# Mutating arrays not supported toy problem
**URL:** https://discourse.julialang.org/t/mutating-arrays-not-supported-toy-problem/78057
**Category:** New to Julia
**Tags:** flux
**Created:** [March 17, 2022, 11:13pm UTC](https://discourse.julialang.org/t/mutating-arrays-not-supported-toy-problem/78057 "2022-03-17T23:13:39Z")
**Posts on this page:** 6
**Page:** 1
### Author:  [@Christopher\_Koh](https://discourse.julialang.org/u/Christopher_Koh)
#### Post date: [March 17, 2022, 11:13pm UTC](https://discourse.julialang.org/t/mutating-arrays-not-supported-toy-problem/78057/1 "2022-03-17T23:13:39Z")
Hi all so taking the sample code from the flux gradient documentation I can take the gradient however if I switch out the given function predict(x) = W\*x .+ b(the commented line) with a dense layer predict(x) = Dense(5,2)(x) it throws a mutating arrays not supported error. Why is this? I haven’t assigned any values to anything simply changed the predict function.
using Flux
W = rand(2, 5)
b = rand(2)
predict(x) = Dense(5,2)(x)
#predict(x) = W\*x .+ b
function loss(x, y)
ŷ = predict(x)
sum((y .- ŷ).^2)
end
x, y = rand(5), rand(2) # Dummy data
loss(x, y) # ~ 3
gs = gradient(() → loss(x, y), params(predict(x)))
Error:
Mutating arrays is not supported – called setindex!(::Vector{Float32}, \_…)
Stacktrace:
[1] error(s::String)
@ Base .\error.jl:33
[2] (::Zygote.var"#442#443"{Vector{Float32}})(#unused#::Nothing)
@ Zygote C:\Users\cfcko.julia\packages\Zygote\3I4nT\src\lib\array.jl:71
[3] (::Zygote.var"#2339#back#444"{Zygote.var"#442#443"{Vector{Float32}}})(Δ::Nothing)
@ Zygote C:\Users\cfcko.julia\packages\ZygoteRules\AIbCs\src\adjoint.jl:67
[4] Pullback
@ .\array.jl:353 [inlined]
[5] (::typeof(∂(fill!)))(Δ::Vector{Float32})
@ Zygote C:\Users\cfcko.julia\packages\Zygote\3I4nT\src\compiler\interface2.jl:0
[6] Pullback
@ C:\Users\cfcko.julia\packages\Flux\qAdFM\src\utils.jl:385 [inlined]
[7] (::typeof(∂(create\_bias)))(Δ::Vector{Float32})
@ Zygote C:\Users\cfcko.julia\packages\Zygote\3I4nT\src\compiler\interface2.jl:0
[8] Pullback
@ C:\Users\cfcko.julia\packages\Flux\qAdFM\src\layers\basic.jl:128 [inlined]
[9] (::typeof(∂(Dense)))(Δ::NamedTuple{(:weight, :bias, :σ), Tuple{Matrix{Float64}, Vector{Float32}, Nothing}})
@ Zygote C:\Users\cfcko.julia\packages\Zygote\3I4nT\src\compiler\interface2.jl:0
[10] Pullback
@ C:\Users\cfcko.julia\packages\Flux\qAdFM\src\layers\basic.jl:151 [inlined]
[11] (::typeof(∂(#Dense#160)))(Δ::NamedTuple{(:weight, :bias, :σ), Tuple{Matrix{Float64}, Vector{Float32}, Nothing}})
@ Zygote C:\Users\cfcko.julia\packages\Zygote\3I4nT\src\compiler\interface2.jl:0
[12] Pullback (repeats 2 times)
@ C:\Users\cfcko.julia\packages\Flux\qAdFM\src\layers\basic.jl:137 [inlined]
[13] (::typeof(∂(Dense)))(Δ::NamedTuple{(:weight, :bias, :σ), Tuple{Matrix{Float64}, Vector{Float32}, Nothing}})
@ Zygote C:\Users\cfcko.julia\packages\Zygote\3I4nT\src\compiler\interface2.jl:0
[14] Pullback
@ .\In[30]:4 [inlined]
[15] (::typeof(∂(predict)))(Δ::Vector{Float64})
@ Zygote C:\Users\cfcko.julia\packages\Zygote\3I4nT\src\compiler\interface2.jl:0
[16] Pullback
@ .\In[30]:8 [inlined]
[17] Pullback
@ .\In[30]:16 [inlined]
[18] (::typeof(∂(#19)))(Δ::Float64)
@ Zygote C:\Users\cfcko.julia\packages\Zygote\3I4nT\src\compiler\interface2.jl:0
[19] (::Zygote.var"#94#95"{Zygote.Params, typeof(∂(#19)), Zygote.Context})(Δ::Float64)
@ Zygote C:\Users\cfcko.julia\packages\Zygote\3I4nT\src\compiler\interface.jl:357
[20] gradient(f::Function, args::Zygote.Params)
@ Zygote C:\Users\cfcko.julia\packages\Zygote\3I4nT\src\compiler\interface.jl:76
[21] top-level scope
@ In[30]:16
[22] eval
@ .\boot.jl:373 [inlined]
[23] include\_string(mapexpr::typeof(REPL.softscope), mod::Module, code::String, filename::String)
@ Base .\loading.jl:1196
---
### Author:  [@gdalle](https://discourse.julialang.org/u/gdalle)
#### Post date: [March 18, 2022, 6:34am UTC](https://discourse.julialang.org/t/mutating-arrays-not-supported-toy-problem/78057/2 "2022-03-18T06:34:24Z")
Hi Christopher!
I’m not sure what you want to achieve with this new prediction function, but there is a slight problem. Not only does `predict` ignore the values of `W` and `b`, it actually creates a new (randomly initialized) dense layer at each call. Which means you cannot learn the parameters because they keep changing 😱
I suspect this is also the root of your mutation problem, because `params(predict(x))` keeps being modified during each call to `predict`.
In Flux, the way to correct this would be to define the model outside of the prediction function. But since layers are callable, the syntax is actually very easy
```julia
predict = Dense(5, 2)
```
Note that it is very different from
```julia
predict(x) = Dense(5, 2)(x)
```
because in the second one, a new `Dense(5, 2)` object is created for each `x`.
Does that answer you question?
---
### Author:  [@gdalle](https://discourse.julialang.org/u/gdalle)
#### Post date: [March 18, 2022, 6:35am UTC](https://discourse.julialang.org/t/mutating-arrays-not-supported-toy-problem/78057/3 "2022-03-18T06:35:39Z")
For anyone else landing here with a mutation problem that cannot be resolved in this way, see [https://github.com/rakeshvar/Zygote-Mutating-Arrays-WorkAround.jl](https://github.com/rakeshvar/Zygote-Mutating-Arrays-WorkAround.jl)
---
### Author:  [@Christopher\_Koh](https://discourse.julialang.org/u/Christopher_Koh)
#### Post date: [March 18, 2022, 6:49am UTC](https://discourse.julialang.org/t/mutating-arrays-not-supported-toy-problem/78057/4 "2022-03-18T06:49:52Z")
Aaah gotcha perfect thank you. I didn’t want to do anything specific with this function it just happened that I was getting this error in my first attempt at using flux in different code. So I looked up the gradient documentation and they have this snippet as an example of taking a gradient so I was just trying to get it to run with a flux layer instead of the function so I could debug my other code.
---
### Author:  [@gdalle](https://discourse.julialang.org/u/gdalle)
#### Post date: [March 18, 2022, 9:40am UTC](https://discourse.julialang.org/t/mutating-arrays-not-supported-toy-problem/78057/5 "2022-03-18T09:40:20Z")
Glad this solved your problem!
---
### Author:  [@gdalle](https://discourse.julialang.org/u/gdalle)
#### Post date: [March 18, 2022, 9:34pm UTC](https://discourse.julialang.org/t/mutating-arrays-not-supported-toy-problem/78057/6 "2022-03-18T21:34:44Z")
@Christopher_Koh can you maybe indicate that my answer was the solution, so that other people visiting this issue know it is closed?