Modeling the system behavior

In the last section, we confirmed the system requirements. Let's now design the logic for processing the arithmetic expression. The components of the system are shown in Figure 2.1:

Figure 2.1 – Design of an arithmetic expression evaluator

The components shown in the preceding figure work together as follows:

1. The user enters an arithmetic expression at the command-line input and presses the Enter key.
2. The user input is scanned in its entirety and stored in a local variable.
3. The arithmetic expression (from the user) is scanned. The numbers are stored as tokens of the Numeric type. Each arithmetic operator is stored as a token of that appropriate type. For example, the + symbol will be represented as a token of type Add, and the number 1 will be stored as a token of type Num with a value of 1. This is done by the Lexer (or Tokenizer) module.
4. An Abstract Syntax Tree (AST) is constructed from the...