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
  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

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