# Declaring an empty set

**URL:** <https://discourse.julialang.org/t/declaring-an-empty-set/89419>\
**Category:** New to Julia\
**Created:** [October 28, 2022, 2:57am UTC](https://discourse.julialang.org/t/declaring-an-empty-set/89419 "2022-10-28T02:57:03Z")\
**Posts on this page:** 8\
**Page:** 1

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**Author:** ![Muhammad\_Nadeem](https://avatars.discourse-cdn.com/v4/letter/m/bc79bd/32.png) [@Muhammad\_Nadeem](https://discourse.julialang.org/u/Muhammad_Nadeem)\
**Post date:** [October 28, 2022, 2:57am UTC](https://discourse.julialang.org/t/declaring-an-empty-set/89419/1 "2022-10-28T02:57:03Z")

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Hello all,

I need to declare and empty set and then I have to append some of the other sets to it as follows:

```julia
main = []
K = Set()
        K[1] = union(A,C)
        K[2] = union(B,A,C)
        K[3] = union(A,B)
        K[4] = B
        push!(main,K)

```

Where A, B and C are also sets. it is giving me the following error `MethodError MethodError: no method matching setindex!(::Set{Int64}, ::Vector{Int64}, ::Int64)`

Any idea whats is going on? Thank you 🙂

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**Author:** ![hexaeder](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/hexaeder/32/24403_2.png) [@hexaeder](https://discourse.julialang.org/u/hexaeder)\
**Post date:** [October 28, 2022, 4:43am UTC](https://discourse.julialang.org/t/declaring-an-empty-set/89419/2 "2022-10-28T04:43:45Z")

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Should be either `push!(K, union(…))` oder ~~`append!(K, union(…))`~~`union!(K, union(...))` depending on whether you want `K` to be a set of sets or set of values.  
Setindex (`K[i] =…`) is not defined for sets since a set does not have this kind of numbered slots: it‘s an unordered collection.

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<div class="post-metadata">

**Author:** ![Muhammad\_Nadeem](https://avatars.discourse-cdn.com/v4/letter/m/bc79bd/32.png) [@Muhammad\_Nadeem](https://discourse.julialang.org/u/Muhammad_Nadeem)\
**Post date:** [October 28, 2022, 4:59am UTC](https://discourse.julialang.org/t/declaring-an-empty-set/89419/3 "2022-10-28T04:59:04Z")

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So i should write it like this. `push!(K, union(A,C),union(B,A,C),union(A,B))` ?

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<div class="post-metadata">

**Author:** ![hexaeder](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/hexaeder/32/24403_2.png) [@hexaeder](https://discourse.julialang.org/u/hexaeder)\
**Post date:** [October 28, 2022, 5:07am UTC](https://discourse.julialang.org/t/declaring-an-empty-set/89419/4 "2022-10-28T05:07:51Z")

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No you have to push one at a time. Push only takes one additional argument (at least I think so). However you could also push just `union(A,B,C)` because it shoud already contain all the elements. Or even `K = union(A,B,C)`.  
Its a bit hard to say without knowing what you want to achieve exactly.

Anyway I‘d really recommend playing around with that interactively. Either using the Julia repl directly or using rowwise execution in VSCode. That will show you the output at every single step and you will pick up the patterns really quickly! I no time you’ll understand what those errors mean and what exactly triggers them.

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<div class="post-metadata">

**Author:** ![Muhammad\_Nadeem](https://avatars.discourse-cdn.com/v4/letter/m/bc79bd/32.png) [@Muhammad\_Nadeem](https://discourse.julialang.org/u/Muhammad_Nadeem)\
**Post date:** [October 28, 2022, 5:18am UTC](https://discourse.julialang.org/t/declaring-an-empty-set/89419/5 "2022-10-28T05:18:24Z")

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Thanks, but `push!(K,union(A,B))` still doesnt work, it give me this error `MethodError MethodError: Cannot `convert` an object of type Vector{Int64} to an object of type Int64`.  
I actually just want to know how can i declare an empty set and then push other sets in it? Can you give me any toy example?

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<div class="post-metadata">

**Author:** ![hexaeder](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/hexaeder/32/24403_2.png) [@hexaeder](https://discourse.julialang.org/u/hexaeder)\
**Post date:** [October 28, 2022, 5:31am UTC](https://discourse.julialang.org/t/declaring-an-empty-set/89419/6 "2022-10-28T05:31:48Z")

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Sorry there was an error in my previous post. If you declare a set `A = Set{Int}()` it can only take integer elements. Therefore, you cannot `push!` something other than an `Int`. For that you can use `union!` which is the mutation version of `union` and changes the fist argument. In contrast to `push!` this will not try to put the Set into the outer set, but instead add every single element.

```julia
julia> A = Set{Int}();
julia> union!(A, Set([1,2,3]));
juila> A
Set{Int64} with 3 elements:
  2
  3
  1

```

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<div class="post-metadata">

**Author:** ![Rudi79](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/rudi79/32/3884_2.png) [@Rudi79](https://discourse.julialang.org/u/Rudi79)\
**Post date:** [October 28, 2022, 9:16am UTC](https://discourse.julialang.org/t/declaring-an-empty-set/89419/7 "2022-10-28T09:16:00Z")

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Maybe there is a misconception what a set is. Consider

```julia
A = Set([1,2]) 
B = Set([3,4])
C = Set([4,5])
D = union(A,B,C)

```

Note that 4 appears only once in D. Hence you don’t need more than one union to get your desired result.  
At least as far as I understood what you want to achieve by your description.  
If I look at your code it seems that you try to construct a vector of sets. This would look like

```julia
main = []
push!(main, union(A,C))
push!(main, union(A,B,C)) # note that sets do not care about ordering
push!(main, union(A,B))
push!(main, B)

```

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<div class="post-metadata">

**Author:** ![gustaphe](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/gustaphe/32/18174_2.png) [@gustaphe](https://discourse.julialang.org/u/gustaphe)\
**Post date:** [October 28, 2022, 11:35am UTC](https://discourse.julialang.org/t/declaring-an-empty-set/89419/8 "2022-10-28T11:35:23Z")

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Or maybe they are looking for a 1 element `Vector{Set{Set{Int}}}`, in which case they should just replace the indexed assignments by `push!`