## Input variables

Input variables can be used to model that some data exists, without specifying the value or how or when the value changes. Input variables are used in cases where the CIF model is to be connected something else that provides the values.

### Hardware example

The CIF model may for instance be a controller that is to be connected to a piece of physical hardware. The input variables then model sensors, for which the behavior is not modeled in the CIF model. As an example, consider the following CIF specification:

    input bool sensor;

    automaton controller:
      cont t der 1;
      alg bool actuator = on;

      location off:
        initial;
        edge when sensor do t := 0 goto waiting;

      location waiting:
        edge when not sensor goto off;
        edge when t > 1.5 goto on;

      location on:
        edge when not sensor goto off;
    end

Input variable `sensor` models a digital sensor. When the sensor is on, the variable has value `true` and when the sensor is off, the variable has value `false`. When the sensor is on and when it is off is not known, and thus not modeled in the CIF specification.

The `controller` waits (in location `off`) for the sensor to go on, and then resets continuous variable `t` to start measuring `1.5` time units (in location `waiting`). If that much time has passed, it goes to location `on`. If the sensor goes off then, or while it is waiting, it goes back to location `off`. Thus, the `controller` is only `on` if the sensor goes on, `1.5` time units have passed, and the sensor remains on.

The `actuator` algebraic variable models that an actuator is only on if the `controller` is in its on location. The `actuator` is then on if the sensor is one, and `1.5` time units have passed. The actuator is always turned off when the sensor is off.

Without a physical hardware system, the CIF specification is incomplete, as the value of the sensor is not known. The input variable acts as a placeholder for the actual sensor. If the CIF specification were to be coupled to a physical hardware system with a sensor and an actuator, the CIF model would then control the actuator based on the sensor values.

### Co-simulation example

A CIF specification could model a part of a system, while another part of that same system is modeled in another system or language, such as Simulink. Consider for instance a tank filled with water. Water flows out of the tank constantly. By opening a value, water also flows in to the tank. Consider the following CIF specification:

    input real level;

    automaton valve:
      disc bool open = true;

      location:
        initial;
        edge when     open and level >= 10 do open := false;
        edge when not open and level <=  2 do open := true;
    end

The water `level` is modeled as an input variable, meaning it is obtained from the Simulink model. The CIF specification models the valve, and the controller that opens and closes the value, based on the water level. It keeps the level between `2` and `10`.

The CIF model obtains the water level from Simulink by means of input variable `level`. It also provides the `open` variable of the `valve` automaton to Simulink. This variable can be used in Simulink to model how the water level changes over time.

Without the Simulink model, the CIF specification is incomplete, as the water level is not known. The input variable acts as a placeholder for the actual water level. If the CIF specification were to be coupled to a Simulink model that models the water level, the CIF specification would control the valve based on the water level.

### CIF models example

Input variables can also be used to couple CIF models. Consider again the above water level controller example. It models the valve and controls the valve, but does not model the water level. The following CIF specification could be used to model the water level:

    cont level der if valve.open: 1 else -1 end;

    group valve:
      input bool open;
    end

Continuous variable `level` models the water level. The water level increases (derivative `1`) if the valve is open, and decreases (derivative `-1`) otherwise. The `open` variable from the `valve` is used, but this variable is an input variable, and the value is not known in this specification. Input variable `open` acts as a placeholder for a value, and group `valve` acts as a placeholder for automaton `valve`.

The two CIF specifications both model a part of the system. One specification models the water level, and has a placeholder for the variable that indicates whether the valve is open. The other specification models the valve and controls it, but has a placeholder for the water level. If we couple the two CIF specifications, they provide each other with actual variables for each others input variables.

The two specifications can be coupled or merged together:

    cont level = 0.0 der if valve.open: 1 else -1 end;

    automaton valve:
      disc bool open = true;

      location:
        initial;
        edge when     open and level >= 10 do open := false;
        edge when not open and level <=  2 do open := true;
    end

Using input variables to couple CIF specifications is somewhat similar to using imports to split a CIF specification over multiple files. There are however important differences.

The first difference is that when using imports, one CIF specification is coupled to another CIF specification. The coupling is hard-coded in the CIF specification, and can not be changed without changing the import. When using input variables and merging, it is possible to couple a CIF specification to a second CIF specification, and later couple it with a third, different specification instead. The decision about which models to merge can thus be postponed, and does not require changes to any of the CIF specifications.

Another difference is that with imports it is not allowed to have declarations with the same name in different CIF specifications (except for groups), while with the merger this is allowed in many situations. See the documentation of the CIF merger tool for further details on what can be merged using that tool.

### SVG image interaction example

In later tutorial lessons, you will learn about SVG visualization and interaction. The CIF specification can be connected to an image. The image can then be adapted based on the state of the model, allowing for visualization. Additionally, you can set up clickable elements within the image that influence the model’s behavior, allowing interaction with the modeled system through the image. The image then takes the role of the environment of the system being modeled.

An input variable is a connection to the environment, and the environment decides when and how the input variable changes value. If interaction via an image is used, clicks on certain elements of images can be specified to assign values to input variables. Specifically, SVG input mappings with updates allow for this.

As an example, consider an input variable that models a boolean sensor that indicates whether a user pushed a button:

    input bool sensor;

Then the image could have a button, that when clicked assigns `true` to the input variable:

    svgfile "system.svg";
    svgin id "button" do sensor := true;

The details of this will become clearer during the later lessons on SVG visualization and interaction.