Order of parameters

[lungd]

I was expecting to see sys.ps in the same order as I defined p. I think this is true for the states.

using ModelingToolkit, OrdinaryDiffEq, DiffEqCallbacks

@parameters t b a
@variables x(t)
@derivatives D'~t

equ = [D(x) ~ a * b]
u0 = [x => 1.0]

p = [	
	b => 0.2,
	a => 0.1,
]
display(p)

sys = ODESystem(equ)
display(sys.ps)

2-element Array{Pair{Operation,Float64},1}:
 b => 0.2
 a => 0.1
2-element Array{Variable,1}:
 a
 b

How am I supposed to change a specific parameter inside a callback?
Let's say I want to change the last parameter (param a) at a known time.

using ModelingToolkit, OrdinaryDiffEq, DiffEqCallbacks

@parameters t b a
@variables x(t)
@derivatives D'~t

equ = [D(x) ~ a * b]

u0 = [x => 1.0]

p = [	
	b => 0.2,
	a => 0.1,
]

sys = ODESystem(equ)

tspan = (0.0, 2.0)

starttimes = [1]
change!(integrator) = integrator.p[end] = 0.3
displ(integrator) = display(integrator.p)
cb1 = PresetTimeCallback(starttimes, change!)
cb2 = PresetTimeCallback(starttimes, displ)

prob = ODEProblem(sys, u0, tspan, p)
sol = solve(prob,Tsit5(),callback=CallbackSet(cb1, cb2))

But this code changes b due to the 'wrong' order of the parameters.
I could change the callback like the following if I know the identifier of the last parameter:

function change!(integrator)
	for i=1:length(sys.ps)
		if string(sys.ps[i]) == string(a)
			integrator.p[i] = 0.3
		end
	end
end

But there is a more elegant solution for sure...

[ChrisRackauckas]

Essentially a duplicate of #318 . You can't really assume that the parameter you made is going to match the order of your map, or even that it will exist because we do a lot of symbolic transforms and such. But what we can do is give you an integrator.p[indexmap(a)] = ....

That would at least be a decent short term solution, with a longer term solution being more integrated ways to define callbacks.

[lungd]

Thank you for clarifying.
Actually, I was looking for a way to define a time-dependent input.
I had problems using a function because I was checking the value of t inside the function but that's not possible (non-boolean (Operation) used in boolean context).
Figured out, I could use a param and change it with a callback function. Then, I realized the mismatch of the last parameters.

I will create a new issue for my actual problem.

[lungd]

Duplicate of #318

[ChrisRackauckas]

I had problems using a function because I was checking the value of t inside the function but that's not possible (non-boolean (Operation) used in boolean context).

You can have p(t) be a function.