Add and test errors when changing independent variable

Author: hersle

src/systems/diffeqs/basic_transformations.jl

@@ -51,10 +51,22 @@ function liouville_transform(sys::AbstractODESystem; kwargs...)
 end
 
 function change_independent_variable(sys::AbstractODESystem, iv, eq = nothing; verbose = false, simplify = true, dummies = false, kwargs...)
+    if !iscomplete(sys)
+        error("Cannot change independent variable of incomplete system $(nameof(sys))")
+    elseif isscheduled(sys)
+        error("Cannot change independent variable of structurally simplified system $(nameof(sys))")
+    end
+
+    iv = unwrap(iv)
     iv1 = get_iv(sys) # e.g. t
-    iv2name = nameof(operation(unwrap(iv))) # TODO: handle namespacing?
+
+    if !iscall(iv) || !isequal(only(arguments(iv)), iv1)
+        error("New independent variable $iv is not a function of the independent variable $iv1 of the system $(nameof(sys))")
+    end
+
+    iv2func = iv # e.g. a(t)
+    iv2name = nameof(operation(iv))
     iv2, = @independent_variables $iv2name # e.g. a
-    iv2func, = @variables $iv2name(iv1) # e.g. a(t)
     D1 = Differential(iv1)
     D2 = Differential(iv2)
 
@@ -98,8 +110,12 @@ function change_independent_variable(sys::AbstractODESystem, iv, eq = nothing; v
         end
     end
 
-    isnothing(div2_div1) && error("No equation for $D1($iv2func) was specified.")
     verbose && println("Found $div2 = $div2_div1")
+    if isnothing(div2_div1)
+        error("No equation for $D1($iv2func) was specified.")
+    elseif isequal(div2_div1, 0)
+        error("Cannot change independent variable from $iv1 to $iv2 with singular transformation $(div2 ~ div2_div1).")
+    end
 
     # 3) Add equations for dummy variables
     div1_div2 = 1 / div2_div1

test/basic_transformations.jl

@@ -99,3 +99,17 @@ end
     sol = solve(prob, Tsit5(); reltol = 1e-5)
     @test all(isapprox.(sol[Mx.y], sol[Mx.x - g*(Mx.x/v)^2/2]; atol = 1e-10)) # compare to analytical solution (x(t) = v*t, y(t) = v*t - g*t^2/2)
 end
+
+@testset "Change independent variable (errors)" begin
+    @variables x(t) y z(y) w(t) v(t)
+    M = ODESystem([D(x) ~ 0, v ~ x], t; name = :M)
+    @test_throws "incomplete" ModelingToolkit.change_independent_variable(M, M.x)
+    M = complete(M)
+    @test_throws "singular" ModelingToolkit.change_independent_variable(M, M.x)
+    @test_throws "structurally simplified" ModelingToolkit.change_independent_variable(structural_simplify(M), y)
+    @test_throws "No equation" ModelingToolkit.change_independent_variable(M, w)
+    @test_throws "No equation" ModelingToolkit.change_independent_variable(M, v)
+    @test_throws "not a function of the independent variable" ModelingToolkit.change_independent_variable(M, y)
+    @test_throws "not a function of the independent variable" ModelingToolkit.change_independent_variable(M, z)
+    M = ODESystem([D(x) ~ t], t; name = :M) |> complete
+end