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2013-03-19-book-problems.md

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Erlang

Day 1: Appearing Human

Find

Do

  • Write a function that uses recursion to return the number of words in a string.
numwords([]) -> 0;
numwords([32]) -> 0;
numwords([_]) -> 1;
numwords([32|Rest]) -> 1 + numwords(Rest);
numwords([_|Rest]) -> numwords(Rest).

% Using higher order functions and added dedup
dedup(_, []) -> [];
dedup(C, [C,C|T]) -> dedup(C, [C|T]);
dedup(C, [H|T]) -> [H | dedup(C, T)].
countwords(S) -> length(lists:filter(fun(C) -> C == 32 end, dedup(32, S))) + 1.
  • Write a function that uses recursion to count to ten.
count(11) -> ok;
count(A) -> io:fwrite("~w\n", [A]), count(A + 1).
count() -> count(0).
  • Write a function that uses matching to selectively print success or error: message given input of the form {error, Message} or success.
printStatus(success) -> io:fwrite("success\n");
printStatus({error, Message}) -> io:fwrite("error: ~s\n", [Message]);
printStatus(S) -> io:fwrite("unknown: ~p\n", [S]).

Day 2: Changing Forms

Do

  • Consider a list of keyword-value tuples, such as [{erlang, "a functional language"}, {ruby, "an OO language"}]. Write a function that accepts the list and a keyword and returns the associated value for the keyword.
dict([], _) -> null;
dict([{Key, Value}|_], Key) -> Value;
dict([_|T], Key) -> dict(T, Key).
  • Consider a shopping list that looks like [{item, quantity, price}, ...]. Write a list comprehension that builds a list of items of the form [{item, total_price}, ...], where total_price is quantity times price.
calcPrice(ShoppingList) -> [ {Item, Quantity * Price} || {Item, Quantity, Price} <- ShoppingList ].

Bonus problem

  • Write a program that reads a tic-tac-toe board presented as a list or a tuple of size nine. Return the winner (x or o) if a winner has been determined, cat if there are no more possible moves, or no_winner if no player has won yet.
% Applies function to a list and returns the first result that's not false
first([], _) -> false;
first([H|T], F) -> case F(H) of
  false -> first(T, F);
  Anything -> Anything
end.

tictactoe(Board) ->
  [C11, C12, C13, C21, C22, C23, C31, C32, C33] = Board,

  Combs = [
    % Rows
    [C11, C12, C13],
    [C21, C22, C23],
    [C31, C32, C33],
    % Columns
    [C11, C21, C31],
    [C12, C22, C32],
    [C13, C23, C33],
    % Diagonals
    [C11, C22, C33],
    [C13, C22, C31]
  ],

  CountSpaces = fun(Cell, Spaces) ->
    case Cell of
      "x" -> Spaces;
      "o" -> Spaces;
      _ -> Spaces + 1
    end
  end,

  Winner = fun([A, B, C]) ->
    case [A, B, C] of
      ["x", "x", "x"] -> "x";
      ["o", "o", "o"] -> "o";
      _ -> false
    end
  end,

  case first(Combs, Winner) of
    false ->
      case lists:foldl(CountSpaces, 0, Board) of
        0 -> "cat";
        _ -> "no_winner"
      end;
    Anything -> Anything
  end.

Day 3: The Red Pill

Find

Do

  • Monitor the translate_service and restart it should it die.
  • Make the Doctor process restart itself if it should die.
  • Make a monitor for the Doctor monitor. If either monitor dies, restart it.

Bonus Problems

  • Create a basic OTP server that logs messages to a file.
  • Make the translate_service work across a network.