ModelingToolkit - component defaults

[ChrisRackauckas]

@BLI

OK – not a big problem [although somewhat unsystematic?]

I think we want all people to be putting it on the symbols, but defaults was exposed from earlier versions. We need to figure out whether it should stay public API exposed.

@BLI

Question 1: if I specify defaults, do I need to specify defaults for all states and parameters, or is it possible to specify defaults for a subset? [And… if the system is not in state space form but needs to be “reduced” to state space form by index reduction, etc. – how can I know how many states there are? Normally, I’d have to do structural_simplify first to see the names of the states??]

Question 2: how do I specify parameters that differ from default values? I don’t want to specify all parameters, but just those that are different.

• Do I simply add parameters or initial values that differ from the defaults?
• Or do I need to specify the complete set of values? [Which would reduce the advantage of specifying defaults.]

I think these go hand-in-hand. If you specify a subset, then at construction time you need to specify the values or you will get an error. For example, if your components are missing a parameter r, then at construction time you’ll be told component1.r and component2.r are not defined etc. (that’s not the exact error message, but it’ll point it out like that). And then you need to do:

ODEProblem(unitstep_fol_factory(name=:fol),[],(0.0,5.0),[component1.r => 2.0, component2.r => 3.0])

to fill in the missing values. So basically, you only need to know values at numerical problem construction time, but at numerical problem construction time you do need to make sure that a whole consistent set of u0 and p are given, otherwise it will error and tell you which values are missing.

I think the docs are a little confusing here because it’s still a little in flux. I think that in general a u0 and p map are not both needed, so this should probably just be:

@named fol = unitstep_fol_factory()
ODEProblem(fol,[component1.r => 2.0, component2.r => 3.0],(0.0,5.0))

What’s a state vs parameter vs observed variable is is kind of an internal solver detail since structural_simplify can change the designation on variables as is required for performance. It’s probably about time to clean that up.