# Generically create an instance of \`AA \<: AbstractArray\` with contents 1,2,3

**URL:** <https://discourse.julialang.org/t/generically-create-an-instance-of-aa-abstractarray-with-contents-1-2-3/86570>\
**Category:** General Usage\
**Tags:** question, array\
**Created:** [August 30, 2022, 7:43pm UTC](https://discourse.julialang.org/t/generically-create-an-instance-of-aa-abstractarray-with-contents-1-2-3/86570 "2022-08-30T19:43:32Z")\
**Posts on this page:** 4\
**Page:** 1

<div class="post-metadata">

**Author:** ![tictaccat](https://avatars.discourse-cdn.com/v4/letter/t/9f8e36/32.png) [@tictaccat](https://discourse.julialang.org/u/tictaccat)\
**Post date:** [August 30, 2022, 7:43pm UTC](https://discourse.julialang.org/t/generically-create-an-instance-of-aa-abstractarray-with-contents-1-2-3/86570/1 "2022-08-30T19:43:32Z")

</div>

This is a fairly concrete question: the user passes in a type `AA <: AbstractArray` and I want to make a concrete instantiation of this type which contains some specified data, e.g. [1,2,3]. Here is the best I’ve come up with using the interface specified here: [Interfaces · The Julia Language](https://docs.julialang.org/en/v1/manual/interfaces/#man-interface-array)

```julia
arr = similar(AA, 3)
arr .= 1:3

```

However, this will not work if e.g. `arr` is mutable (whereas in theory this makes sense for immutable arrays too).

Some special cases of this problem of instantiating an array type with concrete data are covered by e.g. the `zeros` and `ones` functions (although they are only defined in `Base` for `Array` so I’m not sure if I can even assume their existence…), but is there a general way of solving this?

---

<div class="post-metadata">

**Author:** ![rdeits](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/rdeits/32/286_2.png) [@rdeits](https://discourse.julialang.org/u/rdeits)\
**Post date:** [August 30, 2022, 7:46pm UTC](https://discourse.julialang.org/t/generically-create-an-instance-of-aa-abstractarray-with-contents-1-2-3/86570/2 "2022-08-30T19:46:10Z")

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How about converting an existing array to the desired type? Perhaps:

```julia
using StaticArrays: SVector

convert(AA, SVector(1, 2, 3))

```

or just call the constructor:

```julia
AA(SVector(1, 2, 3))

```

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<div class="post-metadata">

**Author:** ![lmiq](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/lmiq/32/18314_2.png) [@lmiq](https://discourse.julialang.org/u/lmiq)\
**Post date:** [August 30, 2022, 8:06pm UTC](https://discourse.julialang.org/t/generically-create-an-instance-of-aa-abstractarray-with-contents-1-2-3/86570/3 "2022-08-30T20:06:19Z")

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Not sure how general this is, but one option is:

```julia
using Setfield
function newarr(A::T) where T
    arr = similar(A, 3)
    for i in 1:3
       @set! arr[i] = i
    end
    return T(arr)
end

```

It works for some very different types:

```julia
julia> using SparseArrays, StaticArrays

julia> A = sparsevec([1,5,10], rand(3))
10-element SparseVector{Float64, Int64} with 3 stored entries:
  [1] = 0.269524
  [5] = 0.715611
  [10] = 0.629861

julia> newarr(A)
3-element SparseVector{Float64, Int64} with 3 stored entries:
  [1] = 1.0
  [2] = 2.0
  [3] = 3.0

julia> A = rand(SVector{3,Float64})
3-element SVector{3, Float64} with indices SOneTo(3):
 0.7228423544347192
 0.771679776967447
 0.13509032343576854

julia> newarr(A)
3-element SVector{3, Float64} with indices SOneTo(3):
 1.0
 2.0
 3.0

```

(for the static arrays only if the input has length 3, but I’m not sure if there is any other interpretation for that in this case)

---

<div class="post-metadata">

**Author:** ![aplavin](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/aplavin/32/222056_2.png) [@aplavin](https://discourse.julialang.org/u/aplavin)\
**Post date:** [August 30, 2022, 8:45pm UTC](https://discourse.julialang.org/t/generically-create-an-instance-of-aa-abstractarray-with-contents-1-2-3/86570/4 "2022-08-30T20:45:05Z")

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> [@tictaccat](#):
>
> user passes in a type `AA <: AbstractArray` and I want to make a concrete instantiation of this type which contains some specified data

For many array types, this only makes limited sense. For example, if `AA` in your example is an OffsetArray, or a KeyedArray, then you don’t have any information on the offset or axis keys.

So, the question is what kind of array types do you have in mind? If they implement `similar(T, ...)`, then your original solution would work and always return a mutable array (not always of the specified type, eg for static arrays).