Array cumulation

General Usage
4

For-loops are very fast in Julia. If you need to do it many times, you can just write a small function like so:

function cumulate!(B, A, idx)
    for (i, Aᵢ) in zip(idx, A)
        B[i] += Aᵢ
    end
end

julia> cumulate!(B, A, idx)

julia> B
2×2 Array{Float64,2}:
 1.1  0.0
 0.0  0.0
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