1  Introduction

In Ptolemy II, models specify computations by composing actors. Many computations, however, are awkward to specify this way. A common situation is where we wish to evaluate a simple algebraic expression, such as "sin(2π (x-1))." It is possible to express this computation by composing actors in a block diagram, but it is far more convenient to give it textually. actions in state machines, and for the calculation performed by the Expression actor. In fact, the expression language is part of the generic infrastructure in Ptolemy II, and it can be used by programmers extending the Ptolemy II system. In this chapter, we describe how to use expressions from the perspective of a user rather than a programmer.

1.1  Expression Evaluator

Vergil provides an interactive expression evaluator, which is accessed through the File:New menu. This operates like an interactive command shell, and is shown in figure 1. It supports a command history. To access the previously entered expression, type the up arrow or Control-P. To go back, type the down arrow or Control-N. The expression evaluator is useful for experimenting with expressions.

Figure 1: The Expression Evaluator