add get_variables

didnt commit uniqueness

add in substitution logic

add subs

Update src/utils.jl

Co-Authored-By: Christopher Rackauckas <accounts@chrisrackauckas.com>
Update src/utils.jl

Co-Authored-By: Christopher Rackauckas <accounts@chrisrackauckas.com>
add generic method for is_singleton [ci skip]

clean up get_variables [ci skip]

clean up substitution

Update src/utils.jl

Co-Authored-By: Christopher Rackauckas <accounts@chrisrackauckas.com>
fix up get variable and substitution


fix up getting and substitution

Author: ChrisRackauckas

src/utils.jl

@@ -144,3 +144,29 @@ function vars!(vars, O)
 
     return vars
 end
+
+# variable extraction
+is_singleton(e) = false
+is_singleton(e::Operation) = e.op isa Variable
+
+get_variables(e::Constant, vars = Operation[]) = vars
+function get_variables(e::Operation, vars = Operation[])
+    if is_singleton(e)
+        push!(vars, e)
+    else
+        foreach(x -> get_variables(x, vars), e.args)
+    end
+    return unique(vars)
+end
+
+# variable substitution
+substitute_expr!(expr::Constant, s::Pair{Operation, Operation}) = nothing
+function substitute_expr!(expr::Operation, s::Pair{Operation, Operation})
+    if !is_singleton(expr)
+        expr.args .= replace(expr.args, s)
+        for arg in expr.args
+            substitute_expr!(arg, s)
+        end
+    end
+    return nothing
+end

test/variable_utils.jl

@@ -0,0 +1,12 @@
+using ModelingToolkit, Test
+@parameters α β δ
+expr = (((1 / β - 1) + δ) / α) ^ (1 / (α - 1))
+ref = [β, δ, α]
+sol = ModelingToolkit.get_variables(expr)
+@test all([sol[i].op.name == ref[i].op.name for i in 1:3])
+
+@parameters γ
+s = α => γ
+new = (((1 / β - 1) + δ) / γ) ^ (1 / (γ - 1))
+ModelingToolkit.substitute_expr!(expr, s)
+@test isequal(expr, new)