Access StaticArray fields in a StructArray

General Usage
,
1

StaticArrays defines properties x, y, z on SVector{3}:

using StaticArrays

v = SA[1,2,3]
v.x # returns 1

However these accessors don’t work on StructArrays:

using StructArrays

A = StructArray([v, 2v])
A.x # gives ERROR: type Tuple has no field x

I thought this was because propertynames(v) returns (:data,) so I defined

Base.propertynames(x::SVector{3}) = (:x, :y, :z)

but I still get the same error with A.x. How can I get this to work, and can we get this to work by default?

2

It requires special handling, by default StructArrays accesses fields and not properties. See my PR to add this feature – support named properties in StructArray{SVector} by aplavin · Pull Request #277 · JuliaArrays/StructArrays.jl · GitHub. Created last June, and unfortunately got no response from StructArrays devs at all.

1 Like
3

If you are willing to define methods on types you do not own, you could just define Base.getproperty for Vector{<: SVector}.

julia> function Base.getproperty(v::Vector{<: SVector{3}}, s::Symbol)
           hasproperty(first(v), s) ?
               getproperty.(v, s) :
               getproperty(v, s)
       end

julia> A = [v, 2v]
2-element Vector{SVector{3, Int64}}:
 [1, 2, 3]
 [2, 4, 6]

julia> A.x
2-element Vector{Int64}:
 1
 2

julia> A.y
2-element Vector{Int64}:
 2
 4

julia> A.z
2-element Vector{Int64}:
 3
 6
4

Thanks but I’m trying to understand how to make it work with StructArrays…

@aplavin I wonder why it doesn’t use properties… But even with fields, the following should work no?

Base.fieldnames(::Type{<:SVector{3}}) = (:x, :y, :z)

v = SA[1,2,3]
fieldnames(typeof(v)  # returns (:x, :y, :z) as expected

but I still get the same error.

It would be great to have your PR merged but I wonder if there’s not a larger fix to make here. It seems to me that this should “just work” using properties (or maybe fields) for any type that is unwrapped by StructArray.

5

A major point of StructArrays is efficiency – there, accessing a column like arr.x is free. This is difficult to achieve with arr::Vector.
But should be possible with stuff like view and reinterpret, would be a nice excercise btw!

6

It’s impossible (?) to do generically. How would a StructArray look like, if its elements have computed properties? Even for StructArray{SVector} – there are both data and x, y, z properties that refer to the same data.

Currently, StructArray stores each field of its content separately: all a fields go to one array, all b fields go to another, etc. With fields, it’s unambiguous.

Whew, I can imagine this definition breaking some fundamental assumptions in Julia… (:
Changing fieldnames this way doesn’t change the fact that the only actual field of an SVector is data.

1 Like
7

That was just a test to try and understand how StructArrays works, I’m not saying that StaticArrays should overload fieldnames :). But I do wonder why the overload makes no difference… maybe because static arrays are special cased?

I see that now. We don’t want the redundancy of storing the properties in addition to the fields, and we cannot in general reconstruct the fields from the properties.