# Smart RecursiveArrayTools.VectorOfArray which allows binary operations with AbstractVector

**URL:** <https://discourse.julialang.org/t/smart-recursivearraytools-vectorofarray-which-allows-binary-operations-with-abstractvector/56327>\
**Category:** General Usage\
**Created:** [March 2, 2021, 12:29pm UTC](https://discourse.julialang.org/t/smart-recursivearraytools-vectorofarray-which-allows-binary-operations-with-abstractvector/56327 "2021-03-02T12:29:27Z")\
**Posts on this page:** 2\
**Page:** 1

<div class="post-metadata">

**Author:** ![liuyxpp](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/liuyxpp/32/9870_2.png) [@liuyxpp](https://discourse.julialang.org/u/liuyxpp)\
**Post date:** [March 2, 2021, 12:29pm UTC](https://discourse.julialang.org/t/smart-recursivearraytools-vectorofarray-which-allows-binary-operations-with-abstractvector/56327/1 "2021-03-02T12:29:27Z")

</div>

This is a follow-up question from [How to solve set of nonlinear fixed-point equations using Anderson acceleration in NLsolve.jl?](https://discourse.julialang.org/t/how-to-solve-set-of-nonlinear-fixed-point-equations-using-anderson-acceleration-in-nlsolve-jl/55994). Suppose I have a VectorOfArray object

```julia
z = VectorOfArray([rand(4), rand(4)])

```

Then the following operations are not allowed

```julia
t .- rand(8) # ERROR: DimensionMismatch("arrays could not be broadcast to a common size; got a dimension with lengths 4 and 8")
t - rand(8) # ERROR: DimensionMismatch("dimensions must match: a has dims (Base.OneTo(32), Base.OneTo(2)), b has dims (Base.OneTo(64),), mismatch at 1")

```

In practice, it will be more complicated where the elements inside the VectorOfArray shall be any AbstractArray.

It is sensible to do following using `copyto!`

```julia
function Base.copyto!(vf::AbstractOfArray{T}, v::AbstractVector) where {T<:MyAbstractArrayType}
    total_elements = 0
    for i in 1:length(vf)
        total_elements += length(vf[1])
    end
    total_elements == length(v) || error("Length mismatch.")

    j = 1
    for i in 1:length(vf)
        n = length(vf[i])
        vf[i] .= v[j:j+n-1]
        j += n
    end
end

z1 = VectorOfArray([rand(4), rand(4)])
z2 = VectorOfArray([rand(4), rand(4)])
copyto!(z1, rand(8))
# Use RecursiveArrayTools broadcasting
z2 .- z1
# Or
z2 - z1

```

Or shall I define a new operator method as

```julia
function Base.-(vf::AbstractOfArray{T}, v::AbstractVector) where {T<:MyAbstractArrayType}
    total_elements = 0
    for i in 1:length(vf)
        total_elements += length(vf[1])
    end
    total_elements == length(v) || error("Length mismatch.")

    out = deepcopy(vf)

    j = 1
    for i in 1:length(vf)
        n = length(vf[i])
        out[i] = vf[i] - v[j:j+n-1]
        j += n
    end
    return out
end

```

However, this approach again relies on whether we can do `-` with `vf[`i]`and`v` which may fail.

I want to know the best practice to solve this kind of issue. Thanks!

---

<div class="post-metadata">

**Author:** ![ChrisRackauckas](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/chrisrackauckas/32/77_2.png) [@ChrisRackauckas](https://discourse.julialang.org/u/ChrisRackauckas)\
**Post date:** [March 2, 2021, 1:02pm UTC](https://discourse.julialang.org/t/smart-recursivearraytools-vectorofarray-which-allows-binary-operations-with-abstractvector/56327/2 "2021-03-02T13:02:26Z")

</div>

Best thing to do would be to extend the broadcast overload.