Dim 0 control

ext/plot_utils.jl

@@ -55,6 +55,8 @@ Determine which components should be plotted based on the `description` and styl
 # Notes
 - Duals are only plotted if `sol` contains path constraint dual variables.
 - A style must not be `:none` for the component to be included.
+- Control is **never** plotted when `control_dimension(sol) == 0`, even if `:control` is
+  listed in `description`. This supports solutions of problems without a control input.
 
 # Example
 ```julia-repl
@@ -73,7 +75,7 @@ function do_plot(
 )
     do_plot_state = :state ∈ description && state_style != :none
     do_plot_costate = :costate ∈ description && costate_style != :none
-    do_plot_control = :control ∈ description && control_style != :none
+    do_plot_control = :control ∈ description && control_style != :none && CTModels.control_dimension(sol) > 0
     ocp = CTModels.model(sol)
     do_plot_path =
         :path ∈ description &&

src/Display/print.jl

@@ -53,6 +53,13 @@ Print the mathematical definition of an optimal control problem.
 Displays the problem in standard mathematical notation with objective,
 dynamics, and constraints.
 
+When `u_dim == 0` (no control input), all control-dependent parts of the
+output are suppressed:
+- The objective is rendered as `J(x, v)` instead of `J(x, u, v)`.
+- Dynamics arguments omit the control: `f(t, x, v)` instead of `f(t, x, u, v)`.
+- The "where" clause lists only `x` (and `v` if variable-dependent).
+- Box constraints on control are not listed.
+
 # Returns
 
 - `Bool`: `true` if something was printed.
@@ -91,10 +98,11 @@ function __print_mathematical_definition(
     # args
     t_ = is_time_dependent ? t_name * ", " : ""
     _v = is_variable_dependent ? ", " * v_name : ""
+    _u = u_dim > 0 ? ", " * u_name * "(" * t_name * ")" : ""
 
     # other names
     bounds_args_names = x_name * "(" * t0_name * "), " * x_name * "(" * tf_name * ")" * _v
-    mixed_args_names = t_ * x_name * "(" * t_name * "), " * u_name * "(" * t_name * ")" * _v
+    mixed_args_names = t_ * x_name * "(" * t_name * ")" * _u * _v
     state_args_names = x_name * "(" * t_name * ")"
     control_args_names = u_name * "(" * t_name * ")"
     variable_args_names = v_name
@@ -112,7 +120,9 @@ function __print_mathematical_definition(
 
     # J
     printstyled(io, "    minimize  "; color=:blue)
-    print(io, "J(" * x_name * ", " * u_name * _v * ") = ")
+    # Only include control in objective if u_dim > 0
+    u_in_obj = u_dim > 0 ? ", " * u_name : ""
+    print(io, "J(" * x_name * u_in_obj * _v * ") = ")
 
     # Mayer
     has_a_mayer_cost && print(io, "g(" * bounds_args_names * ")")
@@ -205,22 +215,26 @@ function __print_mathematical_definition(
     end
     x_name_space *= " ∈ " * x_space
 
-    # control name and space
-    if u_dim == 1
-        u_name_space = u_name * "(" * t_name * ")"
-    else
-        u_name_space = u_name * "(" * t_name * ")"
-        if ui_names != [u_name * CTBase.ctindices(i) for i in range(1, u_dim)]
-            u_name_space *= " = ("
-            for i in 1:u_dim
-                u_name_space *= ui_names[i] * "(" * t_name * ")"
-                i < u_dim && (u_name_space *= ", ")
+    # control name and space (only if u_dim > 0)
+    u_name_space = ""
+    if u_dim > 0
+        if u_dim == 1
+            u_name_space = u_name * "(" * t_name * ")"
+        else
+            u_name_space = u_name * "(" * t_name * ")"
+            if ui_names != [u_name * CTBase.ctindices(i) for i in range(1, u_dim)]
+                u_name_space *= " = ("
+                for i in 1:u_dim
+                    u_name_space *= ui_names[i] * "(" * t_name * ")"
+                    i < u_dim && (u_name_space *= ", ")
+                end
+                u_name_space *= ")"
             end
-            u_name_space *= ")"
         end
+        u_name_space *= " ∈ " * u_space
     end
-    u_name_space *= " ∈ " * u_space
 
+    # Build the "where" clause based on what's present
     if is_variable_dependent
         # space
         v_space = "R" * (v_dim == 1 ? "" : CTBase.ctupperscripts(v_dim))
@@ -239,13 +253,21 @@ function __print_mathematical_definition(
             end
         end
         v_name_space *= " ∈ " * v_space
-        # print
-        print(
-            io, "    where ", x_name_space, ", ", u_name_space, " and ", v_name_space, ".\n"
-        )
+        # print with or without control
+        if u_dim > 0
+            print(
+                io, "    where ", x_name_space, ", ", u_name_space, " and ", v_name_space, ".\n"
+            )
+        else
+            print(io, "    where ", x_name_space, " and ", v_name_space, ".\n")
+        end
     else
-        # print
-        print(io, "    where ", x_name_space, " and ", u_name_space, ".\n")
+        # print with or without control
+        if u_dim > 0
+            print(io, "    where ", x_name_space, " and ", u_name_space, ".\n")
+        else
+            print(io, "    where ", x_name_space, ".\n")
+        end
     end
     return true
 end

src/Init/control.jl

@@ -17,7 +17,17 @@ Throws `Exceptions.IncorrectArgument` if the control dimension is not 1.
 """
 function initial_control(ocp::AbstractModel, control::Real)
     dim = control_dimension(ocp)
-    if dim == 1
+    if dim == 0
+        throw(
+            Exceptions.IncorrectArgument(
+                "Initial control dimension mismatch";
+                got="scalar value",
+                expected="no control (dimension 0)",
+                suggestion="Remove the control argument or set control=nothing",
+                context="initial_control with scalar input for zero-dimensional control",
+            ),
+        )
+    elseif dim == 1
         return t -> control
     else
         throw(
@@ -41,7 +51,17 @@ Throws `Exceptions.IncorrectArgument` if the vector length does not match the co
 """
 function initial_control(ocp::AbstractModel, control::Vector{<:Real})
     dim = control_dimension(ocp)
-    if length(control) != dim
+    if dim == 0 && !isempty(control)
+        throw(
+            Exceptions.IncorrectArgument(
+                "Initial control dimension mismatch";
+                got="vector of length $(length(control))",
+                expected="no control (dimension 0)",
+                suggestion="Remove the control argument or set control=nothing",
+                context="initial_control with vector input for zero-dimensional control",
+            ),
+        )
+    elseif length(control) != dim
         throw(
             Exceptions.IncorrectArgument(
                 "Initial control dimension mismatch";
@@ -60,11 +80,14 @@ $(TYPEDSIGNATURES)
 
 Return a default control initialisation function when no control is provided.
 
-Returns a constant function yielding `0.1` (scalar) or `fill(0.1, dim)` (vector).
+Returns a constant function yielding `Float64[]` (empty) if `dim == 0`, 
+`0.1` (scalar) if `dim == 1`, or `fill(0.1, dim)` (vector) otherwise.
 """
 function initial_control(ocp::AbstractModel, ::Nothing)
     dim = control_dimension(ocp)
-    if dim == 1
+    if dim == 0
+        return t -> Float64[]
+    elseif dim == 1
         return t -> 0.1
     else
         return t -> fill(0.1, dim)

src/OCP/Building/model.jl

@@ -258,15 +258,33 @@ $(TYPEDSIGNATURES)
 
 Converts a mutable `PreModel` into an immutable `Model`.
 
-This function finalizes a pre-defined optimal control problem (`PreModel`) by verifying that all necessary components (times, state, control, dynamics) are set. It then constructs a `Model` instance, incorporating optional components like objective and constraints if they are defined.
+This function finalizes a pre-defined optimal control problem (`PreModel`) by verifying that all
+necessary components (times, state, dynamics, objective) are set. It then constructs a `Model`
+instance, incorporating optional components like control, variable, and constraints.
+
+!!! note
+    Control is **optional**: calling `control!` is not required. When omitted, the model is
+    built with `control_dimension == 0` (an `EmptyControlModel`). This is useful for problems
+    where the dynamics depend only on the state, such as pure state-space systems.
 
 # Arguments
 - `pre_ocp::PreModel`: The pre-defined optimal control problem to be finalized.
 
 # Returns
 - `Model`: A fully constructed model ready for solving.
 
-# Example
+# Example without control
+```julia-repl
+julia> pre_ocp = PreModel()
+julia> times!(pre_ocp, 0.0, 1.0, 100)
+julia> state!(pre_ocp, 2, "x", ["x1", "x2"])
+julia> dynamics!(pre_ocp, (t, x, u) -> [-x[2], x[1]])
+julia> objective!(pre_ocp, :min, mayer=(x0, xf) -> xf[1]^2)
+julia> model = build(pre_ocp)
+julia> control_dimension(model)  # 0
+```
+
+# Example with control
 ```julia-repl
 julia> pre_ocp = PreModel()
 julia> times!(pre_ocp, 0.0, 1.0, 100)
@@ -289,12 +307,6 @@ function build(pre_ocp::PreModel; build_examodel=nothing)::Model
         suggestion="Call state!(pre_ocp, dimension) before building",
         context="build function - state validation",
     )
-    @ensure __is_control_set(pre_ocp) Exceptions.PreconditionError(
-        "Control must be set before building model",
-        reason="control has not been defined yet",
-        suggestion="Call control!(pre_ocp, dimension) before building",
-        context="build function - control validation",
-    )
     @ensure __is_dynamics_set(pre_ocp) Exceptions.PreconditionError(
         "Dynamics must be set before building model",
         reason="dynamics have not been defined yet",

src/OCP/Components/constraints.jl

@@ -278,14 +278,19 @@ julia> constraint!(ocp, :control, rg=1:2, lb=[0.0], ub=[1.0], label=:control_con
 # Throws
 
 - `Exceptions.PreconditionError`: If state has not been set
-- `Exceptions.PreconditionError`: If control has not been set
 - `Exceptions.PreconditionError`: If times has not been set
+- `Exceptions.PreconditionError`: If control has not been set **and** `type == :control`
 - `Exceptions.PreconditionError`: If variable has not been set (when type=:variable)
 - `Exceptions.PreconditionError`: If constraint with same label already exists
 - `Exceptions.PreconditionError`: If both lb and ub are nothing
 - `Exceptions.IncorrectArgument`: If lb and ub have different lengths
 - `Exceptions.IncorrectArgument`: If lb > ub element-wise
 - `Exceptions.IncorrectArgument`: If dimensions don't match expected sizes
+
+!!! note
+    Control is only required for `type == :control` constraints. All other types
+    (`:state`, `:boundary`, `:path`, `:variable`) are valid even when no control
+    is defined (control dimension 0).
 """
 function constraint!(
     ocp::PreModel,
@@ -298,26 +303,30 @@ function constraint!(
     codim_f::Union{Dimension,Nothing}=nothing,
 )
 
-    # checks: times, state and control must be set before adding constraints
+    # checks: times and state must be set before adding constraints
     @ensure __is_state_set(ocp) Exceptions.PreconditionError(
         "State must be set before adding constraints",
         reason="state has not been defined yet",
         suggestion="Call state!(ocp, dimension) before adding constraints",
         context="constraint! function - state validation",
     )
-    @ensure __is_control_set(ocp) Exceptions.PreconditionError(
-        "Control must be set before adding constraints",
-        reason="control has not been defined yet",
-        suggestion="Call control!(ocp, dimension) before adding constraints",
-        context="constraint! function - control validation",
-    )
     @ensure __is_times_set(ocp) Exceptions.PreconditionError(
         "Times must be set before adding constraints",
         reason="time horizon has not been defined yet",
         suggestion="Call times!(ocp, t0, tf) or times!(ocp, N) before adding constraints",
         context="constraint! function - times validation",
     )
 
+    # checks: control must be set for :control constraint type
+    if type == :control
+        @ensure __is_control_set(ocp) Exceptions.PreconditionError(
+            "Control must be set for type=:control constraints",
+            reason="control has not been defined yet but constraint type requires it",
+            suggestion="Call control!(ocp, dimension) before adding :control constraints, or use a different constraint type",
+            context="constraint! function - control validation for type=:control",
+        )
+    end
+
     # checks: variable must be set if using type=:variable
     @ensure (type != :variable || __is_variable_set(ocp)) Exceptions.PreconditionError(
         "Variable must be set for type=:variable constraints",

src/OCP/Components/control.jl

@@ -208,3 +208,30 @@ Get the control function associated with the solution.
 function value(model::ControlModelSolution{TS})::TS where {TS<:Function}
     return model.value
 end
+
+"""
+$(TYPEDSIGNATURES)
+
+Return an empty string, since no control is defined.
+"""
+function name(::EmptyControlModel)::String
+    return ""
+end
+
+"""
+$(TYPEDSIGNATURES)
+
+Return an empty vector since there are no control components defined.
+"""
+function components(::EmptyControlModel)::Vector{String}
+    return String[]
+end
+
+"""
+$(TYPEDSIGNATURES)
+
+Return `0` since no control is defined.
+"""
+function dimension(::EmptyControlModel)::Dimension
+    return 0
+end

src/OCP/Components/dynamics.jl

@@ -8,13 +8,13 @@ Set the full dynamics of the optimal control problem `ocp` using the function `f
 - `f::Function`: A function that defines the complete system dynamics.
 
 # Preconditions
-- The state, control, and times must be set before calling this function.
+- The state and times must be set before calling this function.
+- Control is **optional**: problems without control input (dimension 0) are supported.
 - No dynamics must have been set previously.
 
 # Behavior
 This function assigns `f` as the complete dynamics of the system. It throws an error
-if any of the required fields (`state`, `control`, `times`) are not yet set, or if
-dynamics have already been set.
+if the state or times are not yet set, or if dynamics have already been set.
 
 # Errors
 Throws `Exceptions.PreconditionError` if called out of order or in an invalid state.
@@ -26,12 +26,6 @@ function dynamics!(ocp::PreModel, f::Function)::Nothing
         suggestion="Call state!(ocp, dimension) before dynamics!",
         context="dynamics! function - state validation",
     )
-    @ensure __is_control_set(ocp) Exceptions.PreconditionError(
-        "Control must be set before defining dynamics",
-        reason="control has not been defined yet",
-        suggestion="Call control!(ocp, dimension) before dynamics!",
-        context="dynamics! function - control validation",
-    )
     @ensure __is_times_set(ocp) Exceptions.PreconditionError(
         "Times must be set before defining dynamics",
         reason="time horizon has not been defined yet",
@@ -63,7 +57,8 @@ subset of state indices specified by the range `rg`.
 - `f::Function`: A function describing the dynamics over the specified state indices.
 
 # Preconditions
-- The state, control, and times must be set before calling this function.
+- The state and times must be set before calling this function.
+- Control is **optional**: problems without control input (dimension 0) are supported.
 - The full dynamics must not yet be complete.
 - No overlap is allowed between `rg` and existing dynamics index ranges.
 
@@ -74,7 +69,7 @@ configuration for adding partial dynamics.
 
 # Errors
 Throws `Exceptions.PreconditionError` if:
-- The state, control, or times are not yet set.
+- The state or times are not yet set.
 - The dynamics are already defined completely.
 - Any index in `rg` overlaps with an existing dynamics range.
 
@@ -91,12 +86,6 @@ function dynamics!(ocp::PreModel, rg::AbstractRange{<:Int}, f::Function)::Nothin
         suggestion="Call state!(ocp, dimension) before partial dynamics!",
         context="partial_dynamics! function - state validation",
     )
-    @ensure __is_control_set(ocp) Exceptions.PreconditionError(
-        "Control must be set before defining partial dynamics",
-        reason="control has not been defined yet",
-        suggestion="Call control!(ocp, dimension) before partial dynamics!",
-        context="partial_dynamics! function - control validation",
-    )
     @ensure __is_times_set(ocp) Exceptions.PreconditionError(
         "Times must be set before defining partial dynamics",
         reason="time horizon has not been defined yet",