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Add empty for AbstractArrays and their types #33482

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Add empty for AbstractArrays and their types #33482

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9 changes: 7 additions & 2 deletions base/abstractarray.jl
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Original file line number Diff line number Diff line change
Expand Up @@ -672,9 +672,10 @@ similar(::Type{T}, shape::Tuple{Union{Integer, OneTo}, Vararg{Union{Integer, One
similar(::Type{T}, dims::Dims) where {T<:AbstractArray} = T(undef, dims)

"""
empty(v::AbstractVector, [eltype])
empty(a::AbstractArray, [eltype])
empty(a::Type{AbstractArray}, [eltype])
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Suggested change
empty(a::Type{AbstractArray}, [eltype])
empty(a::Type{<:AbstractArray}, [eltype])


Create an empty vector similar to `v`, optionally changing the `eltype`.
Create an empty array similar to `a`, optionally changing the `eltype`.

# Examples

Expand All @@ -687,6 +688,10 @@ julia> empty([1.0, 2.0, 3.0], String)
```
"""
empty(a::AbstractVector{T}, ::Type{U}=T) where {T,U} = Vector{U}()
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change this method also then?

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I left that in for inlining purposes due to all the existing code depending on it. If that is not necessary, then I'll change it as well.

function empty(a::Type{<:AbstractArray{T,N}}, ::Type{U}=T) where {T,N,U}
convert(AbstractArray{U}, a(undef, Tuple(Iterators.repeated(0,N))))
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Unfortunately I'm not sure T(undef, dims) is a required part of the AbstractArray API. The operation of creating a new array from a type but with a different element type could potentially be handled by similar (see #20815).

Also this should be faster:

Suggested change
convert(AbstractArray{U}, a(undef, Tuple(Iterators.repeated(0,N))))
convert(AbstractArray{U}, a(undef, ntuple(_ -> 0, N)))

BTW, better use the short function syntax as for other definitions.

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Thanks for the ntuple correction! I actually found that function some days ago and was gonna modify the PR accordingly but you were faster. :)

About the T(undef, dims), yeah, sorry; that was stupid since similar has the dims argument. I completely forgot about it.
I also love your referenced PR; just today I ran into a case where it would have been useful.

BTW, better use the short function syntax as for other definitions.

Do you mean I should always use empty(x) = ...? I didn't use it as the functions were getting too wide (over 92 columns) and I thought the short syntax should be avoided when the function spans multiple lines.

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About the T(undef, dims), yeah, sorry; that was stupid since similar has the dims argument. I completely forgot about it.
I also love your referenced PR; just today I ran into a case where it would have been useful.

As I noted, the problem with similar is that it requires an instance, not a type.

Do you mean I should always use empty(x) = ...? I didn't use it as the functions were getting too wide (over 92 columns) and I thought the short syntax should be avoided when the function spans multiple lines.

Yes it's fine to break after = (see examples in this file).

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As I noted, the problem with similar is that it requires an instance, not a type.

That was the problem with it, I remember... So first we will have to keep the constructor by type, if I'm seeing this correctly? Until #20815 is merged.

Yes it's fine to break after = (see examples in this file).

I was thinking more in an aesthetic/readability sense. Only because it has been done in the past does not imply it has to be done again. But sure, I will change it!

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Actually I'm not even sure calling the constructor like that is guaranteed to work. Somebody else will have to confirm.

end
empty(a::AbstractArray{T}, ::Type{U}=T) where {T,U} = empty(typeof(a), U)

# like empty, but should return a mutable collection, a Vector by default
emptymutable(a::AbstractVector{T}, ::Type{U}=T) where {T,U} = Vector{U}()
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12 changes: 12 additions & 0 deletions test/abstractarray.jl
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Expand Up @@ -892,11 +892,23 @@ end
v = [1, 2, 3]
v2 = empty(v)
v3 = empty(v, Float64)
v4 = empty(typeof(v))
v5 = empty(typeof(v), Float64)
@test !isempty(v)
empty!(v)
@test isempty(v)
@test isempty(v2::Vector{Int})
@test isempty(v3::Vector{Float64})
@test isempty(v4::Vector{Int})
@test isempty(v5::Vector{Float64})

m = [1 2; 3 4]
# These call `empty(typeof(m))`, so we don't test that case.
m2 = empty(m)
m3 = empty(m, Float64)
@test !isempty(m)
@test isempty(m2::Matrix{Int})
@test isempty(m3::Matrix{Float64})
end

@testset "CartesianIndices" begin
Expand Down