# Store in place results from function returning multiple arrays

**URL:** <https://discourse.julialang.org/t/store-in-place-results-from-function-returning-multiple-arrays/84336>\
**Category:** General Usage\
**Tags:** functions\
**Created:** [July 16, 2022, 10:02pm UTC](https://discourse.julialang.org/t/store-in-place-results-from-function-returning-multiple-arrays/84336 "2022-07-16T22:02:55Z")\
**Posts on this page:** 9\
**Page:** 1

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**Author:** ![rafael.guerra](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/rafael.guerra/32/216610_2.png) [@rafael.guerra](https://discourse.julialang.org/u/rafael.guerra)\
**Post date:** [July 16, 2022, 10:02pm UTC](https://discourse.julialang.org/t/store-in-place-results-from-function-returning-multiple-arrays/84336/1 "2022-07-16T22:02:55Z")

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What is the best practice to store in place the results of a function returning multiple arrays?

A simple non-MWE:

```julia
f(x,y,z) = 1 ./ x, y'.*y, 2*z # the actual function is long and complex, with matrix operations

n = 10
x = y = z = rand(n)
fx, fy, fz = Vector{Float64}(undef,n), Matrix{Float64}(undef,n, n), Vector{Float64}(undef,n)

fx, fy, fz .= f(x,y,z) # ERROR: MethodError: no method matching copyto!

```

Should we define some `f!!!()` function that mutates in place the first three arguments? Example:

```julia
f!!!(fx,fy,fz,x,y,z) = @. begin fx = 1/x; fy = y'*y; fz = 2*z; return fx, fy, fz; end

f!!!(fx,fy,fz,x,y,z)

```

Thanks.

_ **NB:** edited original example because it had several typos._

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<div class="post-metadata">

**Author:** ![aplavin](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/aplavin/32/222056_2.png) [@aplavin](https://discourse.julialang.org/u/aplavin)\
**Post date:** [July 16, 2022, 10:13pm UTC](https://discourse.julialang.org/t/store-in-place-results-from-function-returning-multiple-arrays/84336/2 "2022-07-16T22:13:37Z")

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StructArrays can help:

```julia
julia> f(x) = (x+1, x+2)

julia> sa = StructArray((zeros(10), zeros(10)))

julia> @btime map!(f, $sa, 1:10)
  11.008 ns (0 allocations: 0 bytes)
10-element StructArray(::Vector{Float64}, ::Vector{Float64}) with eltype Tuple{Float64, Float64}:
 (2.0, 3.0)
 (3.0, 4.0)
 (4.0, 5.0)
 (5.0, 6.0)
 (6.0, 7.0)
 (7.0, 8.0)
 (8.0, 9.0)
 (9.0, 10.0)
 (10.0, 11.0)
 (11.0, 12.0)

```

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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:** [July 16, 2022, 10:19pm UTC](https://discourse.julialang.org/t/store-in-place-results-from-function-returning-multiple-arrays/84336/3 "2022-07-16T22:19:14Z")

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My suggestion: don’t return an array. Let the function work on scalars, and return a Tuple of scalars. And then mutate your arrays in-place in a loop.

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**Author:** ![rafael.guerra](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/rafael.guerra/32/216610_2.png) [@rafael.guerra](https://discourse.julialang.org/u/rafael.guerra)\
**Post date:** [July 16, 2022, 10:20pm UTC](https://discourse.julialang.org/t/store-in-place-results-from-function-returning-multiple-arrays/84336/4 "2022-07-16T22:20:14Z")

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In my example, I would need an heterogeneous structure (_the function returns vectors and a matrix_):

`sa = StructArray((zeros(n), zeros(n,n), zeros(n))) # ERRORS`

but it seems that with StructArrays all field arrays must have same shape?

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<div class="post-metadata">

**Author:** ![aplavin](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/aplavin/32/222056_2.png) [@aplavin](https://discourse.julialang.org/u/aplavin)\
**Post date:** [July 16, 2022, 10:24pm UTC](https://discourse.julialang.org/t/store-in-place-results-from-function-returning-multiple-arrays/84336/5 "2022-07-16T22:24:07Z")

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Of course, all subarrays need to have the same shape, as `sa[i] = (sa.field_1[i], sa.field_2[i])`.

Maybe I misunderstood your question: thought that `f` is a scalar, and you want to broadcast it. If `f` returns already allocated arrays, there’s no way to make it inplace.

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**Author:** ![Benny](https://avatars.discourse-cdn.com/v4/letter/b/49beb7/32.png) [@Benny](https://discourse.julialang.org/u/Benny)\
**Post date:** [July 16, 2022, 10:29pm UTC](https://discourse.julialang.org/t/store-in-place-results-from-function-returning-multiple-arrays/84336/6 "2022-07-16T22:29:10Z")

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If you intend to avoid allocation, this seems impossible because you have already allocated new arrays inside the function. At that point you might as well just destructuringly assign those returned arrays to the variables: `fx, fy, fz = f(x,y,z)`. If you want to do in-place mutation, you need to pass the `fx, fy, fz` instances into the function, like a `function f!(xout, yout, zout, x, y, z) ... nothing end`, and use in-place versions of whatever you do in the function.

`fx, fy, fz .= f(x,y,z)` errors because what you’re really doing is creating a Tuple `(fx, fy, fz)` and trying to `broadcast!`-reassign the Tuple’s elements. Even if a Tuple were mutable, the variables `fx`, `fy`, and `fz` aren’t being reassigned. An Array is mutable, so we can see what happens:

```julia
julia> x,y,z = (0,0,0) # destructuring assignment to variables x,y,z
(0, 0, 0)

julia> x,y,z
(0, 0, 0)

julia> [x,y,z] .= (1,2,3) # elementwise assignment to array [0,0,0]
3-element Array{Int64,1}:
 1
 2
 3

julia> x,y,z
(0, 0, 0)

```

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<div class="post-metadata">

**Author:** ![rafael.guerra](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/rafael.guerra/32/216610_2.png) [@rafael.guerra](https://discourse.julialang.org/u/rafael.guerra)\
**Post date:** [July 16, 2022, 10:36pm UTC](https://discourse.julialang.org/t/store-in-place-results-from-function-returning-multiple-arrays/84336/7 "2022-07-16T22:36:55Z")

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@DNF, thanks for the advice. Unfortunately, the function is quite long and complex involving matrix operations and it seems improbable one can make it scalar.

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<div class="post-metadata">

**Author:** ![rafael.guerra](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/rafael.guerra/32/216610_2.png) [@rafael.guerra](https://discourse.julialang.org/u/rafael.guerra)\
**Post date:** [July 16, 2022, 10:43pm UTC](https://discourse.julialang.org/t/store-in-place-results-from-function-returning-multiple-arrays/84336/8 "2022-07-16T22:43:02Z")

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Thanks all for the explanations, it is a bit more clear now. We need scalar functions to have the dot fusion to work with broadcasting, or we have to pass pre-allocated arrays and mutate them inside the function.

Now what is the recommended syntax for my example: one exclamation mark (!) or three (!!!) in the function name? And the order of the function arguments that are mutated, should they appear all in front, or interleaved as in Benny’s response?

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<div class="post-metadata">

**Author:** ![Benny](https://avatars.discourse-cdn.com/v4/letter/b/49beb7/32.png) [@Benny](https://discourse.julialang.org/u/Benny)\
**Post date:** [July 16, 2022, 10:48pm UTC](https://discourse.julialang.org/t/store-in-place-results-from-function-returning-multiple-arrays/84336/9 "2022-07-16T22:48:31Z")

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I believe the convention is to use 1 `!` character always, and to put the (possibly) mutated arguments first, sorry about that, I’ll amend my post.