3.2 The Driver
In this section, we will presume the table has been created by magic.
Rows in the table represent states, columns are input characters, and
the entries in the table are states.
The lexical analyzer (the driver) considers the input source program
as a long sequence of characters with two pointers: a current and a lookahead
pointer:
When lexical analysis begins to find the next token, current
and lookahead both point to the same character:
In the
algorithm,
four actions are
performed on these pointers:
- GetChar: Moves the lookahead pointer ahead one
character and returns that character.
- Retract: Moves the lookahead pointer
back one character.
Before Retract
After Retract 
- Accept: Moves the current pointer ahead to the
lookahead pointer:
After Accept
- Return: Returns a token consisting of a class and
a value, as well as performs any actions associated with that state,
e.g., installing an identifier into the name table.
The driver program scans the input program and comsults the entry at
Table[State, InputChar] for the new state. The entry at
Table[State, InputChar] consists of a new state and perhaps an
action to be performed before moving to the new state:
Algorithm
In the algorithm retract is
necessary when a token is found because the algorithm will have scanned
one character too far.
Example 3.3 shows a piece of a state table and the execution
of the algorithm on an input string.
Rows represent states, columns are input characters, and the entries in the table are states.
