Frequently Asked Questions

Getting the index for a symbol

Since ordering of symbols is not guaranteed after symbolic transformations, one should normally refer to values by their name. For example, sol[lorenz.x] from the solution. But what if you need to get the index? The following helper function will do the trick:

indexof(sym,syms) = findfirst(isequal(sym),syms)
indexof(σ,parameters(sys))

Transforming value maps to arrays

ModelingToolkit.jl allows (and recommends) input maps like [x => 2.0, y => 3.0] because symbol ordering is not guaranteed. However, what if you want to get the lowered array? You can use the internal function varmap_to_vars. For example:

pnew = varmap_to_vars([β=>3.0, c=>10.0, γ=>2.0],parameters(sys))

How do I handle if statements in my symbolic forms?

For statements that are in the if then else form, use IfElse.ifelse from the IfElse.jl package to represent the code in a functional form. For handling direct if statements, you can use equivalent boolean mathematical expressions. For example if x > 0 ... can be implementated as just (x > 0) *, where if x <= 0 then the boolean will evaluate to 0 and thus the term will be excluded from the model.

ERROR: TypeError: non-boolean (Num) used in boolean context?

If you see the error:

ERROR: TypeError: non-boolean (Num) used in boolean context

then it's likely you are trying to trace through a function which cannot be directly represented in Julia symbols. The techniques to handle this problem, such as @register, are described in detail in the Symbolics.jl documentation.