# Zygote in Turing: Mutating arrays is not supported

**URL:** <https://discourse.julialang.org/t/zygote-in-turing-mutating-arrays-is-not-supported/41155>\
**Category:** New to Julia\
**Tags:** turing, zygote\
**Created:** [June 10, 2020, 5:59pm UTC](https://discourse.julialang.org/t/zygote-in-turing-mutating-arrays-is-not-supported/41155 "2020-06-10T17:59:19Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![Manoj\_Gopalkrishnan](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/manoj_gopalkrishnan/32/14587_2.png) [@Manoj\_Gopalkrishnan](https://discourse.julialang.org/u/Manoj_Gopalkrishnan)\
**Post date:** [June 10, 2020, 5:59pm UTC](https://discourse.julialang.org/t/zygote-in-turing-mutating-arrays-is-not-supported/41155/1 "2020-06-10T17:59:19Z")

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I am trying to use Zygote in Turing. Here’s my MWE

```julia
@model function mwe(B, ::Type{T} = Vector{Float64}) where {T}
    μ = mapslices(lsexp, B; dims=[1])[1,:]
end
mwe_model = mwe([1.0 2; 3 4])
mwe_ch = sample(mwe_model, NUTS(.65),10)

```

and the error is

```julia
Mutating arrays is not supported

Stacktrace:
 [1] error(::String) at .\error.jl:33
 [2] (::Zygote.var"#1052#1053")(::Nothing) at C:\Users\Manoj\.julia\packages\Zygote\YeCEW\src\lib\array.jl:64
 [3] (::Zygote.var"#2785#back#1054"{Zygote.var"#1052#1053"})(::Nothing) at C:\Users\Manoj\.julia\packages\ZygoteRules\6nssF\src\adjoint.jl:49
 [4] materialize! at .\broadcast.jl:823 [inlined]
 [5] concatenate_setindex! at .\abstractarray.jl:2058 [inlined]
 [6] (::typeof(∂(concatenate_setindex!)))(::Nothing) at C:\Users\Manoj\.julia\packages\Zygote\YeCEW\src\compiler\interface2.jl:0
 [7] (::Zygote.var"#174#175"{typeof(∂(concatenate_setindex!)),Tuple{Tuple{Nothing,Nothing},Int64}})(::Nothing) at C:\Users\Manoj\.julia\packages\Zygote\YeCEW\src\lib\lib.jl:182
 [8] (::Zygote.var"#347#back#176"{Zygote.var"#174#175"{typeof(∂(concatenate_setindex!)),Tuple{Tuple{Nothing,Nothing},Int64}}})(::Nothing) at C:\Users\Manoj\.julia\packages\ZygoteRules\6nssF\src\adjoint.jl:49
 [9] inner_mapslices! at .\abstractarray.jl:2039 [inlined]
 [10] (::typeof(∂(inner_mapslices!)))(::Nothing) at C:\Users\Manoj\.julia\packages\Zygote\YeCEW\src\compiler\interface2.jl:0
 [11] #mapslices#115 at .\abstractarray.jl:2029 [inlined]
 [12] (::typeof(∂(#mapslices#115)))(::Nothing) at C:\Users\Manoj\.julia\packages\Zygote\YeCEW\src\compiler\interface2.jl:0 (repeats 2 times)
 [13] macro expansion at .\In[19]:2 [inlined]

```

I tried to read about this error, and found a long discussion here: [Facilitate array construction without mutation · Issue #377 · FluxML/Zygote.jl · GitHub](https://github.com/FluxML/Zygote.jl/issues/377) . I was not sure what the fix is.

I’m hoping someone can tell me how to fix this. Note: use of mapslices is fairly trivial here because this is a minimal example which does nothing much, but I do need something very much like that in my actual code. Nonmutating options would be fine if you can suggest some. I am still fairly new to Julia and not sure what mutates and what doesn’t mutate.

---

<div class="post-metadata">

**Author:** ![Manoj\_Gopalkrishnan](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/manoj_gopalkrishnan/32/14587_2.png) [@Manoj\_Gopalkrishnan](https://discourse.julialang.org/u/Manoj_Gopalkrishnan)\
**Post date:** [June 12, 2020, 6:21am UTC](https://discourse.julialang.org/t/zygote-in-turing-mutating-arrays-is-not-supported/41155/2 "2020-06-12T06:21:52Z")

</div>

I realized my example leaves out description of lsexp. This is logsumexp written as:

```julia
function lsexp(w)
    N = length(w)
    offset, maxind = findmax(w)
    w .= exp.(w .- offset)
    Σ = _sum_all_but(w, maxind)
    log1p(Σ) + offset
end

function _sum_all_but(w, i)
    w[i] -= 1
    s = sum(w)
    w[i] += 1
    s
end

```

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<div class="post-metadata">

**Author:** ![mcabbott](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/mcabbott/32/6603_2.png) [@mcabbott](https://discourse.julialang.org/u/mcabbott)\
**Post date:** [June 12, 2020, 6:56am UTC](https://discourse.julialang.org/t/zygote-in-turing-mutating-arrays-is-not-supported/41155/3 "2020-06-12T06:56:19Z")

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You may like [SliceMap.jl](https://github.com/mcabbott/SliceMap.jl) which has Zygote-compatible replacements for `mapslices`.

But these parts also mutate `w`, which you will need to avoid:

> [@Manoj\_Gopalkrishnan](#):
>
> `w .= exp.(w .- offset)`

> [@Manoj\_Gopalkrishnan](#):
>
> ```julia
> w[i] -= 1
> s = sum(w)
> w[i] += 1
> 
> ```

Another approach would be to provide a gradient for the function `lsexp`, so that Zygote does not need to look inside. This must have been worked out somewhere, perhaps [Crash on 0.7.0-rc3.0 · Issue #2 · FluxML/Zygote.jl · GitHub](https://github.com/FluxML/Zygote.jl/issues/2) has what you want?

In fact, you should probably write a function `lsexp(B; dims)` which works directly on the whole array. This is very likely to be more efficient than making slices & acting on each of them.