Merge pull request #2631 from AayushSabharwal/as/var-discovery

feat: allow creating NonlinearSystem without specifying unknowns/parameters

Author: ChrisRackauckas

src/systems/nonlinear/nonlinearsystem.jl

@@ -153,6 +153,32 @@ function NonlinearSystem(eqs, unknowns, ps;
         connector_type, metadata, gui_metadata, checks = checks)
 end
 
+function NonlinearSystem(eqs; kwargs...)
+    eqs = collect(eqs)
+    allunknowns = OrderedSet()
+    ps = OrderedSet()
+    for eq in eqs
+        collect_vars!(allunknowns, ps, eq.lhs, nothing)
+        collect_vars!(allunknowns, ps, eq.rhs, nothing)
+    end
+    new_ps = OrderedSet()
+    for p in ps
+        if istree(p) && operation(p) === getindex
+            par = arguments(p)[begin]
+            if Symbolics.shape(Symbolics.unwrap(par)) !== Symbolics.Unknown() &&
+               all(par[i] in ps for i in eachindex(par))
+                push!(new_ps, par)
+            else
+                push!(new_ps, p)
+            end
+        else
+            push!(new_ps, p)
+        end
+    end
+
+    return NonlinearSystem(eqs, collect(allunknowns), collect(new_ps); kwargs...)
+end
+
 function calculate_jacobian(sys::NonlinearSystem; sparse = false, simplify = false)
     cache = get_jac(sys)[]
     if cache isa Tuple && cache[2] == (sparse, simplify)

test/nonlinearsystem.jl

@@ -257,3 +257,12 @@ end
     sol = solve(prob)
     @test_nowarn sol[unknowns(ns)]
 end
+
+# Issue#2625
+@parameters p d
+@variables X(t)
+alg_eqs = [0 ~ p - d * X]
+
+sys = @test_nowarn NonlinearSystem(alg_eqs; name = :name)
+@test isequal(only(unknowns(sys)), X)
+@test all(isequal.(parameters(sys), [p, d]))