# Vector of pointers to struct array elements

**URL:** <https://discourse.julialang.org/t/vector-of-pointers-to-struct-array-elements/60417>\
**Category:** Performance\
**Tags:** question\
**Created:** [May 2, 2021, 10:49am UTC](https://discourse.julialang.org/t/vector-of-pointers-to-struct-array-elements/60417 "2021-05-02T10:49:42Z")\
**Posts on this page:** 7\
**Page:** 1

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**Author:** ![vitelot](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/vitelot/32/24663_2.png) [@vitelot](https://discourse.julialang.org/u/vitelot)\
**Post date:** [May 2, 2021, 10:49am UTC](https://discourse.julialang.org/t/vector-of-pointers-to-struct-array-elements/60417/1 "2021-05-02T10:49:43Z")

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Hi, I’m trying to avoid multiple vector allocations to speed up the weighted sampling of a vector of structs:

```julia
using StatsBase
function chooseBox(room::Vector{Box})
    v = [x.k for x in room]
    sample(room, Weights(v))
end

```

The `mutable struct Box` contains a field `k::Int` which changes during the simulation.  
If I were in C I would use a vector of pointers initialized at the first function call, something like  
`v=[&x.k for x in room]` (sorry for the language mixing)

I’ve browsed around here looking for something about pointers and did not find what I need.  
Probably there is a Julia-like elegant solution but I do not get it.  
Thanks in advance for any hint 🙂

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**Author:** ![baggepinnen](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/baggepinnen/32/693_2.png) [@baggepinnen](https://discourse.julialang.org/u/baggepinnen)\
**Post date:** [May 2, 2021, 11:45am UTC](https://discourse.julialang.org/t/vector-of-pointers-to-struct-array-elements/60417/2 "2021-05-02T11:45:41Z")

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Hello and welcome to the community!  
You could probably use [GitHub - JuliaArrays/MappedArrays.jl: Lazy in-place transformations of arrays](https://github.com/JuliaArrays/Mappedarrays.jl) for this, where the map would be a function that gets the field `k`. That way, you never have to allocate any new vector. You may pay a slight performance hit for doing so though, so it’s worth benchmarking.

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**Author:** ![lmiq](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/lmiq/32/18314_2.png) [@lmiq](https://discourse.julialang.org/u/lmiq)\
**Post date:** [May 2, 2021, 12:07pm UTC](https://discourse.julialang.org/t/vector-of-pointers-to-struct-array-elements/60417/3 "2021-05-02T12:07:38Z")

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I think that defining a vector of pointers like that (here or in C) won’t give you any benefit if `Box` contains only one integer element `k`, or very few immutable elements.

But you could define your own struct to handle a pointer to the whole vector, like this:

```julia
julia> struct Box
         k::Int
       end

julia> struct Room{T} <: AbstractVector{T}
         v::Vector{Box}
       end
       Base.getindex(room::Room{Int},i) = room.v[i].k
       Base.size(room::Room{Int}) = size(room.v)
       Base.length(room::Room{Int}) = length(room.v)

julia> room = [Box(rand(Int)) for i in 1:1000];

julia> using StatsBase, BenchmarkTools

julia> function chooseBox(room::Vector{Box})
           v = [x.k for x in room]
           sample(room, Weights(v))
       end
chooseBox (generic function with 1 method)

julia> function chooseBox2(room::Vector{Box})
           v = Room{Int}(room)
           sample(room, Weights(v))
       end
chooseBox2 (generic function with 1 method)

julia> @btime chooseBox($room)
  525.382 ns (2 allocations: 7.97 KiB)
Box(-4332088731253364447)

julia> @btime chooseBox2($room)
  773.944 ns (1 allocation: 32 bytes)
Box(-4332088731253364447)

julia> @btime chooseBox2($room)
  331.156 ns (1 allocation: 32 bytes)
Box(-4332088731253364447)

```

Note that `chooseBox2` doesn’t allocate almost anything, probably only a reference to the original vector. But for some reason I don’t understand it takes either less or more than time than the first one (770 or 330 ns).

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<div class="post-metadata">

**Author:** ![vitelot](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/vitelot/32/24663_2.png) [@vitelot](https://discourse.julialang.org/u/vitelot)\
**Post date:** [May 2, 2021, 12:20pm UTC](https://discourse.julialang.org/t/vector-of-pointers-to-struct-array-elements/60417/4 "2021-05-02T12:20:27Z")

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Thank you. Sorry, I edited the initial post adding that the struct Box is mutable and in fact, the values of k change in time (rich-get-richer mechanism). If k would be constant I would fill the vector with the values of k once at the first instance of the function.

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**Author:** ![stevengj](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/stevengj/32/71_2.png) [@stevengj](https://discourse.julialang.org/u/stevengj)\
**Post date:** [May 2, 2021, 12:32pm UTC](https://discourse.julialang.org/t/vector-of-pointers-to-struct-array-elements/60417/5 "2021-05-02T12:32:24Z")

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> [@vitelot](#):
>
> `struct Box` is mutable

You don’t to have a `mutable struct` to modify the array contents. In @lmiq’s example, if you have a `v::Vector{Box}`, you can still do `v[i] = Box(newk)`, for example.

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<div class="post-metadata">

**Author:** ![vitelot](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/vitelot/32/24663_2.png) [@vitelot](https://discourse.julialang.org/u/vitelot)\
**Post date:** [May 2, 2021, 1:17pm UTC](https://discourse.julialang.org/t/vector-of-pointers-to-struct-array-elements/60417/6 "2021-05-02T13:17:05Z")

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Thank you @lmiq ,  
This is what I meant with “elegant” solution. Probably it’s the `sample` function that creates a copy of the vector, which would be a little weird imho. I need to look into it.

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<div class="post-metadata">

**Author:** ![lmiq](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/lmiq/32/18314_2.png) [@lmiq](https://discourse.julialang.org/u/lmiq)\
**Post date:** [May 2, 2021, 1:28pm UTC](https://discourse.julialang.org/t/vector-of-pointers-to-struct-array-elements/60417/7 "2021-05-02T13:28:48Z")

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No It does not. That option does only one allocation of 32 bytes, which is likely the pointer to the `room` vector in the new `Room` struct. Nothing else.

edit: actually the allocation there is on the `Weights` function:

```julia
julia> v = rand(1000);

julia> @btime Weights($v);
  74.145 ns (1 allocation: 32 bytes)

```