# Filling pre-allocated arrays with minimal function allocations

**URL:** <https://discourse.julialang.org/t/filling-pre-allocated-arrays-with-minimal-function-allocations/119720>\
**Category:** New to Julia\
**Created:** [September 22, 2024, 8:31pm UTC](https://discourse.julialang.org/t/filling-pre-allocated-arrays-with-minimal-function-allocations/119720 "2024-09-22T20:31:06Z")\
**Posts on this page:** 4\
**Page:** 1

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**Author:** ![lkeeble](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/lkeeble/32/212129_2.png) [@lkeeble](https://discourse.julialang.org/u/lkeeble)\
**Post date:** [September 22, 2024, 8:31pm UTC](https://discourse.julialang.org/t/filling-pre-allocated-arrays-with-minimal-function-allocations/119720/1 "2024-09-22T20:31:06Z")

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Hello. I am writing some in which I take a time series of some coordinates x, y, z and must transform them to another coordinate system via some set of functions X = X(x, y, z), Y = Y(x, y, z) and Z = Z(x, y, z). I would like to do this with minimal memory allocations. As a first step in trying to optimise my code, I would like to understand why the following function is not zero allocating:

```julia
using BenchmarkTools
function func!(x1::AbstractVector{Float64}, x0::AbstractVector{Float64})
    x1[1] = x0[1]
    x1[2] = x0[2]
    x1[3] = x0[3]
end

x1 = zeros(3)
x2 = ones(3)

@btime func!(x1, x2);

```

which gives the following output

```plaintext
julia> @btime func!(x1, x2);
  19.027 ns (1 allocation: 16 bytes)

```

I thought that since I have pre-allocated the arrays, there would be no allocations here. Could someone explain why this is not the case?

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<div class="post-metadata">

**Author:** ![AMJ](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/amj/32/214096_2.png) [@AMJ](https://discourse.julialang.org/u/AMJ)\
**Post date:** [September 22, 2024, 8:52pm UTC](https://discourse.julialang.org/t/filling-pre-allocated-arrays-with-minimal-function-allocations/119720/2 "2024-09-22T20:52:07Z")

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You should put `$` signs before your variables when you want to pass them to `@btime` like this:

```julia
julia> @btime func!($x1, $x2)
  3.472 ns (0 allocations: 0 bytes)

```

In general you should make your global variabls `const` as much as possible so the compilers can make better optimizations:

```julia
const x3 = zeros(3)
const x4 = ones(3)

julia> @btime func!(x3, x4) # notice variables without $ sign
  3.460 ns (0 allocations: 0 bytes) 

```

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<div class="post-metadata">

**Author:** ![lkeeble](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/lkeeble/32/212129_2.png) [@lkeeble](https://discourse.julialang.org/u/lkeeble)\
**Post date:** [September 22, 2024, 8:57pm UTC](https://discourse.julialang.org/t/filling-pre-allocated-arrays-with-minimal-function-allocations/119720/3 "2024-09-22T20:57:05Z")

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Ah, I see. So the function is indeed zero-allocating, but I wan’t profiling the function correctly with the arguments of btime(). Thank you.

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**Author:** ![mikmoore](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/mikmoore/32/31109_2.png) [@mikmoore](https://discourse.julialang.org/u/mikmoore)\
**Post date:** [September 23, 2024, 6:50pm UTC](https://discourse.julialang.org/t/filling-pre-allocated-arrays-with-minimal-function-allocations/119720/4 "2024-09-23T18:50:58Z")

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For the special case of 3-vectors (or other relatively small arrays with a constant size that is less than ~100 elements) you should strongly consider StaticArrays.jl. In particular, `SVector` is non-allocating (not even initially) and even faster (at small sizes) than `Vector`.

`SVector` is not mutable (`MVector` is, but is usually worse than `SVector`), so your code above wouldn’t work as written. But I find that code written with immutables is usually “nicer” and more performant than mutating code.