Oo Tic Tac Toe


  1. Build a CLI Tic Tac Toe game!
  2. Encapsulate Tic Tac Toe in a TicTacToe object.


You're going to be building a 2 player CLI version of Tic Tac Toe by building a TicTacToe object. The game play will be very similar to other versions of TicTacToe.


In order to get everything working you should refer to these instructions as you go and pay close attention to the test output.

Run bundle within this lab's directory before getting started.

Project Structure

├── Gemfile
├── Gemfile.lock
├── README.md
├── bin
│   └── tictactoe
├── lib
│   └── tic_tac_toe.rb
└── spec
    ├── 01_tic_tac_toe_spec.rb
    ├── 02_play_spec.rb
    ├── 03_cli_spec.rb
    └── spec_helper.rb


These files set up some tools and gems for our project and can mostly be ignored. Make sure to run bundle before starting this project so that you have all the required gems.


This is our main executable and will be how we initialize and start our game.


Our main TicTacToe class will be defined here with all the data and logic required to play a game of tic tac toe via instances of TicTacToe.


There are three test files that should be completed in order. 01_tic_tac_toe_spec.rb tests our helper methods within TicTacToe. 02_play_spec.rb tests the main #play method. 03_cli_spec.rb tests the CLI.

Your Object-Oriented Tic Tac Toe

We're going to be building a very well encapsulated object for Tic Tac Toe where each instance method will represent a discrete, single responsibility or functionality of a Tic Tac Toe game.

We'll be following the Tic Tac Toe conventions of representing the board as an array with 9 elements where " " represents an empty cell in the board.

We'll be getting user input via gets and a player will choose a position by entering 1-9. Our program will then fill the appropriate position on the board with the player's token.

We will keep track of which player's turn it is and how many turns have been played. We will check to see, at every turn, if there is a winner. If there is a winner, we'll congratulate them. If there is a tie, we will inform our players.

The Logic: lib/tictactoe.rb

TicTacToe class

Open up lib/tic_tac_toe.rb. You'll be defining the main game class, TicTacToe in lib/tic_tac_toe.rb. Until the TicTacToe class is defined, everything will break.

Every method you build will be encapsulated by this class.

#initialize and @board

The first test in 01_tic_tac_toe_spec.rb will ensure the requirement that when a new game of Tic Tac Toe is started — that is, when a new instance of TicTacToe is initialized — the instance of the game must set the starting state of the board, an array with 9 empty strings " ", within an instance variable named @board.

In other words, your #initialize method should set a @board variable equal to a new, empty array that represents the game board.


Define a WIN_COMBINATIONS constant within the TicTacToe class, and set it equal to a nested array filled with the index values for the eight winning combinations possible in Tic Tac Toe.

Top-Tip: When you see this line, TicTacToe::WIN_COMBINATIONS, in the test suite, that means the test suite is accessing the constant WIN_COMBINATIONS that was declared inside the TicTacToe class.

# within the body of TicTacToe

  [0,1,2], # Top row
  [3,4,5]  # Middle row
  # et cetera, creating a nested array for each win combination

# the rest of the TicTacToe class definition

Helper Methods

The next bunch of methods we will be describing are helper methods - methods that will be called by other methods in your code. This keeps our code DRY and well encapsulated — each method has a single responsibility — which makes the code easier to maintain and expand.


Define a method that prints the current board representation based on the @board instance variable.


Define a method into which we can pass user input (in the form of a string, e.g., "1", "5", etc.) and have it return to us the corresponding index of the @board array. Remember that, from the player's point of view, the board contains spaces 1-9. But the indices in an array start their count at 0. If the user inputs 5, your method must correctly translate that from the player's perspective to the array's perspective — accounting for the fact that @board[5] is not where the user intended to place their token.


Your #move method must take in two arguments: the index in the @board array that the player chooses and the player's token (either "X" or "O"). The second argument, the player's token, should default to "X".


The #position_taken? method will be responsible for evaluating the user's desired move against the Tic Tac Toe board and checking to see whether or not that position is already occupied. Note that this method will be running after #input_to_index, so it will be checking index values. When it is passed the index value for a prospective move, #position_taken? will check to see if that position on the @board is vacant or if it contains an "X" or an "O". If the position is free, the method should return false (i.e., "the position is not taken"); otherwise, it will return true.


Build a method valid_move? that accepts a position to check and returns true if the move is valid and false or nil if not. A valid move means that the submitted position is:

  1. Present on the game board.
  2. Not already filled with a token.


This method returns the number of turns that have been played based on the @board variable.


The #current_player method should use the #turn_count method to determine if it is "X"'s or "O"'s turn.


Build a method #turn to encapsulate the logic of a single complete turn composed of the following routine:

  1. Ask the user for their move by specifying a position between 1-9.
  2. Receive the user's input.
  3. Translate that input into an index value.
  4. If the move is valid, make the move and display the board.
  5. If the move is invalid, ask for a new move until a valid move is received.

Note: If the user enters an invalid move, we need to repeat the entire sequence of events listed above: asking for input, translating it into an index, checking whether the move is valid and, if it is, making the move. Theoretically, we could code all that into our else but that would definitely violate DRY principles. And what if the user enters an invalid move a second or third or tenth time? What we really need to do is restart the turn method each time an invalid move is entered. Luckily, Ruby (and many other programming languages) allows us to call a method from inside itself. This process of calling a method from inside itself is commonly used in recursion.

All these procedures will be wrapped into our #turn method, but the majority of the logic for these procedures will be defined and encapsulated in our helper methods. You will need to call those methods from inside the turn method to get the tests passing. Pay close attention to the sequence of events and when and where the helper methods should be called. If there are redundancies in your code the tests may not pass.

Hint: don't forget to pay attention to which argument(s) each of your helper methods requires.

You can imagine the pseudocode for the #turn method:

ask for input
get input
translate input into index
if index is valid
  make the move for index
  show the board
  restart turn


Your #won? method should return false or nil if there is no win combination present in the board and return an array containing the winning combination indexes if there is a win. Use your WIN_COMBINATIONS constant in this method.


The #full? method should return true if every element in the board contains either an "X" or an "O".


Build a method #draw? that returns true if the board is full and has not been won and false otherwise.


Build a method #over? that returns true if the board has been won or is full (i.e., is a draw).


Given a winning @board, the #winner method should return the token, "X" or "O", that has won the game.

Putting it all together: the #play method


The play method is the main method of the Tic Tac Toe application and is responsible for the game loop. A Tic Tac Toe game must allow players to take turns, checking if the game is over after every turn. At the conclusion of the game, whether because it was won or ended in a draw, the game should report to the user the outcome of the game. You can imagine the pseudocode:

until the game is over
  take turns

if the game was won
  congratulate the winner
else if the game was a draw
  tell the players it ended in a draw

Here again, much of the functionality you need is built into our helper methods — don't forget to use them!

Run the tests for the #play method by typing rspec spec/02_play_spec.rb in your terminal.

The CLI: bin/tictactoe

Your bin/tictactoe CLI should:

  1. Instantiate an instance of TicTacToe
  2. Start the game by calling #play on that instance.

Run the tests by typing rspec spec/03_cli_spec.rb in your terminal.

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