ModelingToolkit: variables in an expression and expression simplification

[cvanaret]

Hi,
I have two questions about the ModelingToolkit package:

• is there a function that determines the list of variables that occur in an expression?
  Example: x^2 + y - exp(y) contains x and y

• is there a function that simplifies an expression? After writing a linear combination of expressions in my code, a “1x - 0y” pops up, and I would like the second term to be simplified (otherwise, the result of my first question may be irrelevant )

Thanks,

Charlie

[ChrisRackauckas]

@cvanaret

is there a function that determines the list of variables that occur in an expression?
Example: x^2 + y - exp(y) contains x and y

No, we should add one.

@cvanaret

is there a function that simplifies an expression? After writing a linear combination of expressions in my code, a “1 x - 0 y” pops up, and I would like the second term to be simplified (otherwise, the result of my first question may be irrelevant )

simplify_expressions IIRC. It’s pretty bad and we actually need Rewrite.jl integration to be completed.

[cvanaret]

Got it, thanks Chris!
There’s a simplified_expr, but it doesn’t do the trick.

[ChrisRackauckas]

@cvanaret

There’s a simplified_expr , but it doesn’t do the trick.

no, that’s different. That for producing Julia Exprs.

simplify_constants

[cvanaret]

Yeah, this one simplifies the multiplicative constants (it returns x - 0).

[ChrisRackauckas]

Yeah, it needs to be better. FWIW, fixing the case you mentioned there is easy, so it might be a nice thing to add. But in general, we need a rewrite system

[cvanaret]

Hi,

I just found out that

ModelingToolkit.vars

does the trick. It returns a Set of all variables that appear in an expression.

Charlie