Tic Tac Toe CLI: Adding Player Movement to the Game Board

Objectives

Define a method to convert a user's input to an array index.
Define a method that updates an array passed to it.
Define a method with a default value.
Use a method in a CLI.
Accept user input via gets.
Use user input in a method.

Overview

In this lab we'll be adding a input_to_index method and a move method to Tic Tac Toe to update the board with a player's token. The input_to_index method will take the user's input ("1"-"9") and convert it to the index of the board array (0-8). The move method represents a user moving into a position, like the middle cell, in Tic Tac Toe. We already have a method, #display_board, that prints out the tic tac toe board to the console and maps each location of the board to an array index. Then, we'll build a CLI that asks the player for the position on the board that they like to fill out with an "X" or an "O", convert the position to an index, update the board, and displays the updated board.

Project Structure

In previous exercises, we've learned to build programs that the user interacts with via the command line. Such interaction is considered to occur through the CLI, or command line interface. Conventionally, CLI programs have a bin directory which contains an executable file. This file contains the code that is responsible for running the program.

Take a look at the file structure of this project, mapped out below:

bin
  |–– move
lib
  |–– move.rb
spec
  |–– 01_input_to_index_spec.rb
  |–– 02_move_spec.rb
  |–– 03_cli_spec.rb
  |–– spec_helper.rb
...

We have our bin directory and it contains our executable file, move. Remember that executable files conventionally are not given a file extension like .rb.

Open up bin/move and take a look at the following code:

#!/usr/bin/env ruby

require_relative '../lib/move.rb'

First, we have our shebang line that tells the terminal which interpreter to use to execute the remainder of the file. Second, we are requiring the move.rb file, from within the lib directory. This gives our executable file access to whatever code we write in the move.rb file.

We'll be writing our #display_board, #input_to_index and #move methods in lib/move.rb and writing the code that interacts with the command line in the bin/move file.

Desired Behavior

Now that we understand the broad strokes of what we're trying to accomplish, as well as where we'll be writing our code, let's take a look at an example of the functionality:

When we run our executable file with ruby bin/move in the terminal, inside the directory of this project, we should see something like the following:

Welcome to Tic Tac Toe!
Where would you like to go?
2
   | X |   
-----------
   |   |   
-----------
   |   |

Our program will:

Print out a welcome message.
Ask the user to input the position on the board they would like to fill.
Convert the input to an array index.
Fill out that position with either an "X" or an "O".
Print the updated board.

Okay, we're ready to start coding!

Instructions

Part I: Defining our Methods

The first part of this lab is test-driven. Run learn to get started and use the test output to guide you in defining the move method.

Notice that we've already given you the #display_board method, since we've already built that out in a previous exercise.

`#input_to_index`

Your #input_to_index method must take one argument, the user's input (should be a string that is "1" - "9").

Regarding the player's input: if the user's input is "5", the player wants to fill out position 5 with their character. This means that your method must fill out the correct array index with the player's character.

The player's input is the string '5'. The first thing you'll need to do is convert the string to its integer value, because array indexes are always integers (think '5' vs 5). Give #to_i a try, as in '5'.to_i.

Also remember, from the player's point of view, the board contains spaces 1-9. An array's indexes start their count at 0. You'll have to account for that in your #move method by doing some math. In this one case, make sure to put spaces between the elements you type; something like 5 + 1 - not 5+1 or 5 +1. There is an edge case which might come up and break your code if you forget the spaces.

So if the player types in a "2", what index in the board array do you want to access?

That's right, we would want to access index 1 of the board array.

`#move`

Your #move method must take in three arguments, the board array, the index in the board array that the player would like to fill out with an "X" or an "O", and the player's character (either "X" or "O"). The third argument, the player's character, should have a default of "X".

#move should also return the modified array with the updated index corresponding to the player's token. Don't create a new local variable for the board array, modify the one passed in as the argument and return it.

Part of your #move method will mean updating the board array passed into it. This is a tricky concept that relates to the idea of pass by reference or pass by value. Let's look at a quick example:

board = [" ", " ", " "]
def update_array_at_with(array, index, value)
  array[index] = value
end

update_array_at_with(board, 0, "Red")
# The element at index 0 of array 'board' is set to the value "Red"
board #=> ["Red", " ", " "]

You might be wondering why the update_array_at_with is able to update the reference board defined outside of the method when ruby is a pass by value language. The reason is that we're not updating the reference board. The value of board is an Array object. Inside the method update_array_at_with, we're not changing the reference of array, we're changing one of that object's elements. After that object is modified, even outside of the method, the Array is updated.

Once you have the tests passing, move on to part II.

Part II: The CLI

Open up bin/move. We're ready to code the executable portion of this program.

Our program should first welcome the player by outputting a friendly message to the terminal: "Welcome to Tic Tac Toe!".
Next, establish the starting state of the game, i.e. the empty board. Create a new board by setting a variable board equal to instantiating a new array with 9 elements, each of which is a blank space, " ".
Now, ask the user for input by outputting "Where would you like to go?" to the terminal.
We need to store the user's input. Use gets.strip to store their input to a variable, input.
Now we want to convert the user's input to an array index using our #input_to_index method and store this as the variable index.
Now we're ready to call our #move method. Do so with the arguments of the board, the index the user wants to access and the default player of "X".
Lastly, display the board by calling the #display_board method, passing in the necessary arguments required to run this method.

Now, run your program by typing ruby bin/move in the terminal. Have fun playing (one round of) tic tac toe!

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gregimba/ttt-5-move-rb-ruby-intro-000