# Possible to do non-allocating Setfield operations on OffsetArray(::StaticVector)?

**URL:** https://discourse.julialang.org/t/possible-to-do-non-allocating-setfield-operations-on-offsetarray-staticvector/55031
**Category:** General Usage
**Created:** [February 10, 2021, 10:44pm UTC](https://discourse.julialang.org/t/possible-to-do-non-allocating-setfield-operations-on-offsetarray-staticvector/55031 "2021-02-10T22:44:42Z")
**Posts on this page:** 2
**Page:** 1

<div class="post-metadata">

### Author: ![marius311](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/marius311/32/3953_2.png) [@marius311](https://discourse.julialang.org/u/marius311)
#### Post date: [February 10, 2021, 10:44pm UTC](https://discourse.julialang.org/t/possible-to-do-non-allocating-setfield-operations-on-offsetarray-staticvector/55031/1 "2021-02-10T22:44:42Z")

</div>

I have some code which initializes an array with a loop in a way where each element depends on the last element. I’m trying to make this work with StaticVectors so it can be non-allocating. I’ve found that I can use `Setfield.@set!`(since I need to depend on the previous elements) and that part seems to work fine, e.g.

```julia
function foo(x₁)
    x = @SVector(zeros(Int,5))
    @set! x[1] = x₁
    for i=2:5
        @set! x[i] = 2*x[i-1]+1
    end
    x
end
@code_llvm foo(0)

```

generates what to my naive eye looks like optimal code (which, in a lot of ways, is already pretty amazing to me given what Setfield does; Julia 1.6, btw).

For code clarity in my real case, I _also_ want to have this be an OffsetArray though. Something like:

```julia
function offset_foo(x₀)
    x = OffsetVector(@SVector(zeros(Int,5)), 0:4)
    @set! x[0] = x₀
    for i=1:4
        @set! x[i] = 2*x[i-1]+1
    end
    x
end

```

This allocates unfortunately. I _think_ it may related to bounds checking in `OffsetArray.get/setindex`, but I can’t figure out how to turn it off. Pehaps unsurprisingly, neither of

```julia
@set! @inbounds x[0] = x₀
@inbounds @set! x[0] = x₀

```

seem to work (one is an error, the other doesn’t do anything).

Any ideas how I can turn off bounds checking here? Or if there’s other better approaches to solving this problem?

EDIT: Actually sorry I don’t think its the boundschecking, its just that `@set!` is turning the `OffsetArray(::StaticVector)` into a `OffsetArray(::Vector)`. Still welcome any suggestions for a way around this though.

---

<div class="post-metadata">

### Author: ![jishnub](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/jishnub/32/33620_2.png) [@jishnub](https://discourse.julialang.org/u/jishnub)
#### Post date: [February 28, 2021, 5:23pm UTC](https://discourse.julialang.org/t/possible-to-do-non-allocating-setfield-operations-on-offsetarray-staticvector/55031/2 "2021-02-28T17:23:20Z")

</div>

This behavior might get closer to what you want after [OffsetArrays.jl/pull/213](https://github.com/JuliaArrays/OffsetArrays.jl/pull/213) is merged. After this,

```julia
julia> offset_foo(1)
5-element OffsetArray(::MArray{Tuple{5},Int64,1,5}, 0:4) with eltype Int64 with indices 0:4:
  1
  3
  7
 15
 31

```

The return type is an offset `MArray`, so it is static. It’s not allocation free though, but allocations do come down.

Before:

```julia
julia> @btime offset_foo(1);
  337.063 ns (5 allocations: 640 bytes)

```

After:

```julia
julia> @btime offset_foo(1);
  134.921 ns (5 allocations: 240 bytes)

```