# ForwardDiff: "No Method Matching" Error

**URL:** <https://discourse.julialang.org/t/forwarddiff-no-method-matching-error/53311>\
**Category:** Numerics\
**Tags:** error, forwarddiff\
**Created:** [January 13, 2021, 11:59pm UTC](https://discourse.julialang.org/t/forwarddiff-no-method-matching-error/53311 "2021-01-13T23:59:14Z")\
**Posts on this page:** 10\
**Page:** 1

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**Author:** ![Michael\_Barmann](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/michael_barmann/32/20389_2.png) [@Michael\_Barmann](https://discourse.julialang.org/u/Michael_Barmann)\
**Post date:** [January 13, 2021, 11:59pm UTC](https://discourse.julialang.org/t/forwarddiff-no-method-matching-error/53311/1 "2021-01-13T23:59:14Z")

</div>

I’m testing out the ForwardDiff package for the first time, but unfortunately I’m getting a “No Method Matching” error that I can’t figure out. I was successful in making the following test example by mimicking the one in the documentation ([https://github.com/JuliaDiff/ForwardDiff.jl](https://github.com/JuliaDiff/ForwardDiff.jl)):

```julia
f(x::Vector) = 4*(x[1])^2 + 2*(x[2])^3
x = [2.0,3.0]
grad(y) = ForwardDiff.gradient(f, y)
println(grad(x))

```

which gave [16.0, 54.0], as expected. However, when I tried this on a more complicated objective function called `objective`, I got the following error:

```julia
MethodError: no method matching Float64(::ForwardDiff.Dual{ForwardDiff.Tag{var"#objective#282"{typeof(One_Age_Model_1eta),typeof(f_ICs),typeof(norm1),DataFrame,Array{String,1},Array{String,1},Array{Float64,1},Array{Union{Missing, Float64},1},Array{Union{Missing, Float64},1},Array{Float64,1},Array{Float64,1},Array{Int64,1},Array{Int64,1},Array{Int64,1},Array{Int64,1},Int64,Int64,Int64},Float64},Float64,10})
Closest candidates are:
  Float64(::Real, !Matched::RoundingMode) where T<:AbstractFloat at rounding.jl:200
  Float64(::T) where T<:Number at boot.jl:716
  Float64(!Matched::Irrational{:ℯ}) at irrationals.jl:189

```

The objective function is fairly long, so I’ll just give an outline of my code:

```julia
function objective(x::Vector)

   #The vector x contains a bunch of parameters which are used 
   #to solve an ODE system. (There are also a bunch of other objects 
   #needed to evaluate the function which are defined just before the
   #function definition.) 
   #The ODE solution is compared against some data, and a residual
   #vector is calculated by comparing the observed data with the 
   #ODE solution at a set of discrete points.
   #The function returns an L^2 norm of the residual vector.

    return norm
end

#Defining a gradient function
grad(y) = ForwardDiff.gradient(objective,y)

#Calling the gradient function gives the error shown earlier:
println(grad(x0))

```

Here, `x0` is the input vector I’m using to test. I’ve confirmed that the objective function is working, and `objective(x0)` gives the expected output. So where am I going wrong here? Any help would be greatly appreciated.

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<div class="post-metadata">

**Author:** ![Michael\_Barmann](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/michael_barmann/32/20389_2.png) [@Michael\_Barmann](https://discourse.julialang.org/u/Michael_Barmann)\
**Post date:** [January 14, 2021, 12:34am UTC](https://discourse.julialang.org/t/forwarddiff-no-method-matching-error/53311/3 "2021-01-14T00:34:56Z")

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Hi, thanks for your reply. I did try removing `x::Vector` but I still got the same error. Also, I just updated my post to provide a bit more detail.

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<div class="post-metadata">

**Author:** ![jlchan](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/jlchan/32/10958_2.png) [@jlchan](https://discourse.julialang.org/u/jlchan)\
**Post date:** [January 14, 2021, 12:36am UTC](https://discourse.julialang.org/t/forwarddiff-no-method-matching-error/53311/4 "2021-01-14T00:36:58Z")

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Sorry - misread your question. Let me try the new description.

Can you create a similar-looking function as a MWE?

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**Author:** ![marius311](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/marius311/32/3953_2.png) [@marius311](https://discourse.julialang.org/u/marius311)\
**Post date:** [January 14, 2021, 12:45am UTC](https://discourse.julialang.org/t/forwarddiff-no-method-matching-error/53311/5 "2021-01-14T00:45:03Z")

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The problem is probably that somewhere you are trying to cast something to Float64, e.g. this gives the same error,

```julia
ForwardDiff.derivative(x -> Float64(x)^2, 2)

```

For ForwardDiff to work you need to have your code written so any generic Real can pass through it. This might be as easy as just removing the type annotations / explicit type conversions, or maybe parameterizing your functions with `T <: Real` where needed.

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<div class="post-metadata">

**Author:** ![Michael\_Barmann](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/michael_barmann/32/20389_2.png) [@Michael\_Barmann](https://discourse.julialang.org/u/Michael_Barmann)\
**Post date:** [January 14, 2021, 12:52am UTC](https://discourse.julialang.org/t/forwarddiff-no-method-matching-error/53311/6 "2021-01-14T00:52:58Z")

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Thanks for the info and suggestion! Based on the error message that seems reasonable. I’ll try this out and report back tomorrow when I get a chance.

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<div class="post-metadata">

**Author:** ![Michael\_Barmann](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/michael_barmann/32/20389_2.png) [@Michael\_Barmann](https://discourse.julialang.org/u/Michael_Barmann)\
**Post date:** [January 14, 2021, 12:53am UTC](https://discourse.julialang.org/t/forwarddiff-no-method-matching-error/53311/7 "2021-01-14T00:53:47Z")

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First I’m going to give @marius311 's suggestion a shot, and failing that I’ll try to come up with a MWE…

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<div class="post-metadata">

**Author:** ![Michael\_Barmann](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/michael_barmann/32/20389_2.png) [@Michael\_Barmann](https://discourse.julialang.org/u/Michael_Barmann)\
**Post date:** [January 14, 2021, 8:11pm UTC](https://discourse.julialang.org/t/forwarddiff-no-method-matching-error/53311/8 "2021-01-14T20:11:29Z")

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Thank you @marius311 for the suggestion! I just did a Find + Replace and changed all the `Float64`’s in that cell to `Real`, and it seemed to work. I do have a question about this: Does `ForwardDiff` require that _all_ arithmetic operations involve only objects of type `Real`?

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<div class="post-metadata">

**Author:** ![marius311](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/marius311/32/3953_2.png) [@marius311](https://discourse.julialang.org/u/marius311)\
**Post date:** [January 14, 2021, 10:04pm UTC](https://discourse.julialang.org/t/forwarddiff-no-method-matching-error/53311/9 "2021-01-14T22:04:47Z")

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Pretty much, yes, or arrays of Reals, see [Limitations of ForwardDiff · ForwardDiff](http://www.juliadiff.org/ForwardDiff.jl/stable/user/limitations/)

If you need gradients through code which is not set up like this, Zygote would be the other thing to look at (currently only reverse-mode though, and more overhead, but very powerful and can do pretty much any code).

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<div class="post-metadata">

**Author:** ![Michael\_Barmann](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/michael_barmann/32/20389_2.png) [@Michael\_Barmann](https://discourse.julialang.org/u/Michael_Barmann)\
**Post date:** [January 14, 2021, 11:23pm UTC](https://discourse.julialang.org/t/forwarddiff-no-method-matching-error/53311/10 "2021-01-14T23:23:51Z")

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Ok, thanks for the info. My intention is to use ForwardDiff to supply the gradient to an optimization algorithm, which will be used to minimize `objective()`. The optimization algorithms I’m planning to try are wrappers of C code, so I guess I’ll find out whether ForwardDiff will work with that. All of the code to evaluate `objective()` is pure Julia code, so hopefully `ForwardDiff` can handle this…

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<div class="post-metadata">

**Author:** ![dpsanders](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/dpsanders/32/3573_2.png) [@dpsanders](https://discourse.julialang.org/u/dpsanders)\
**Post date:** [January 15, 2021, 12:19am UTC](https://discourse.julialang.org/t/forwarddiff-no-method-matching-error/53311/11 "2021-01-15T00:19:25Z")

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Why don’t you use optimization algorithms written in Julia instead, eg Optim.jl?