# Broadcasting to all fields of a struct

**URL:** <https://discourse.julialang.org/t/broadcasting-to-all-fields-of-a-struct/57924>\
**Category:** General Usage\
**Created:** [March 25, 2021, 12:20pm UTC](https://discourse.julialang.org/t/broadcasting-to-all-fields-of-a-struct/57924 "2021-03-25T12:20:24Z")\
**Posts on this page:** 14\
**Page:** 1

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**Author:** ![Philippe\_Maincon1](https://avatars.discourse-cdn.com/v4/letter/p/ec9cab/32.png) [@Philippe\_Maincon1](https://discourse.julialang.org/u/Philippe_Maincon1)\
**Post date:** [March 25, 2021, 12:20pm UTC](https://discourse.julialang.org/t/broadcasting-to-all-fields-of-a-struct/57924/1 "2021-03-25T12:20:24Z")

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I have a data structure, nesting AbstractArrays and Structs, where all the elements (leaves of my datatree) are, for example Float64, and I wish to transform them all into Float32, by rebuilding the data structure. I write the following code, which works:

```julia
using StaticArrays
struct S{R}
    a::SArray{Tuple{2,2},R,2,4}
    b::R
end
s = S(SMatrix{2,2,Float64}(randn(2,2)),randn())

cast(s::S ,cv) = S(cast(s.a,cv),cast(s.b,cv))
#cast(s::S ,cv) = cast.(s,cv)
cast(a::AbstractArray,cv) = cast.(a,cv)
cast(a::Real ,cv) = cv(a)

cv(x) = Float32(x)
@code_warntype cast(s,cv) 
@show cast(s,cv) 

```

However the ambition (as suggested by the line of code commented out) is that I would like to write a `cast`method to recursively apply `cast` to all fields of _any_ composite type.

If the answer is metaprogramming, allright, I’ll do it (`@generated` and `fieldnames`…), but I’d like to know if there is a lazier way.

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<div class="post-metadata">

**Author:** ![Henrique\_Becker](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/henrique_becker/32/15443_2.png) [@Henrique\_Becker](https://discourse.julialang.org/u/Henrique_Becker)\
**Post date:** [March 25, 2021, 1:26pm UTC](https://discourse.julialang.org/t/broadcasting-to-all-fields-of-a-struct/57924/2 "2021-03-25T13:26:25Z")

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> [@Philippe\_Maincon1](#):
>
> write a `cast` method to recursively apply `cast` to all fields of _any_ composite type

You mean, you could, but probably will fail for any struct that is not like yours, this is, for which all the leaves are of the same type that can be converted to whatever you want, right?

It really seems like a generated function is the right choice here, but I myself have never used them, so I cannot offer any practical advice.

Meanwhile, why did you chose to create a `cast` function? `convert` did not seem adequate?

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<div class="post-metadata">

**Author:** ![DNF](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/dnf/32/10191_2.png) [@DNF](https://discourse.julialang.org/u/DNF)\
**Post date:** [March 25, 2021, 1:33pm UTC](https://discourse.julialang.org/t/broadcasting-to-all-fields-of-a-struct/57924/3 "2021-03-25T13:33:07Z")

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I think you shouldn’t call it `cast` but instead extend `convert`. Perhaps something like this:

```julia
function Base.convert(::Type{S{R}}, s::S) where {R}
    vals = getfield.((s,), fieldnames(S))
    return S{R}(vals...)
end

```

```julia
jl> s = S(SMatrix{2,2,Float64}(randn(2,2)),randn())
S{Float64}([-0.33679898580000156 -1.6556986308629353; -1.0290895856851072 0.4700396382426737], 0.041295723749969614)

jl> convert(S{ComplexF32}, s)
S{ComplexF32}(ComplexF32[-0.336799f0 + 0.0f0im -1.6556987f0 + 0.0f0im; -1.0290896f0 + 0.0f0im 0.47003964f0 + 0.0f0im], 0.041295722f0 + 0.0f0im)

```

Note that the use of `getfield` and `fieldnames` isn’t apparently type stable, so there may be a better way to get `vals`, perhaps with a generated function.

If you directly type in `s.a, s.b` into the constructor, it will of course be fast.

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<div class="post-metadata">

**Author:** ![Philippe\_Maincon1](https://avatars.discourse-cdn.com/v4/letter/p/ec9cab/32.png) [@Philippe\_Maincon1](https://discourse.julialang.org/u/Philippe_Maincon1)\
**Post date:** [March 25, 2021, 1:55pm UTC](https://discourse.julialang.org/t/broadcasting-to-all-fields-of-a-struct/57924/4 "2021-03-25T13:55:02Z")

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Hi Henrique,

Yes to your first question.

I prefer not to call it convert because there is the convention that convert is not lossy. For reasons that will bore everyone, my `cv` function may be seriously lossy: I don’t want to create a `convert`method which might be called implicitly in some assignement.

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<div class="post-metadata">

**Author:** ![Henrique\_Becker](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/henrique_becker/32/15443_2.png) [@Henrique\_Becker](https://discourse.julialang.org/u/Henrique_Becker)\
**Post date:** [March 25, 2021, 2:02pm UTC](https://discourse.julialang.org/t/broadcasting-to-all-fields-of-a-struct/57924/5 "2021-03-25T14:02:03Z")

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> [@DNF](#):
>
> Note that the use of `getfield` and `fieldnames` isn’t apparently type stable, so there may be a better way to get `vals` , perhaps with a generated function.

Yes, this kind of thing seems to often call for a generated function to guarantee type-stability.

---

<div class="post-metadata">

**Author:** ![Philippe\_Maincon1](https://avatars.discourse-cdn.com/v4/letter/p/ec9cab/32.png) [@Philippe\_Maincon1](https://discourse.julialang.org/u/Philippe_Maincon1)\
**Post date:** [March 25, 2021, 2:07pm UTC](https://discourse.julialang.org/t/broadcasting-to-all-fields-of-a-struct/57924/6 "2021-03-25T14:07:57Z")

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Hi DNF,

Great snippet

> [@DNF](#):
>
> `getfield.((s,), fieldnames(S))`

which I will use. I will end up metaprogramming and this will appear in the code.

The solution you propose assumes I have easy access to the exact type of the struct I want to create.  
However I will program  
`unparametredtype(s::S{R}) where{R} = S`  
which output I can use to call the constructor.

Almost there!!!

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<div class="post-metadata">

**Author:** ![DNF](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/dnf/32/10191_2.png) [@DNF](https://discourse.julialang.org/u/DNF)\
**Post date:** [March 25, 2021, 2:25pm UTC](https://discourse.julialang.org/t/broadcasting-to-all-fields-of-a-struct/57924/7 "2021-03-25T14:25:49Z")

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> [@Philippe\_Maincon1](#):
>
> The solution you propose assumes I have easy access to the exact type of the struct I want to create.

I must have missed something. I thought you knew the types. Can you give a more concrete example showing what your wanted function _does_ have access to?

---

<div class="post-metadata">

**Author:** ![DNF](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/dnf/32/10191_2.png) [@DNF](https://discourse.julialang.org/u/DNF)\
**Post date:** [March 25, 2021, 2:37pm UTC](https://discourse.julialang.org/t/broadcasting-to-all-fields-of-a-struct/57924/8 "2021-03-25T14:37:28Z")

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> [@Philippe\_Maincon1](#):
>
> I prefer not to call it convert because there is the convention that convert is not lossy.

Convert can be lossy, converting from `Float64` to `Float32`, for example. At least for the use case in your example, `convert` seems like the right name, but perhaps you have a different example in mind?

---

<div class="post-metadata">

**Author:** ![Philippe\_Maincon1](https://avatars.discourse-cdn.com/v4/letter/p/ec9cab/32.png) [@Philippe\_Maincon1](https://discourse.julialang.org/u/Philippe_Maincon1)\
**Post date:** [March 25, 2021, 2:39pm UTC](https://discourse.julialang.org/t/broadcasting-to-all-fields-of-a-struct/57924/9 "2021-03-25T14:39:02Z")

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Well, I want a method `cast(s,cv)` that accepts _any_ struct (and I don’t know how to dispatch for that, by the way…), and recursively applies `cv` to all fields of `s`.

The parameters of `cast` are not a MWE - I want to write exactly that function…

Yes I know the type of `s` (`typeof`) of course. My challenge was (is now solved) to get a name for the constructor I need to call. _I do not easily know the exact type of the output_. The good news is, we can call the constructor for `S` (not `S{hard to get}`), and that I know how to get:  
`unparametredtype(s::S{R}) where{R} = S`

I am working on a parallel solutioj in which I use a named tuple instead of a struct - that’s a lot easier.

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<div class="post-metadata">

**Author:** ![DNF](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/dnf/32/10191_2.png) [@DNF](https://discourse.julialang.org/u/DNF)\
**Post date:** [March 25, 2021, 2:42pm UTC](https://discourse.julialang.org/t/broadcasting-to-all-fields-of-a-struct/57924/10 "2021-03-25T14:42:43Z")

</div>

Indeed, if you want this to work on arbitrary types, you should not overload `convert` (that would be very bad). But then you should expect `cast` to fail pretty frequently, right, since most fields are not convertible to `Float32`, and the constructor syntax is also generally unknown.

Maybe `convertfields` is an appropriate name?

---

<div class="post-metadata">

**Author:** ![Philippe\_Maincon1](https://avatars.discourse-cdn.com/v4/letter/p/ec9cab/32.png) [@Philippe\_Maincon1](https://discourse.julialang.org/u/Philippe_Maincon1)\
**Post date:** [March 25, 2021, 2:44pm UTC](https://discourse.julialang.org/t/broadcasting-to-all-fields-of-a-struct/57924/11 "2021-03-25T14:44:50Z")

</div>

> [@DNF](#):
>
> Convert can be lossy, converting from `Float64` to `Float32` , for example. At least for the use case in your example, `convert` seems like the right name, but perhaps you have a different example in mind?

OK, all cards on the table.

My MWE `Float64` might be in the real world an automatic differentiation object full of partial derivatives data.

So I might need to extract the `Float64`from it (the current challenge), but I never want this to happen implicitly: I would silently get the wrong derivative - I get numb with fear just at the thought! 😀

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<div class="post-metadata">

**Author:** ![DNF](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/dnf/32/10191_2.png) [@DNF](https://discourse.julialang.org/u/DNF)\
**Post date:** [March 25, 2021, 2:46pm UTC](https://discourse.julialang.org/t/broadcasting-to-all-fields-of-a-struct/57924/12 "2021-03-25T14:46:20Z")

</div>

I see. Beware of the [XY problem](https://en.wikipedia.org/wiki/XY_problem) by the way.

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<div class="post-metadata">

**Author:** ![Philippe\_Maincon1](https://avatars.discourse-cdn.com/v4/letter/p/ec9cab/32.png) [@Philippe\_Maincon1](https://discourse.julialang.org/u/Philippe_Maincon1)\
**Post date:** [March 25, 2021, 2:46pm UTC](https://discourse.julialang.org/t/broadcasting-to-all-fields-of-a-struct/57924/13 "2021-03-25T14:46:24Z")

</div>

> [@DNF](#):
>
> Maybe `convertfields` is an appropriate name?

`convertfields`it is! Good name!

---

<div class="post-metadata">

**Author:** ![Philippe\_Maincon1](https://avatars.discourse-cdn.com/v4/letter/p/ec9cab/32.png) [@Philippe\_Maincon1](https://discourse.julialang.org/u/Philippe_Maincon1)\
**Post date:** [March 25, 2021, 2:48pm UTC](https://discourse.julialang.org/t/broadcasting-to-all-fields-of-a-struct/57924/14 "2021-03-25T14:48:43Z")

</div>

> [@DNF](#):
>
> Beware of the [XY problem](https://en.wikipedia.org/wiki/XY_problem) by the way.

Sorry! And thank you for helping!