AayushSabharwal
commented
4 weeks ago While the error message is a little misleading and should be fixed, this isn't a bug. In the first ("not working") system, S_sep_out2 is an unknown and needs an initial value, which can't be calculated given just the defaults and observed in the system:
julia> sys_simplified = structural_simplify(sys)
Model sys with 1 equations
Unknowns (1):
S_sep_out2(t)
Parameters (2):
S_in [defaults to 1]
q_in [defaults to 1]
julia> observed(sys_simplified)
5-element Vector{Equation}:
S(t) ~ q_in
q(t) ~ S_in
q_sep_out2(t) ~ 0.5
S_sep_out1(t) ~ S_sep_out2(t)
q_sep_out1(t) ~ -q_sep_out2(t) + q(t)Since S_sep_out2 doesn't have a guess, it can't be solved for during initialization either. Providing the guess works:
julia> prob = ODEProblem(sys_simplified, [], (0, 1); guesses = [S_sep_out2 => 0.1])
ODEProblem with uType Vector{Float64} and tType Int64. In-place: true
timespan: (0, 1)
u0: 1-element Vector{Float64}:
0.0
julia> sol = solve(prob, Rodas5())
┌ Warning: Rosenbrock methods on equations without differential states do not bound the error on interpolations.
└ @ OrdinaryDiffEqCore ~/Julia/SciML/OrdinaryDiffEq.jl/lib/OrdinaryDiffEqCore/src/solve.jl:108
retcode: Success
Interpolation: specialized 4rd order "free" stiffness-aware interpolation
t: 8-element Vector{Float64}:
0.0
1.0e-6
1.1e-5
0.00011099999999999999
0.0011109999999999998
0.011110999999999996
0.11111099999999996
1.0
u: 8-element Vector{Vector{Float64}}:
[1.0]
[1.0]
[1.0]
[1.0]
[1.0]
[1.0]
[1.0]
[1.0]The solution is constant because this is a system of nonlinear equations, not a DAE/ODE; there are no differential equations.
As for your "working" example, structural_simplify can analytically solve the system and thus you end up with one that has zero unknowns:
julia> sys_simplified = structural_simplify(sys)
Model sys with 0 equations
Unknowns (0):
Parameters (2):
S_in [defaults to 1]
q_in [defaults to 1]And creating the ODEProblem works, because there are no unknowns that need an initial value.
wenflo
ChrisRackauckas
Describe the bug 🐞
Building an ODEProblem from an ODESystem throws an error indicating missing variables, that were optimized away during
structural_simplify. By manually combining two of the equations, I can get it to work it for the MWE below.Expected behavior
Building the ODEProblem should work no matter how the equations are specified.
Minimal Reproducible Example 👇
Failing version:
Working version:
Error & Stacktrace ⚠️
Environment (please complete the following information):
using Pkg; Pkg.status()using Pkg; Pkg.status(; mode = PKGMODE_MANIFEST)versioninfo()Additional context
I'm currently working on a library building on top of ModelingToolkit which aims to simulate biochemical reactions (especially for wastewater treatment). Applying it to one of these plants, this error occurred (in a much more complex model). Doing the rewriting myself would not be feasible due to the complexity of the overall model.