatively, if you want a set that maintains its own sorted order, you could use an OrderedSet from https://juliacollections.github.io/OrderedCollections.jl/latest/ordered_containers.html

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I think the larger question is why you would use a Set if the objects have an order relation.

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This package might also have something useful for you

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Because inserting into a sorted vector is not efficient?

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This assumes there will be a collection of values that needs to be sorted all the time and you also need to insert values between tests. The even larger question is which is the use case?

  • If only membership is needed, the Set would be the right structure.
  • If membership and ordering are important, but there are not many inserts, or the collection size is small, then a sorted Vector is probably the best.
  • If membership, ordering, and repeated/interleaved insertion is needed, then SortedSet is probably the best.
  • There is also the question if arbitrary removal is needed, if it is then Set/SortedSet are prefered over a sorted Vector.

Without knowing the exact use case it is impossible to prescribe a good solution, but Set then sort(collect(...)) does not seem a good choice in many cases, it shines only if: there can be no repetition of elements; there will be a lot of insertions/removals/‘membership tests’; and after all of these, the remaining values need to be returned as a sorted sequence.

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Depending on the use case a PriorityQueue may be a good option.