## Continuous variables

In the lesson that introduced timing, variable `time` was used. Variable `time` uses absolute model time, i.e. the total amount of time that has passed since the start of the simulation. It is usually easier to use relative model time, i.e. a certain amount of time passes after a certain event. This is where continuous variables are ideal. A continuous variable is a variable that changes value automatically, as time progresses. Consider the following CIF specification:

    automaton machine:
      event start, finished;
      cont t = 0 der 1;

      location idle:
        initial;
        edge start do t := 0 goto producing;

      location producing:
        edge finished when t >= 3 goto idle;
    end

This specification models a `machine` that is initially `idle`. The machine can `start` to produce a product. After a while, it is done `producing`. Due to having `finished` the product, it becomes `idle` again, until it is starts to product the next product.

Continuous variable `t` is declared to initially have value `0`. Its derivative is `1`, meaning that every unit of time that passes, the value of `t` increases by `1`. Every time the `start` event happens, the value of continuous variable `t` is reset to `0` using an assignment. As a result of this reset, `t` will be `0` when the automaton enters the `producing` location. The edge for the `finished` event indicates that the event can only happen when `t >= 3` holds. This condition will hold after three time units. This means that automaton `machine` remains in the `producing` location for three time units, before going to the `idle` location. It will thus always take three units after entering the `producing` location, before the guard becomes enabled, and the `finished` event can take place. The state space is as follows:

The states are labeled with the first letters of the names of the current locations of automaton `machine` and the current values of variables `time` and `t`.

Continuous variables always have real values. Similar to discrete variables, if their initial value is not specified, it is `0.0`:

    cont t der 1; // Initial value is 0.0.

The derivative of a continuous variable can be used as a variable as well. The derivative of continuous variable `t` is `t'`. A derivative is read only; it can not be assigned. Similar to algebraic variables, it is always equal to its definition. In the case of variable `t`, its derivative is always `1`. The values of variables `time`, `t`, and `t'` as time progresses are: