Pushdown automaton

In computer science, a pushdown automaton (PDA) is a type of automaton that employs a stack.

Description

Pushdown automata are used in theories about what can be computed by machines. They are more capable than finite-state machines but less capable than Turing machines. Deterministic pushdown automata can recognize all deterministic context-free languages while nondeterministic ones can recognize all context-free languages, with the former often used in parser design.

The term "pushdown" refers to the fact that the stack can be regarded as being "pushed down" like a tray dispenser at a cafeteria, since the operations never work on elements other than the top element. A stack automaton, by contrast, does allow access to and operations on deeper elements. Stack automata can recognize a strictly larger set of languages than pushdown automata.

A nested stack automaton allows full access, and also allows stacked values to be entire sub-stacks rather than just single finite symbols.

See also

• Automata theory
• Context-free grammar
• Counter automaton
• Finite automaton
• Stack (abstract data type)
• Stack machine

External links

• Pushdown automaton @ Wikipedia