# Zygote and setindex!

**URL:** https://discourse.julialang.org/t/zygote-and-setindex/44554
**Category:** General Usage
**Tags:** question
**Created:** [August 8, 2020, 8:12am UTC](https://discourse.julialang.org/t/zygote-and-setindex/44554 "2020-08-08T08:12:27Z")
**Posts on this page:** 1
**Page:** 1

<div class="post-metadata">

### Author: ![jbrea](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/jbrea/32/3879_2.png) [@jbrea](https://discourse.julialang.org/u/jbrea)
#### Post date: [August 8, 2020, 8:12am UTC](https://discourse.julialang.org/t/zygote-and-setindex/44554/1 "2020-08-08T08:12:28Z")

</div>

I would like to use Zygote to differentiate through some code where I would usually use `setindex!`, e.g. `A[i, j] += v` where `v` is a scalar and `A` a matrix. My current approach is to use a custom `OneHotMatrix`

```julia
struct OneHotMatrix <: AbstractMatrix{Float64}
    v::Float64
    i::Int
    j::Int
    size::Tuple{Int, Int}
end
Base.getindex(A::OneHotMatrix, i, j) = i == A.i && j == A.j ? A.v : 0.
Base.size(A::OneHotMatrix) = A.size
function ChainRulesCore.rrule(::Type{<:OneHotMatrix}, v, i, j, size)
    Y = OneHotMatrix(v, i, j, size)
    function OneHotMatrix_pullback(x̄)
        Zero(), x̄[i, j], Zero(), Zero(), Zero()
    end
    Y, OneHotMatrix_pullback
end

```

and write `A + OneHotMatrix(v, i, j, size(A))`. This works but I guess it is overly complicated. Are there simpler approaches for this situation?