# Writing \`broadcast\_c\` for struct with custom indexing
**URL:** https://discourse.julialang.org/t/writing-broadcast-c-for-struct-with-custom-indexing/9071
**Category:** General Usage
**Created:** [February 14, 2018, 10:23pm UTC](https://discourse.julialang.org/t/writing-broadcast-c-for-struct-with-custom-indexing/9071 "2018-02-14T22:23:53Z")
**Posts on this page:** 8
**Page:** 1
### Author:  [@EP-Guy](https://discourse.julialang.org/u/EP-Guy)
#### Post date: [February 14, 2018, 10:23pm UTC](https://discourse.julialang.org/t/writing-broadcast-c-for-struct-with-custom-indexing/9071/1 "2018-02-14T22:23:53Z")
I have a custom type
```julia
struct State{T} <: AbstractArray{T,1}
fields::Dict{Symbol, UnitRange{Int64}}
arr::Array{T}
end
Base.size(S::State) = size(S.arr)
Base.IndexStyle(::Type{<:State}) = IndexLinear()
Base.getindex(S::State{T}, inds::Vararg{Int,1}) where {T} = S.arr[inds...]
Base.getindex(S::State{T}, ind::Symbol) where {T} = S.arr[S.fields[ind]]
Base.setindex!(S::State{T}, val, inds::Vararg{Int,1}) where {T} = (S.arr[inds...] = val)
```
that allows indexing with symbols, e.g.
```julia
julia> state0 = State(Dict([(:r, 1:3), (:v, 4:6)]), [10; 11; 12; 13; 14; 15])
6-element Constants.State{Int64}:
10
11
12
13
14
15
julia> state0[:r]
3-element Array{Int64,1}:
10
11
12
```
I plan on using this with DifferentialEquations.jl, but I’m currently having trouble getting broadcasting to work. For example, I cannot do
```julia
julia> dstate = deepcopy(state0)
6-element Constants.State{Int64}:
10
11
12
13
14
15
julia> dstate[:r] .= 5.0
ERROR: ArgumentError: invalid index: r
```
I found [Broadcast for custom type - #3 by fengyang.wang](https://discourse.julialang.org/t/broadcast-for-custom-type/3497/3) which suggested I do
```julia
Base.Broadcast._containertype(::Type{<:State}) = State
Base.Broadcast.promote_containertype(::Type{State}, _) = State
Base.Broadcast.promote_containertype(_, ::Type{State}) = State
Base.Broadcast.promote_containertype(::Type{State}, ::Type{State}) = State
```
but I’m having trouble implementing the last step, to write a `Base.Broadcast.broadcast_c(f, ::Type{State}, _...) = ...` that makes use of my symbol indexing. It looks like implementing custom broadcasting will be improved in 0.7, but I’m stuck with 0.6 for now.
---
### Author:  [@ChrisRackauckas](https://discourse.julialang.org/u/ChrisRackauckas)
#### Post date: [February 14, 2018, 10:30pm UTC](https://discourse.julialang.org/t/writing-broadcast-c-for-struct-with-custom-indexing/9071/2 "2018-02-14T22:30:29Z")
I think the nicer way to handle this is to make broadcast just apply to the `arr` fields. I would make the `fields` be an `OrderedDict` so you have a canonical numbers for the array and then let broadcast run down the array directly, by passing the slow dictionary access.
BTW,
```julia
arr::Array{T}
```
that’s not type-stable since arrays need the dimension.
One thing to note too is that we have some proof-of-concept implementations of arrays with naming schemes here:
[https://github.com/JuliaDiffEq/LabelledArrays.jl](https://github.com/JuliaDiffEq/LabelledArrays.jl)
On v0.7 we want to make that compile away the cost of named access via either `a.b` overloads or `a[:b]` literals, but for now it does have a small cost.
But even on v0.6 the StaticArray version should be fast.
---
### Author:  [@EP-Guy](https://discourse.julialang.org/u/EP-Guy)
#### Post date: [February 14, 2018, 11:16pm UTC](https://discourse.julialang.org/t/writing-broadcast-c-for-struct-with-custom-indexing/9071/3 "2018-02-14T23:16:54Z")
I’m looking at LabelledArrays.jl, except I’m getting an error running the example code for constructing LMArrays and LArrays (also, I think the single line constructor macro names are wrong in the README).
Copy pasting
```julia
names = @SArray [:a,:b,:c]
values = @MArray [1,2,3]
```
gives
```julia
julia> A = LMArray(names,values)
3-element LArray{Tuple{3},Int64,1,3}:
Error showing value of type LArray{Tuple{3},Int64,1,3}:
ERROR: ArgumentError: invalid index: (1, 1)
```
Additionally, would this allow indexing by multiple names at once? e.g. `A[:a,:b]`? I often need access to several elements at once for easy vector math (`:r -> :rx, :ry, :rz`).
---
### Author:  [@ChrisRackauckas](https://discourse.julialang.org/u/ChrisRackauckas)
#### Post date: [February 15, 2018, 12:31am UTC](https://discourse.julialang.org/t/writing-broadcast-c-for-struct-with-custom-indexing/9071/4 "2018-02-15T00:31:30Z")
That error is just in the display. It’s being made correctly but it’s not printing correctly. That’s fine though: the library is not released and still being worked on so think of it as a preview.
> [@EP-Guy](#):
>
> Additionally, would this allow indexing by multiple names at once? e.g. A[:a,:b]? I often need access to several elements at once for easy vector math (:r -\> :rx, :ry, :rz).
It doesn’t handle this. That’s a good idea though.
---
### Author:  [@EP-Guy](https://discourse.julialang.org/u/EP-Guy)
#### Post date: [February 15, 2018, 5:44am UTC](https://discourse.julialang.org/t/writing-broadcast-c-for-struct-with-custom-indexing/9071/5 "2018-02-15T05:44:35Z")
Do I have to do anything special to use LMArray in an in-place function to integrate? Trying to solve an ODEProblem this gives a ‘no method matching’ error that looks related to uType.
---
### Author:  [@ChrisRackauckas](https://discourse.julialang.org/u/ChrisRackauckas)
#### Post date: [February 16, 2018, 9:45pm UTC](https://discourse.julialang.org/t/writing-broadcast-c-for-struct-with-custom-indexing/9071/6 "2018-02-16T21:45:59Z")
Looks like this library still needs work. Now that I know that someone is interested I’ll move it up the priority list. Ping me every once in awhile here or in the chat to force me to do it 😛
---
### Author:  [@yakir12](https://discourse.julialang.org/u/yakir12)
#### Post date: [December 12, 2018, 11:46am UTC](https://discourse.julialang.org/t/writing-broadcast-c-for-struct-with-custom-indexing/9071/7 "2018-12-12T11:46:50Z")
The friendliest of pings: 🏓
---
### Author:  [@ChrisRackauckas](https://discourse.julialang.org/u/ChrisRackauckas)
#### Post date: [December 12, 2018, 3:36pm UTC](https://discourse.julialang.org/t/writing-broadcast-c-for-struct-with-custom-indexing/9071/8 "2018-12-12T15:36:31Z")
LabelledArrays works and is registered now.
Julia changed its broadcasting implementation. You can now read about it here. [Interfaces · The Julia Language](https://docs.julialang.org/en/v1/manual/interfaces/index.html#man-interfaces-broadcasting-1)