Modeling Toolkit (v8.76) - Change parameters by reference with remake

Specific Domains Modelling & Simulations
1

Hello all. I have a question about using remake on an ODEproblem constructed using modeling toolkit (MTK). I’ll note that I’m using MTK v8.76.0. I am aware that v9 introduces new structure referencing (Frequently Asked Questions · ModelingToolkit.jl), but it sounds like this is experimental so I would prefer not to rely on it yet.

The basic goal is to create and ODEproblem from a MTK model, and then remake the problem with a subset of the parameters changed. Specifically, I would like to be able to change the values of a subset of parameters by named reference rather than by index. I found the following syntax in the SciML docs (Optimizing through an ODE solve and re-creating MTK Problems · ModelingToolkit.jl) (full code below).

prob2 = remake(prob; p = [:r => 42.0]);

However this does not work. The new problem does not have the new parameter value. I can do this all using extracted parameter indexes.

p = prob.p;
p[r_ind] = 42.0;
prob3 = remake(prob; p = p);
r_val = prob3.p[r_ind]

This is however really clunky and more prone to introducing errors.

This should be a standard issue, and I am pulling the syntax directly from the SciML docs. The docs are really well done so I’m assuming I have either a typo or a mis-understanding of how the structures are handled in the background.

Any thoughts on where I am going wrong or suggestions for a better way to do this?

FULL CODE BELOW

using DifferentialEquations, ModelingToolkit, Plots

## Construct MTK model.
LV = function(; name)
	@variables t, [description = "time"]
	Dt = Differential(t)

    pars = @parameters begin
        r = 1.0, [description = "growth rate"]
        K = 2.0, [description = "carrying capacity"]
    end

    vars = @variables begin
        x(t) = 0.1
    end

    eqs = [Dt(x) ~ r*x*(1.0-x/K)]

    ODESystem(eqs, t, vars, pars; name=name)
end

## Instantiate model and ode problem
@mtkbuild LV1 = LV();
prob = ODEProblem(LV1, [], (0.0, 20.0), []);
sol = solve(prob, Tsit5());

##### Change one parameter and return with new value
# This does NOT work. It does not change the value of 'r'.
r_ind = ModelingToolkit.parameter_index(LV1, :r);
r_val = prob.p[r_ind]
prob2 = remake(prob; p = [:r => 42.0]);
r_val = prob2.p[r_ind]

##### Try again, but with direct indexing. This does work, but is clunky.
p = prob.p;
p[r_ind] = 42.0;
prob3 = remake(prob; p = p);
r_val = prob3.p[r_ind]
2

I’m not sure what else to say other than these are almost precisely the breaking changes that led to ModelingToolkit v9. The reason for the initialization system, missing values for parameter equations, and the new SymbolicIndexingInterface.jl were made to solve this issue. There won’t be a backporting of that to MTK v8 because the breaking changes that were made were to facilitate making these features and thus required for solving these problems. I wish I had an easier answer but I don’t think it can be robustly done on v8.

But also, what is the issue you are having with the update? That may be the simpler thing to solve.

3

Thank you for the response. I’m new to MTK and so haven’t seen the progress and did not realize the extent to which structure access was new to v9. Reasons for staying on v8 were 1) organizational and 2) documentation made some of the v9 structure management sound experimental.

Thank you again for the response.