# Assigning a user-defined function to RobotDynamics.dynamics()

**URL:** <https://discourse.julialang.org/t/assigning-a-user-defined-function-to-robotdynamics-dynamics/102842>\
**Category:** General Usage\
**Tags:** function\
**Created:** [August 15, 2023, 8:54pm UTC](https://discourse.julialang.org/t/assigning-a-user-defined-function-to-robotdynamics-dynamics/102842 "2023-08-15T20:54:40Z")\
**Posts on this page:** 4\
**Page:** 1

<div class="post-metadata">

**Author:** ![shawn\_T](https://avatars.discourse-cdn.com/v4/letter/s/858c86/32.png) [@shawn\_T](https://discourse.julialang.org/u/shawn_T)\
**Post date:** [August 15, 2023, 8:54pm UTC](https://discourse.julialang.org/t/assigning-a-user-defined-function-to-robotdynamics-dynamics/102842/1 "2023-08-15T20:54:40Z")

</div>

Hi,

I am using the RobotDynamics Julia interface to define an optimal control problem, specifically, I would like to dynamically create a function that matches the style of function that RobotDynamics.dynamics() supports.

Basically it means instead of defining the function directly in the file as below,

```julia
function RobotDynamics.dynamics!(model::MyModel, x, u)
    ẋ = [u[1], x[1] + u[1]]
    return ẋ
end

```

I have customized a similar function and stored it into the field “dynamics” in the struct below.

```julia
mutable struct Optimizer
    ...
    dynamics::Function
    ...
end

RobotDynamics.dynamics!(model::user_problem, x, u) = opt.dynamics

```

The problem is, during the optimization process, it seems that Julia incorrectly recognized my function as a type so the multiplication cannot be done, shown in the error report below. I am not sure why this is the case, so I am not sure if my method is actually doable or not.

As for the extra argument “t”, I think it is automatically transformed in the bottom code.

```julia
#in RobotDynamics\src\integration.jl
function integrate(::RK4, model, x, u, t, h)
    k1 = dynamics(model, x, u, t) * h #line 281
    k2 = dynamics(model, x + k1 / 2, u, t + h / 2) * h
    k3 = dynamics(model, x + k2 / 2, u, t + h / 2) * h
    k4 = dynamics(model, x + k3, u, t + h) * h
    x + (k1 + 2k2 + 2k3 + k4) / 6
end

MethodError: no method matching *(::typeof(My_repository_name.my_function_name), ::Float64)
at line 281

```

```julia
#in RobotDynamics.jl/src/dynamics.jl line 83
@inline dynamics(model::ContinuousDynamics, x, u, t) = dynamics(model, x, u)

```

Any suggestion on the source of the problem would help, thank you.

---

<div class="post-metadata">

**Author:** ![contradict](https://avatars.discourse-cdn.com/v4/letter/c/ac91a4/32.png) [@contradict](https://discourse.julialang.org/u/contradict)\
**Post date:** [August 15, 2023, 9:12pm UTC](https://discourse.julialang.org/t/assigning-a-user-defined-function-to-robotdynamics-dynamics/102842/2 "2023-08-15T21:12:40Z")

</div>

This line

> [@shawn\_T](#):
>
> ```julia
> RobotDynamics.dynamics!(model::user_problem, x, u) = opt.dynamics
> 
> ```

returns an unevaluated function that is independent of the arguments. You need to call `opt.dynamics` when implementing ` RobotDynamics.dynamics!`.

Try this, then you don’t need the extra function to remove the time argument.

```julia
RobotDynamics.dynamics!(model::user_problem, x, u, t) = opt.dynamics(model, x, u)

```

I’m a little worried about capturing `opt` in the closure if you need high performance, but without seeing more of your code I can’t suggest a better alternative.

---

<div class="post-metadata">

**Author:** ![shawn\_T](https://avatars.discourse-cdn.com/v4/letter/s/858c86/32.png) [@shawn\_T](https://discourse.julialang.org/u/shawn_T)\
**Post date:** [August 15, 2023, 10:21pm UTC](https://discourse.julialang.org/t/assigning-a-user-defined-function-to-robotdynamics-dynamics/102842/3 "2023-08-15T22:21:31Z")

</div>

Thank you, it is working now, though I am not sure why the original method returns a function that is unevaluated and how adding the argument list corrects it.

---

<div class="post-metadata">

**Author:** ![contradict](https://avatars.discourse-cdn.com/v4/letter/c/ac91a4/32.png) [@contradict](https://discourse.julialang.org/u/contradict)\
**Post date:** [August 15, 2023, 10:27pm UTC](https://discourse.julialang.org/t/assigning-a-user-defined-function-to-robotdynamics-dynamics/102842/4 "2023-08-15T22:27:24Z")

</div>

Another way to write these two lines is

```julia
# Equivalent to
# RobotDynamics.dynamics!(model::user_problem, x, u) = opt.dynamics
function RobotDynamics.dynamics!(model::user_problem, x, u)
    return opt.dynamics
end

```

Which returns the value `opt.dynamics`

and

```julia
# Equivalent to
# RobotDynamics.dynamics!(model::user_problem, x, u, t) = opt.dynamics(model, x, u)
function RobotDynamics.dynamics!(model::user_problem, x, u, t)
    return opt.dynamics(model, x, u)
end

```

Which returns the result of calling the function `opt.dynamics` with the provided arguments.