[Feature]: Count how many variables are in a ComposableExpression

[Moelf]

Feature Request

Sometimes when looking at the result in HoF, I'm interested in selecting the first equation that used "variable" in both component:

In this case, I'd like to select the equation with complexity = 10.

I thought I just need to check .constant, but then I found this counter-example:

So I checked the code base:

SymbolicRegression.jl/src/ComposableExpression.jl

Lines 113 to 115 in ddb7efb

	function DE.count_scalar_constants(ex::AbstractComposableExpression)
	return DE.count_scalar_constants(convert(Expression, ex))
	end

but looks like there's no way to count the non-constant terms

[MilesCranmer]

A lot of the Base operations on collections are overloaded to do tree traversal on the inner Node type (it's a binary tree). So you can solve this with:

ex::ComposableExpression
tree = ex.tree
target_feature = 1
has_variable = any(node -> node.degree == 0 && node.feature == target_feature, tree)

[MilesCranmer]

See https://ai.damtp.cam.ac.uk/dynamicexpressions/dev/examples/base_operations/ for more examples

[MilesCranmer]

I'm assuming this is fixed; feel free to re-open if not!

[Moelf]

that doesn't quite work, clearly the base_func here is a constant

[Moelf]

maybe it should have been:

node.degree != 0 && node.feature == 1

?

[Moelf]

then it doesn't work on nested equations:

[MilesCranmer]

Ah, sorry, I need(ed) coffee!

It needs to first check !node.constant before checking node.feature. So the correct call would be:

has_variable = any(node -> node.degree == 0 && !node.constant && node.feature == target_feature, tree)

A specific method for checking feature equality would no doubt be a good idea.

I suppose you can also use node == Node{Float64}(feature=target_feature) and it will handle this automatically. (Though might want to create the checked node once, since it will allocate on the heap)

[Moelf]

ahh, I see how that works now, yeah that works.

If there's any place where something like this would be useful I'm happy to add a few utility functions, but I'm not sure what to call it. (something like is_a_function_of(tree, target_variable)?)