program MontyHall;
(* Monty Hall problem *)

const
  n_games = 10000;

  unit is_game_won: function (sw: boolean): boolean;
  var
    car, player, player0, Monty: integer;
  begin
    car := entier(random * 3); (* Randomly place car behind a door. *)
    player0 := entier(random * 3); (* Player randomly chooses a door. *)
    Monty := entier(random * 3); (* Monty opens door revealing a goat. *)
    while (Monty = car) or (Monty = player0)
    do
      Monty := entier(random * 3)
    od;
    if sw
    then (* Player switches to remaining door. *)
      player := entier(random * 3);
      while (player = player0) or (player = Monty)
      do
        player := entier(random * 3)
      od
    else
      player := player0 (* Player sticks with original door. *)
    fi;
    result := (player = car)
  end;

var
  n_wins, game: integer;
begin
  n_wins := 0;
  for game := 1 to n_games
  do
    if is_game_won(false) then n_wins := n_wins + 1 fi
  od;
  write("NOT switching doors wins car in ");
  writeln(n_wins / n_games * 100: 4: 1, "% of games.");
  n_wins := 0;
  for game := 1 to n_games
  do
    if is_game_won(true) then n_wins := n_wins + 1 fi
  od;
  write("But switching doors wins car in ");
  writeln(n_wins / n_games * 100: 4: 1, "% of games.");
end