# \[ForwardDiff Question\]: Jacobian of a function that calls another function
**URL:** https://discourse.julialang.org/t/forwarddiff-question-jacobian-of-a-function-that-calls-another-function/43276
**Category:** General Usage
**Tags:** question, forwarddiff
**Created:** [July 18, 2020, 7:03am UTC](https://discourse.julialang.org/t/forwarddiff-question-jacobian-of-a-function-that-calls-another-function/43276 "2020-07-18T07:03:29Z")
**Posts on this page:** 7
**Page:** 1
### Author:  [@Jaepil\_Lee](https://discourse.julialang.org/u/Jaepil_Lee)
#### Post date: [July 18, 2020, 7:03am UTC](https://discourse.julialang.org/t/forwarddiff-question-jacobian-of-a-function-that-calls-another-function/43276/1 "2020-07-18T07:03:29Z")
I don’t understand why my vector-valued function does not produce Jacobian using ForwardDiff.jl. Could you help me how can I make it work? Ultimately I am gonna call that resulting Jacobian P and try to compute P’_P\P_(some vector). Below is the truncated work of mine that keeps what I intend to do.
```julia
using ForwardDiff, Parameters
@with_kw struct Primitives
β::Float64 = 0.96 # discount rate
end
function u(θ; M,A,E,N,B,W,j)
δ_1, δ_2 = θ
u = (δ_1^2*A) + δ_2^2*A^2+j
return u
end
function Z(θ)
Z1 = zeros(3)
β = 0.96
for a=2:4
Z1[a-1] = u(θ,M=0,A=a+0,E=1,N=0,B=0,W=0,j=1) + β*u(θ,M=0,A=a+0,E=1,N=0,B=0,W=0,j=2))
end
return Z1
end
Z([1.0,1.0]) # this yields correct Vector{Float64}
θ_init = [1.0,1.0] # Vector{Float64}
ForwardDiff.jacobian(θ->Z(θ), θ_init) # this is where I get an error
```
I get the following error message:
MethodError: no method matching Float64(::ForwardDiff.Dual{ForwardDiff.Tag{var"#30#31",Float64},Float64,2}}
I am certain that my function Z accepts Vector{Float64} as an argument and its output is also a Vector{Float64}. For function u(), it also accepts Vector{Float64} as an argument.
Thank you for your input 🙂
---
### Author:  [@mcabbott](https://discourse.julialang.org/u/mcabbott)
#### Post date: [July 18, 2020, 11:02am UTC](https://discourse.julialang.org/t/forwarddiff-question-jacobian-of-a-function-that-calls-another-function/43276/2 "2020-07-18T11:02:00Z")
ForwardDiff works by running the function with its own `Dual` number types. The error message tells you that your function is trying to convert one of these to `Float64`, which won’t work (as it would have to throw away the information you are looking for). You will generally need to make things more generic to allow this, e.g. `struct Primitives{T} β::T end` and also `zeros(typeof(θ), 3)` (which is a good idea anyway, to allow e.g. `Float32`).
Also, please check that your code runs (at least until the error in question!) on a fresh session of Julia, and write ``` before & after to quote it.
---
### Author:  [@Jaepil\_Lee](https://discourse.julialang.org/u/Jaepil_Lee)
#### Post date: [July 19, 2020, 3:05am UTC](https://discourse.julialang.org/t/forwarddiff-question-jacobian-of-a-function-that-calls-another-function/43276/3 "2020-07-19T03:05:50Z")
Thank you for your reply. I have modified the question according to your suggestion. The above script is all I have and I verified that after your suggestion, I have the error message even before ForwardDiff.jacobian:
```julia
using Parameters, ForwardDiff
@with_kw struct Primitives{T}
β::T # discount rate
end
function u(θ; M,A,E,N,B,W,j)
δ_1, δ_2 = θ
u = (δ_1^2*A) + δ_2^2*A^2+j
return u
end
function Z(θ)
Z1 = zeros(typeof(θ), 3)
@unpack β = Primitives()
for a=22:24
Z1[a-21] = (u(θ,M=0,A=a+0,E=1,N=0,B=0,W=0,j=1)+β*u(θ,M=0,A=a+0,E=1,N=0,B=0,W=0,j=2))
end
return Z1
end
Z([1.0,1.0])
θ_init = [1.0,1.0]
ForwardDiff.jacobian(θ->Z(θ), θ_init)
```
```julia
Z([1.0,1.0])
```
returns
```julia
MethodError: no method matching zero(::Type{Array{Float64,1}})
Closest candidates are:
zero(!Matched::Type{Missing}) at missing.jl:103
zero(!Matched::Type{LibGit2.GitHash}) at D:\buildbot\worker\package_win64\build\usr\share\julia\stdlib\v1.4\LibGit2\src\oid.jl:220
zero(!Matched::Type{Pkg.Resolve.VersionWeight}) at D:\buildbot\worker\package_win64\build\usr\share\julia\stdlib\v1.4\Pkg\src\Resolve\versionweights.jl:15
...
zeros(::Type{Array{Float64,1}}, ::Tuple{Int64}) at array.jl:505
zeros(::Type{Array{Float64,1}}, ::Int64) at array.jl:500
Z(::Array{Float64,1}) at experimentHouse.jl:38
top-level scope at experimentHouse.jl:45
```
Also, `ForwardDiff.jacobian(θ->Z(θ), θ_init)` returnst the following:
```julia
MethodError: no method matching zero(::Type{Array{ForwardDiff.Dual{ForwardDiff.Tag{var"#21#22",Float64},Float64,2},1}})
Closest candidates are:
zero(!Matched::Type{Missing}) at missing.jl:103
zero(!Matched::Type{LibGit2.GitHash}) at D:\buildbot\worker\package_win64\build\usr\share\julia\stdlib\v1.4\LibGit2\src\oid.jl:220
zero(!Matched::Type{Pkg.Resolve.VersionWeight}) at D:\buildbot\worker\package_win64\build\usr\share\julia\stdlib\v1.4\Pkg\src\Resolve\versionweights.jl:15
...
zeros(::Type{Array{ForwardDiff.Dual{ForwardDiff.Tag{var"#21#22",Float64},Float64,2},1}}, ::Tuple{Int64}) at array.jl:505
zeros(::Type{Array{ForwardDiff.Dual{ForwardDiff.Tag{var"#21#22",Float64},Float64,2},1}}, ::Int64) at array.jl:500
Z(::Array{ForwardDiff.Dual{ForwardDiff.Tag{var"#21#22",Float64},Float64,2},1}) at experimentHouse.jl:38
(::var"#21#22")(::Array{ForwardDiff.Dual{ForwardDiff.Tag{var"#21#22",Float64},Float64,2},1}) at experimentHouse.jl:48
vector_mode_dual_eval(::var"#21#22", ::Array{Float64,1}, ::ForwardDiff.JacobianConfig{ForwardDiff.Tag{var"#21#22",Float64},Float64,2,Array{ForwardDiff.Dual{ForwardDiff.Tag{var"#21#22",Float64},Float64,2},1}}) at apiutils.jl:37
vector_mode_jacobian(::var"#21#22", ::Array{Float64,1}, ::ForwardDiff.JacobianConfig{ForwardDiff.Tag{var"#21#22",Float64},Float64,2,Array{ForwardDiff.Dual{ForwardDiff.Tag{var"#21#22",Float64},Float64,2},1}}) at jacobian.jl:140
jacobian(::Function, ::Array{Float64,1}, ::ForwardDiff.JacobianConfig{ForwardDiff.Tag{var"#21#22",Float64},Float64,2,Array{ForwardDiff.Dual{ForwardDiff.Tag{var"#21#22",Float64},Float64,2},1}}, ::Val{true}) at jacobian.jl:17
jacobian(::Function, ::Array{Float64,1}, ::ForwardDiff.JacobianConfig{ForwardDiff.Tag{var"#21#22",Float64},Float64,2,Array{ForwardDiff.Dual{ForwardDiff.Tag{var"#21#22",Float64},Float64,2},1}}) at jacobian.jl:15
jacobian(::Function, ::Array{Float64,1}) at jacobian.jl:15
top-level scope at experimentHouse.jl:48
```
As I am new to the `Dual` number types, I am more lost here… why do we use write curly brackets in the end of `Primitives{T}`? Does it free \beta to change its types when needed?
---
### Author:  [@longemen3000](https://discourse.julialang.org/u/longemen3000)
#### Post date: [July 19, 2020, 3:36am UTC](https://discourse.julialang.org/t/forwarddiff-question-jacobian-of-a-function-that-calls-another-function/43276/4 "2020-07-19T03:36:47Z")
Change `typeof(x) ` for `eltype(x) `
---
### Author:  [@mcabbott](https://discourse.julialang.org/u/mcabbott)
#### Post date: [July 19, 2020, 6:24am UTC](https://discourse.julialang.org/t/forwarddiff-question-jacobian-of-a-function-that-calls-another-function/43276/5 "2020-07-19T06:24:49Z")
Sorry my mistake re `typeof` / `eltype`. And on reading a bit more carefully, the original `Primitives` might be fine too.
> [@Jaepil\_Lee](#):
>
> why do we use write curly brackets in the end of `Primitives{T}` ? Does it free \beta to change its types when needed?
Yes, this would allow `Primitives` to store `β` of any type, although the type (and value, since it’s not a mutable struct) are fixed for the life of that object. Making `T` part of the wrapper’s type means there’s no need for the compiler to allow for surprises upon unwrapping. But this is only essential if it’s on the path from `θ_init` to `Z1`, i.e. if you created one with input from `θ`.
---
### Author:  [@Jaepil\_Lee](https://discourse.julialang.org/u/Jaepil_Lee)
#### Post date: [July 20, 2020, 1:18am UTC](https://discourse.julialang.org/t/forwarddiff-question-jacobian-of-a-function-that-calls-another-function/43276/6 "2020-07-20T01:18:26Z")
Thank you so much for your reply. It works now 🙂
---
### Author:  [@Jaepil\_Lee](https://discourse.julialang.org/u/Jaepil_Lee)
#### Post date: [July 20, 2020, 1:41am UTC](https://discourse.julialang.org/t/forwarddiff-question-jacobian-of-a-function-that-calls-another-function/43276/7 "2020-07-20T01:41:29Z")
Thanks for the clarification!