Ackermann function (Erlang)
From LiteratePrograms
The Ackermann function or Ackermann-Péter function is defined recursively for non-negative integers m and n as follows:
- 0 \mbox{ and } n = 0 \\ A(m-1, A(m, n-1)) & \mbox{if } m > 0 \mbox{ and } n > 0. \end{cases}"/>
In the theory of computation, the Ackermann function is a simple example of a recursive function that is not primitively recursive. Note that this function grows very quickly -- even A(4, 3) cannot be feasibly computed on ordinary computers.
Using recursion and pattern matching this function is extreamly easy to impliment in Erlang.
<<ackermann function>>= ackermann(0, N) -> N + 1; ackermann(M, 0) when M > 0 -> ackermann(M-1, 1); ackermann(M, N) when M > 0, N > 0 -> ackermann(M-1, ackermann(M, N - 1)).
Then all that is needed is some glue code to allow the function to be accessed as a script.
<<ackermann>>= #!/usr/bin/env escript main([A, P]) -> io:fwrite("~p~n", [xackermann(string:to_integer(A), string:to_integer(P))]). xackermann({A, _}, {B, _}) -> ackermann(A, B). ackermann function