Diagram of a counter automaton. Each cell of its stack either contains an "A" or a space symbol.

In computer science, more particular in the theory of formal languages, a counter automaton, or counter machine, is a pushdown automaton with only two symbols, ${\displaystyle A}$ and the initial symbol in ${\displaystyle \Gamma }$, the finite set of stack symbols.^[1]: 171

Equivalently, a counter automaton is a nondeterministic finite automaton with an additional memory cell that can hold one nonnegative integer number (of unlimited size), which can be incremented, decremented, and tested for being zero.^[2]: 351

Properties

The class of counter automata can recognize a proper superset of the regular^{[note 1]} and a subset of the deterministic context free languages.^[2]: 352

For example, the language ${\displaystyle \{a^{n}b^{n}:n\in \mathbb {N} \}}$ is a non-regular^{[note 2]} language accepted by a counter automaton: It can use the symbol ${\displaystyle A}$ to count the number of ${\displaystyle a}$s in a given input string ${\displaystyle x}$ (writing an ${\displaystyle A}$ for each ${\displaystyle a}$ in ${\displaystyle x}$), after that, it can delete an ${\displaystyle A}$ for each ${\displaystyle b}$ in ${\displaystyle x}$.

A two-counter automaton, that is, a two-stack Turing machine with a two-symbol alphabet, can simulate an arbitrary Turing machine.^[1]: 172

Notes

References