ModelingToolkit: how to get variable index of ODE solution as interpolating function

General Usage
1

Since the ordering of symbols is not guaranteed, the solution of ode’s from ModelingToolkit is usually accessed by name. However, sol[foo] only gives a vector (against sol.t). How to find the index of some variable foo in the solutions’s interpolating function interface sol(t)? example from doc:

using ModelingToolkit, OrdinaryDiffEq

@parameters t σ ρ β
@variables x(t) y(t) z(t)
D = Differential(t)

eqs = [D(D(x)) ~ σ*(y-x),
       D(y) ~ x*(ρ-z)-y,
       D(z) ~ x*y - β*z]

sys = ODESystem(eqs)
sys = ode_order_lowering(sys)

u0 = [D(x) => 2.0,
      x => 1.0,
      y => 0.0,
      z => 0.0]

p  = [σ => 28.0,
      ρ => 10.0,
      β => 8/3]

tspan = (0.0,100.0)
prob = ODEProblem(sys,u0,tspan,p,jac=true)
sol = solve(prob,Tsit5())

how do I get the index of, say, the index of x from e.g. sol(1.2)? Looks like the faq’s index finding applies only to parameters and not on the ODESolution object.

Thanks!

Edit: looks like sys.states gives a vector of Term, one may compare against that with
[s.f.name for s in sys.states]. It appears to be the same index given in sol(t)

2

sol(1.2,idxs=x)

1 Like
3

thanks!