Intermediate Automata

Abstract

The power of finite automata is limited by their inability to generalise. Thus a finite automaton may be designed to add two numbers, but not to add an arbitrary collection of numbers; or to multiply two numbers less than 10, but not to multiply any two numbers. In this chapter, we consider two more sophisticated classes of automata that overcome some of these limitations. In the first of these classes, the machine uses a pushdown stack to extend its capacity. If the length of the stack is decided in advance, as in applications of this device in computing, a pushdown automaton could be simulated by a finite automaton; however, it is more helpful to refer to a model that uses a stack in its structure. The general pushdown automaton uses an unbounded stack, which neither overflows nor loses information, however many items are loaded onto it.