Dutch national flag problem: Difference between revisions

=={{header|Erlang}}==

<lang erlang>-module(dutch).

-export([random_balls/1, is_dutch/1, dutch/1]).

ball(red) -> 1;

ball(white) -> 2;

ball(blue) -> 3.

random_ball() -> lists:nth(random:uniform(3), [red, white, blue]).

random_balls(N) -> random_balls(N,[]).

random_balls(0,L) -> L;

random_balls(N,L) when N > 0 ->

B = random_ball(),

random_balls(N-1, [B|L]).

is_dutch([]) -> true;

is_dutch([_]) -> true;

is_dutch([B|[H|L]]) -> (ball(B) < ball(H)) and is_dutch([H|L]);

is_dutch(_) -> false.

dutch(L) -> dutch([],[],[],L).

dutch(R, W, B, []) -> R ++ W ++ B;

dutch(R, W, B, [red | L]) -> dutch([red|R], W, B, L);

dutch(R, W, B, [white | L]) -> dutch(R, [white|W], B, L);

dutch(R, W, B, [blue | L]) -> dutch(R, W, [blue|B], L).</lang>

Sample usage:

<lang erlang>main(_) ->

L = random_balls(10),

case is_dutch(L) of

true -> io:format("The random sequence ~p is already in the order of the Dutch flag!~n", [L]);

false -> io:format("The starting random sequence is ~p;~nThe ordered sequence is ~p.~n", [L, dutch(L)])

end.</lang>

Sample output:

<pre>The starting random sequence is [white,white,blue,blue,white,red,white,blue,

blue,white];

The ordered sequence is [red,white,white,white,white,white,blue,blue,blue,

blue].</pre>