# Getting gradients for DiffEq with single derivative

Hey All,

I’ve been meaning to calculate the gradient of an ODEProblem using Zygote, OrdinaryDiffEq, and DiffEqSensitivity. I’ve been running into a wide scala of errors along the way but found a way to do it.

So initially I follow the docs here: https://docs.sciml.ai/latest/analysis/sensitivity/
This is my code:

``````using DiffEqSensitivity, OrdinaryDiffEq, Zygote

function ode(u, p, t)
CL, V, F, kₐ, dose = p
k = CL / V
return (F * kₐ * dose * exp(-kₐ * t)) - (k * u)
end

p = [1., 10., .5, 1., 100.]
u0 = 0.
t = [0.5, 1., 2., 3., 6., 9., 12., 24., 36., 48., 72.]
y = [0., 1.9, 3.3, 6.6, 9.1, 10.8, 8.6, 5.6, 4, 2.7, 0.8]

prob = ODEProblem(fiip, u0, (0.0, 150.0), p)
sol = solve(prob, Tsit5())

function predict(p)
_prob = remake(prob, p=p)
return sum(abs2, ŷ .- y) # loss function
end

``````

This returns the error: `ERROR: LoadError: MethodError: no method matching similar(::Float64, ::Int64)`
The error and the stack trace aren’t very helpful to figure out what causes this error.

In the end I found it was because u0 in this case was a Float64, instead of a Array{Float64}.
Changing the line `u0 = [0.]` and the ode function to

``````function ode(u, p, t)
CL, V, F, kₐ, dose = p
k = CL / V
return (F * kₐ * dose * exp(-kₐ * t)) .- (k * u) # - to .-
end
``````

the gradients actually get calculated.

Why does u0 have to be an Array? This is not really documented in the doc I listed so it might nice to either add that or add a more understandable error message in the ODEProblem function when gradient is called on it.

Also I do not like that I have to change my ode function (and in the process supporting u0 with multiple dimensions), so I figured I could add a line like `u = first(u)` but that also breaks the script with the following error:
`ERROR: LoadError: MethodError: Cannot `convert` an object of type Float64 to an object of type Array{Float64,1}`

Does this mean only u0 is an array and other instances of u are just normal Floats? Again the question, why does u0 need to be an Array if that is the case?

Maybe someone is willing to discuss this?

DifferentialEquations.jl allows for `u0` to be a scalar, but its adjoint definition currently requires that `u0` is some kind of mutable object. This is issue https://github.com/SciML/DiffEqSensitivity.jl/issues/113 . FWIW, if you’re at the size where you’re using scalar differential equations, you don’t want to use adjoints and should instead use ForwardDiff for the gradient.

Hey Chris,

Thanks for your reply! I’ll look into ForwardDiff. Does DiffEqParamEstim use this for parameter estimation?

The system is all automatic (see https://docs.sciml.ai/latest/analysis/sensitivity/), so if you use an optimization method that uses ForwardDiff (like Optim), then it’ll use ForwardDiff. If you use an optimization method that uses a reverse-mode AD (Flux), then it’ll use an adjoint method.

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