Tic-tac-toe

Mix.install([
  {:jason, "~> 1.4"},
  {:kino, "~> 0.9", override: true},
  {:youtube, github: "brooklinjazz/youtube"},
  {:hidden_cell, github: "brooklinjazz/hidden_cell"}
])

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Overview

You’re going to create a game of Tic-tac-toe.

In Tic-tac-toe, players take turns placing either an X or an O onto a 3 by 3 grid.

We can represent a grid like this as a list of lists.

grid = [
  ["X", "O", nil],
  [nil, "X", "O"],
  [nil, nil, "X"]
]

Usually we say grids have an x and a y axis. Each location in the grid is called a coordinate.

We’ll use an {x, y} tuple to represent a coordinate on the board.

For example, the "X" in the top left corner of the board above would be at coordinate {0, 2}.

TicTacToe

You’re going to create a TicTacToe module which can read coordinates from a board, and fill them in.

For example, reading the coordinate {0, 2} on the following board returns "X".

board = [
  ["X", "O", nil],
  [nil, "X", "O"],
  [nil, nil, "X"]
]

TicTacToe.at(board, {0, 2})
"X"

Filling the coordinate will return a newly updated board.

board = [
  ["X", "O", nil],
  [nil, "X", "O"],
  [nil, nil, "X"]
]

TicTacToe.fill(board, {0, 1}, "O")
[
  ["X", "O", nil],
  ["O", "X", "O"],
  [nil, nil, "X"]
]

Implement the TicTacToe module as documented.

Hint

Remember that Enum.at can access a list at an index, however our coordinates are flipped on the board, so the first list is at y = 2.

Therefore, if y is 0, then we want to access the row list at index 2.

Example Solution

defmodule TicTacToe do
  def at(board, coordinate) do
    {x, y} = coordinate

    Enum.at(board, 2 - y) |> Enum.at(x)
  end

  def fill(board, coordinate, symbol) do
    {x, y} = coordinate
    List.update_at(board, 2 - y, fn row ->
      List.replace_at(row, x, symbol)
    end)
  end
end

defmodule TicTacToe do
  @moduledoc """
    Documentation for `TicTacToe`
  """

  @doc """
  Read a coordinate on a board.

  ## Examples

      We've used a board with letter symbols for sake of example.

      iex> board = [
      ...> ["A", "B", "C"],
      ...> ["D", "E", "F"],
      ...> ["G", "H", "I"]
      ...> ]
      iex> TicTacToe.at(board, {0, 0})
      "G"
      iex> TicTacToe.at(board, {2, 1})
      "F"
      iex> TicTacToe.at(board, {0, 2})
      "A"
  """
  def at(board, coordinate) do
    {x, y} = coordinate

    Enum.at(board, x) |> Enum.at(2 - y)
  end

  @doc """
  Fill in a coordinate on a board with the provided symbol and return a new updated board.

  ## Examples

      iex> board = [
      ...> [nil, nil, nil], 
      ...> [nil, nil, nil],
      ...> [nil, nil, nil]
      ...> ]
      iex> TicTacToe.fill(board, {0, 0}, "X")
      [[nil, nil, nil], [nil, nil, nil], ["X", nil, nil]]
      iex> TicTacToe.fill(board, {1, 1}, "O")
      [[nil, nil, nil], [nil, "O", nil], [nil, nil, nil]]
  """
  def fill(board, coordinate, symbol) do
  end
end

Bonus: Winner

Create a TicTacToe.winner/1 function which accepts a board and determines if there is a winner. A player wins if they have three symbols in a row, three symbols in a column, or three symbols diagonally.

For example:

board = [
  ["X", "O", "O"]
  ["X", "O", nil]
  ["X", nil, nil]
]

TicTacToe.winner(board) # returns "X".
"X"

Example Solution

defmodule TicTacToe do
  def winner(board) do
    cond do
      winner?(board, "X") -> "X"
      winner?(board, "O") -> "O"
      true -> nil
    end
  end

  def winner?(board, symbol) do
    row_winner =
      Enum.any?(board, fn row ->
        Enum.all?(row, fn element -> element == symbol end)
      end)

    column_winner =
      Enum.any?(0..2, fn index ->
        Enum.all?(board, fn row ->
          Enum.at(row, index) == symbol
        end)
      end)

    diagonal_winner =
      Enum.all?(0..2, fn index ->
        board
        |> Enum.at(index)
        |> Enum.at(index) == symbol
      end) or
        Enum.all?(0..2, fn index ->
          board
          |> Enum.at(index)
          |> Enum.at(2 - index) == symbol
        end)

    row_winner or column_winner or diagonal_winner
  end
end

Note it’s also possible to use pattern matching to solve this problem.

defmodule TicTacToe do
  def winner(board) do
    cond do
      winner?(board, "X") -> "X"
      winner?(board, "O") -> "O"
      true -> nil
    end
  end

  def winner?(board, x) do
    match?([[^x, ^x, ^x], [_, _, _], [_, _, _]], board) or
      match?([[_, _, _], [^x, ^x, ^x], [_, _, _]], board) or
      match?([[_, _, _], [_, _, _], [_, _, _]], board) or
      match?([[^x, _, _], [^x, _, _], [^x, _, _]], board) or
      match?([[_, ^x, _], [_, ^x, _], [_, ^x, _]], board) or
      match?([[_, _, ^x], [_, _, ^x], [_, _, ^x]], board) or
      match?([[^x, _, _], [_, ^x, _], [_, _, ^x]], board) or
      match?([[_, _, ^x], [_, ^x, _], [^x, _, _]], board)
  end
end

To get started, copy the code below into the TicTacToe module above.

@doc """
Determine if a player has won the game.

## Examples

  Row Wins:

  iex> TicTacToe.winner([["X", "X", "X"], [nil, nil, nil], [nil, nil, nil]])
  "X"
  iex> TicTacToe.winner([[nil, nil, nil], ["X", "X", "X"], [nil, nil, nil]])
  "X"
  iex> TicTacToe.winner([[nil, nil, nil], [nil, nil, nil], ["X", "X", "X"]])
  "X"

  Column Wins:

  iex> TicTacToe.winner([["X", nil, nil], ["X", nil, nil], ["X", nil, nil]])
  "X"
  iex> TicTacToe.winner([[nil, "X", nil], [nil, "X", nil], [nil, "X", nil]])
  "X"
  iex> TicTacToe.winner([[nil, nil, "X"], [nil, nil, "X"], [nil, nil, "X"]])
  "X"

  Diagonal Wins

  iex> TicTacToe.winner([["X", nil, nil], [nil, "X", nil], [nil, nil, "X"]])
  "X"
  iex> TicTacToe.winner([[nil, nil, "X"], [nil, "X", nil], ["X", nil, nil]])
  "X"

  No Winner

  iex> TicTacToe.winner([[nil, nil, nil], [nil, nil, nil], [nil, nil, nil]])
  nil
  iex> TicTacToe.winner([[nil, "X", nil], [nil, "X", nil], [nil, nil, nil]])
  nil

"""
def winner(board) do
end

Commit Your Progress

DockYard Academy now recommends you use the latest Release rather than forking or cloning our repository.

Run git status to ensure there are no undesirable changes. Then run the following in your command line from the curriculum folder to commit your progress.

$ git add .
$ git commit -m "finish Tic-tac-toe exercise"
$ git push

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