# How to efficiently transform vector of structures?

**URL:** <https://discourse.julialang.org/t/how-to-efficiently-transform-vector-of-structures/16816>\
**Category:** Performance\
**Tags:** question, design\
**Created:** [October 26, 2018, 5:59am UTC](https://discourse.julialang.org/t/how-to-efficiently-transform-vector-of-structures/16816 "2018-10-26T05:59:22Z")\
**Posts on this page:** 6\
**Page:** 1

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**Author:** ![Martin\_Florek](https://avatars.discourse-cdn.com/v4/letter/m/f4b2a3/32.png) [@Martin\_Florek](https://discourse.julialang.org/u/Martin_Florek)\
**Post date:** [October 26, 2018, 5:59am UTC](https://discourse.julialang.org/t/how-to-efficiently-transform-vector-of-structures/16816/1 "2018-10-26T05:59:22Z")

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How to efficiently transform vector of structures without unnecessary allocation? We have two structures:

```julia
struct ApiData
  name::String
  reload::Vector{ApiReload}
  values::Vector{Float64}
end

struct ApiReload
  status::String
  time::Sting #Transform to sturcture "TimeOfDay"
end

```

which make the vector **a** = `Vector{ApiData}`. Now we want to transform the vector **a** into vector **b** which has the form of three structures:

```julia
struct Data
  name::String
  reload::Vector{Reload}
  values::Vector{Float64}
end

struct Reload
  status::String
  time::TimeOfDay
end

struct TimeOfDay
  minutes::Array{Int}
  hours::Array{Int}
end

```

Vector **b** has the form of `Vector{Data}`, with percentage members in the `Data` structure (name =1%, reload=4% and values 95%).

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<div class="post-metadata">

**Author:** ![laborg](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/laborg/32/5474_2.png) [@laborg](https://discourse.julialang.org/u/laborg)\
**Post date:** [October 26, 2018, 7:48am UTC](https://discourse.julialang.org/t/how-to-efficiently-transform-vector-of-structures/16816/2 "2018-10-26T07:48:38Z")

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What have you tried and what do you mean with _unnecessary_ allocation? Wouldn’t it be sufficient to preallocate the vector **b** and fill it in a loop?

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<div class="post-metadata">

**Author:** ![Martin\_Florek](https://avatars.discourse-cdn.com/v4/letter/m/f4b2a3/32.png) [@Martin\_Florek](https://discourse.julialang.org/u/Martin_Florek)\
**Post date:** [October 26, 2018, 8:42am UTC](https://discourse.julialang.org/t/how-to-efficiently-transform-vector-of-structures/16816/3 "2018-10-26T08:42:13Z")

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@laborg In fact, I have a third structure, yet. For C API.

```julia
struct C_ApiData
  name::Ptr{Cwchar_t}
  reloadLength::Csize_t
  reload::Vector{C_ApiReload}
  valuesLength::Csize_t
  values::Vector{Cdouble}
end

```

And I’m looking for the most optimal way (design Pattern).

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<div class="post-metadata">

**Author:** ![laborg](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/laborg/32/5474_2.png) [@laborg](https://discourse.julialang.org/u/laborg)\
**Post date:** [October 26, 2018, 9:06am UTC](https://discourse.julialang.org/t/how-to-efficiently-transform-vector-of-structures/16816/4 "2018-10-26T09:06:57Z")

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You haven’t specified how fields should be transformed, so I’m just guessing on the similarity of type names, but then its not clear how `ApiReload` with a single `String` should map to `Reload` having `Array{Int}` for minutes and hours.

Now, looking at your last line:

> `Vector{Data}` , with percentage members in the `Data` structure…

should the transform contain any calculations as well, or is it merely a structural transformation?

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<div class="post-metadata">

**Author:** ![Martin\_Florek](https://avatars.discourse-cdn.com/v4/letter/m/f4b2a3/32.png) [@Martin\_Florek](https://discourse.julialang.org/u/Martin_Florek)\
**Post date:** [October 26, 2018, 9:29am UTC](https://discourse.julialang.org/t/how-to-efficiently-transform-vector-of-structures/16816/5 "2018-10-26T09:29:15Z")

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Yes, there will be a `parse` function that transforms `String` to structure `TimeOfDay`. Further e.g. `a[1].reload[1].status` to ` enum`

> @enum Status lunch=1 evening=2

etcetera …

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<div class="post-metadata">

**Author:** ![laborg](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/laborg/32/5474_2.png) [@laborg](https://discourse.julialang.org/u/laborg)\
**Post date:** [October 26, 2018, 10:30am UTC](https://discourse.julialang.org/t/how-to-efficiently-transform-vector-of-structures/16816/6 "2018-10-26T10:30:20Z")

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I’d go with:

```julia
parsetime(t::String) = TimeOfDay([1],[1])
transform(x::ApiReload) = Reload(x.status, parsetime(x.time))
transform(x::ApiData) = Data(x.name, transform.(x.reload), x.values)

a = ApiData[
  ApiData("ap1",
    ApiReload[ApiReload("st1","ti1"), ApiReload("st2","ti2")],
    Float64[1.,2.,3.,4.]),
  ApiData("ap2",
      ApiReload[ApiReload("st3","ti3")],
      Float64[1.,2.])
  ]

b = transform.(a)

```

If you want less allocations and more speed you can replace the broadcasting by manual preallocation of the vectors and explicit loops.

It would be easier for others to help if you present your current solution and describe what you’d like to improve. (nota bene: I’m still learning Julia, so I might have missed something, but till now I’ve never come across a generic structural transformation design pattern).