# Scalar parameter doesn't update in Flux, but array does

**URL:** <https://discourse.julialang.org/t/scalar-parameter-doesnt-update-in-flux-but-array-does/32548>\
**Category:** General Usage\
**Tags:** flux\
**Created:** [December 21, 2019, 11:26am UTC](https://discourse.julialang.org/t/scalar-parameter-doesnt-update-in-flux-but-array-does/32548 "2019-12-21T11:26:19Z")\
**Posts on this page:** 11\
**Page:** 1

<div class="post-metadata">

**Author:** ![jonasaugust](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/jonasaugust/32/11942_2.png) [@jonasaugust](https://discourse.julialang.org/u/jonasaugust)\
**Post date:** [December 21, 2019, 11:26am UTC](https://discourse.julialang.org/t/scalar-parameter-doesnt-update-in-flux-but-array-does/32548/1 "2019-12-21T11:26:19Z")

</div>

When I try to minimize a quadratic function `f(x)` in a single, scalar parameter `x` via gradient descent in Flux, there is no progression (no change in `x`), but the single-component array version of the same works correctly:

```julia
using Flux
data = Iterators.repeated([], 5) # 5 iterations of empty data
opt = Descent()
let x = 3.0, f(x) = x^2
    println("No updates observed in scalar version:")
    Flux.train!(() -> f(x), params(x), data, opt, cb = () -> @show x, f(x))
end
let x = [3.0], f(x) = x[1]^2
    println("Updates seen as expected in 1-d array version:")
    Flux.train!(() -> f(x), params(x), data, opt, cb = () -> @show x, f(x))
end

```

Under Flux 0.10 (CPU, not GPU) in Julia 1.3 macos Mojave on a 2014 Macbook Pro 15, I get this output in a restarted IJulia notebook:

```julia
┌ Info: CUDAdrv.jl failed to initialize, GPU functionality unavailable (set JULIA_CUDA_SILENT or JULIA_CUDA_VERBOSE to silence or expand this message)
└ @ CUDAdrv /Users/jonas/.julia/packages/CUDAdrv/3EzC1/src/CUDAdrv.jl:69

No updates observed in scalar version:
(x, f(x)) = (3.0, 9.0)
(x, f(x)) = (3.0, 9.0)
(x, f(x)) = (3.0, 9.0)
(x, f(x)) = (3.0, 9.0)
(x, f(x)) = (3.0, 9.0)
Updates seen as expected in 1-d array version:
(x, f(x)) = ([2.4], 5.76)
(x, f(x)) = ([1.92], 3.6864)
(x, f(x)) = ([1.536], 2.359296)
(x, f(x)) = ([1.2288000000000001], 1.5099494400000002)
(x, f(x)) = ([0.9830400000000001], 0.9663676416000002)

```

Shouldn’t both give essentially the same result (except for the single component array / scalar distinction in the output)?

---

<div class="post-metadata">

**Author:** ![baggepinnen](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/baggepinnen/32/693_2.png) [@baggepinnen](https://discourse.julialang.org/u/baggepinnen)\
**Post date:** [December 21, 2019, 12:28pm UTC](https://discourse.julialang.org/t/scalar-parameter-doesnt-update-in-flux-but-array-does/32548/2 "2019-12-21T12:28:08Z")

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What does `params(x) ` return for scalar x? I guess nothing.

---

<div class="post-metadata">

**Author:** ![baggepinnen](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/baggepinnen/32/693_2.png) [@baggepinnen](https://discourse.julialang.org/u/baggepinnen)\
**Post date:** [December 21, 2019, 12:29pm UTC](https://discourse.julialang.org/t/scalar-parameter-doesnt-update-in-flux-but-array-does/32548/3 "2019-12-21T12:29:07Z")

</div>

Try wrapping scalar x in a `Params` struct before using it.

---

<div class="post-metadata">

**Author:** ![jonasaugust](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/jonasaugust/32/11942_2.png) [@jonasaugust](https://discourse.julialang.org/u/jonasaugust)\
**Post date:** [December 21, 2019, 1:29pm UTC](https://discourse.julialang.org/t/scalar-parameter-doesnt-update-in-flux-but-array-does/32548/4 "2019-12-21T13:29:55Z")

</div>

Yes, `params(x)` gives an empty list for scalar, very odd:

```julia
x = 3.0
@show params(x)
x_array = [3.0]
@show params(x_array)

```

displays

```julia
params(x) = Params([])
params(x_array) = Params([[3.0]])

```

---

<div class="post-metadata">

**Author:** ![jonasaugust](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/jonasaugust/32/11942_2.png) [@jonasaugust](https://discourse.julialang.org/u/jonasaugust)\
**Post date:** [December 21, 2019, 1:32pm UTC](https://discourse.julialang.org/t/scalar-parameter-doesnt-update-in-flux-but-array-does/32548/5 "2019-12-21T13:32:35Z")

</div>

Wrapping doesn’t seem to help, unfortunately:

```julia
x = 3.0
@show params(Params(x))

```

displays

```julia
params(Params(x)) = Params([])

```

---

<div class="post-metadata">

**Author:** ![baggepinnen](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/baggepinnen/32/693_2.png) [@baggepinnen](https://discourse.julialang.org/u/baggepinnen)\
**Post date:** [December 21, 2019, 2:39pm UTC](https://discourse.julialang.org/t/scalar-parameter-doesnt-update-in-flux-but-array-does/32548/6 "2019-12-21T14:39:07Z")

</div>

You can always manually do `f'(x) ` and call the optimizer yourself instead of using the implicit parameter interface.

---

<div class="post-metadata">

**Author:** ![jonasaugust](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/jonasaugust/32/11942_2.png) [@jonasaugust](https://discourse.julialang.org/u/jonasaugust)\
**Post date:** [December 21, 2019, 4:31pm UTC](https://discourse.julialang.org/t/scalar-parameter-doesnt-update-in-flux-but-array-does/32548/7 "2019-12-21T16:31:59Z")

</div>

Thanks for the suggestion. Unfortunately, taking `f'` and then using the optimizer via update! instead of train! has the same problem:

```julia
using Flux, Flux.Optimise
function mytrain(x, f)
    opt = Descent()
    for i in 1:5
        g = f'(x)
        Optimise.update!(opt, x, g)
        @show x, g, f(x) 
    end
end
println("Using array:")
mytrain([3.0], x -> x[1]^2)
println("Using scalar:")
mytrain(3.0, x -> x^2)

```

has error on the scalar case

```julia
Using array:
(x, g, f(x)) = ([2.4], [0.6000000000000001], 5.76)
(x, g, f(x)) = ([1.92], [0.48], 3.6864)
(x, g, f(x)) = ([1.536], [0.384], 2.359296)
(x, g, f(x)) = ([1.2288000000000001], [0.30720000000000003], 1.5099494400000002)
(x, g, f(x)) = ([0.9830400000000001], [0.24576000000000003], 0.9663676416000002)
Using scalar:

MethodError: no method matching copyto!(::Float64, ::Base.Broadcast.Broadcasted{Base.Broadcast.DefaultArrayStyle{0},Tuple{},typeof(*),Tuple{Float64,Float64}})
Closest candidates are:
  copyto!(!Matched::Union{Base.ReshapedArray{#s27,#s26,AT,#s22} where #s22 where #s26 where #s27 where AT<:GPUArrays.GPUArray, SubArray{#s33,#s32,AT,I,L} where L where I where #s32 where #s33 where AT<:GPUArrays.GPUArray, GPUArrays.GPUArray, LinearAlgebra.Adjoint{#s21,AT} where #s21 where AT<:GPUArrays.GPUArray, LinearAlgebra.Diagonal{#s12,AT} where #s12 where AT<:GPUArrays.GPUArray, LinearAlgebra.LowerTriangular{#s19,AT} where #s19 where AT<:GPUArrays.GPUArray, LinearAlgebra.Transpose{#s20,AT} where #s20 where AT<:GPUArrays.GPUArray, LinearAlgebra.Tridiagonal{#s79,AT} where #s79 where AT<:GPUArrays.GPUArray, LinearAlgebra.UnitLowerTriangular{#s18,AT} where #s18 where AT<:GPUArrays.GPUArray, LinearAlgebra.UnitUpperTriangular{#s13,AT} where #s13 where AT<:GPUArrays.GPUArray, LinearAlgebra.UpperTriangular{#s17,AT} where #s17 where AT<:GPUArrays.GPUArray, PermutedDimsArray{#s31,#s30,#s29,#s28,AT} where #s28 where #s29 where #s30 where #s31 where AT<:GPUArrays.GPUArray}, ::Base.Broadcast.Broadcasted{#s79,Axes,F,Args} where Args<:Tuple where F where Axes where #s79<:Base.Broadcast.AbstractArrayStyle{0}) at /Users/jonas/.julia/packages/GPUArrays/1wgPO/src/broadcast.jl:60
  copyto!(!Matched::AbstractArray, ::Base.Broadcast.Broadcasted{#s627,Axes,F,Args} where Args<:Tuple where F where Axes where #s627<:Base.Broadcast.AbstractArrayStyle{0}) at broadcast.jl:869
  copyto!(!Matched::AbstractArray, ::Base.Broadcast.Broadcasted) at broadcast.jl:863
  ...

Stacktrace:
 [1] materialize! at ./broadcast.jl:822 [inlined]
 [2] apply!(::Descent, ::Float64, ::Float64) at /Users/jonas/.julia/packages/Flux/oX9Pi/src/optimise/optimisers.jl:40
 [3] update!(::Descent, ::Float64, ::Float64) at /Users/jonas/.julia/packages/Flux/oX9Pi/src/optimise/train.jl:10
 [4] mytrain(::Float64, ::var"#38#39") at ./In[18]:6
 [5] top-level scope at In[18]:13

```

I suspect that the scalar case is failing because a Float64 is unboxed / immutable (a value) while the array is boxed / mutable (a reference).

---

<div class="post-metadata">

**Author:** ![DrPapa](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/drpapa/32/6835_2.png) [@DrPapa](https://discourse.julialang.org/u/DrPapa)\
**Post date:** [February 5, 2020, 3:48pm UTC](https://discourse.julialang.org/t/scalar-parameter-doesnt-update-in-flux-but-array-does/32548/8 "2020-02-05T15:48:27Z")

</div>

Did you ever come up with a solution for this? I am facing this problem right now.

---

<div class="post-metadata">

**Author:** ![jling](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/jling/32/212909_2.png) [@jling](https://discourse.julialang.org/u/jling)\
**Post date:** [February 5, 2020, 4:25pm UTC](https://discourse.julialang.org/t/scalar-parameter-doesnt-update-in-flux-but-array-does/32548/9 "2020-02-05T16:25:23Z")

</div>

I guess the hacky way is to just wrap the scalar as `[x]` and do this everywhere

---

<div class="post-metadata">

**Author:** ![DrPapa](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/drpapa/32/6835_2.png) [@DrPapa](https://discourse.julialang.org/u/DrPapa)\
**Post date:** [February 5, 2020, 6:20pm UTC](https://discourse.julialang.org/t/scalar-parameter-doesnt-update-in-flux-but-array-does/32548/10 "2020-02-05T18:20:59Z")

</div>

This is what I have been doing, but my main issue is that the loss function needs to either index into the array or handle broadcasting properly, making this solution even more cumbersome.

---

<div class="post-metadata">

**Author:** ![jling](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/jling/32/212909_2.png) [@jling](https://discourse.julialang.org/u/jling)\
**Post date:** [February 5, 2020, 10:06pm UTC](https://discourse.julialang.org/t/scalar-parameter-doesnt-update-in-flux-but-array-does/32548/11 "2020-02-05T22:06:30Z")

</div>

maybe `@.` macro would make life easier?