Stream: helpdesk (published)

Topic: broadcast without length

Júlio Hoffimann (Mar 07 2021 at 16:29):

Is it possible to define broadcasting behavior for a type without having to define Base.length?

Mason Protter (Mar 07 2021 at 17:25):

Yeah. One way woudl be to just overload Base.broadcasted:

julia> struct MyVector{T}
           data::Vector{T}
       end

julia> v = MyVector([1,2,3])
MyVector{Int64}([1, 2, 3])

julia> Base.broadcasted(f, v::MyVector, args...) = Base.broadcasted(f, v.data, args...)

julia> exp.(v .+ 1)
3-element Vector{Float64}:
  7.38905609893065
 20.085536923187668
 54.598150033144236

julia> length(v)
ERROR: MethodError: no method matching length(::MyVector{Int64})

Mason Protter (Mar 07 2021 at 17:26):

A more robust way would be to define a custom broadcast style as explained here: https://docs.julialang.org/en/v1/manual/interfaces/#man-interfaces-broadcasting

Mason Protter (Mar 07 2021 at 17:27):

Unfortunately, the docs on custom broadcast styles are kinda lacking in examples that aren't basically just standard arrays.

Mason Protter (Mar 07 2021 at 17:29):

Actually, the easiest thing here would probably be to use Base.broadcastable instead. So instead of defining broadcasted above, you'd just do

julia> struct MyVector{T}
           data::Vector{T}
       end

julia> v = MyVector([1,2,3])
MyVector{Int64}([1, 2, 3])

julia> Base.broadcastable(v::MyVector) = v.data

julia> log.(1 .+ 3 .+ v)
3-element Vector{Float64}:
 1.6094379124341003
 1.791759469228055
 1.9459101490553132

Philipp Gabler (Mar 08 2021 at 09:05):

Maybe now it's time for someone to answer this really old SO question of mine?

Chad Scherrer (Mar 08 2021 at 11:58):

Upvoted :+1:

Júlio Hoffimann (Mar 08 2021 at 18:28):

Thank you @Mason Protter and everyone else. I will read the links to learn more about custom broadcasts.

Júlio Hoffimann (Mar 08 2021 at 18:30):

It would be nice if someone with more experience in custom broadcasting styles could write a blog post for the community. This is a topic that Julia can really shine and yet very few of us understand it well even after years playing with the language.

Last updated: Oct 02 2023 at 04:34 UTC