Validating Tic-tac-toe Input

Objectives

1. Use either if statements or boolean expressions to control the return value of a method.
2. Use a "helper method" - a method called within another method - to make your code more readable.

Overview

In our previous tic-tac-toe lab, we built a method, #position_taken?, that checks to see if the user's submitted position is free or already filled with a token. This is a type of validation. Our #position_taken? method protects our game from breaking when the user (accidentally or otherwise) submits a position that isn't available.

Our validation is still incomplete however. What if a user submits a position that isn't even on the board? A more complete validation might look something like this:

1. You must move to a position within the tic-tac-toe board.
2. The position must be vacant, not currently taken by a player.

In this lab, you'll build a #valid_move? method that accepts a board and an index 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.

Helper Methods

We've already defined a method, #position_taken? that handles the second part of our validation procedure (You'll have to re-define it in this lab or copy over the code you've already written). Consequently, we can call that method inside of our #valid_move? method.

The #position_taken? method can thus be referred to as a helper method––a method that handles a discrete unit of behavior and is used inside of other methods to carry out a larger task.

The #position_taken? method can be used directly in a conditional expression, for example:

def some_new_fabulous_method
	if position_taken?
		execute something
	else
		execute something else
	end
end

Instructions

This lab is test-driven, so run the test suite and use the output to help you solve this one. Here are a few things to keep in mind:

• Arrays are indexed starting at 0, and a user playing your game is unlikely to know that. When a user types in that they'd like to fill position 1, they really mean that they'd like to fill the board array at the index of 0.
• That being said, the process of converting user input falls outside the scope of #valid_move?. By the time the user's selection reaches your #valid_move? method, you may assume it has already been converted from the user's point of view (squares 1 through 9) to the array's point of view (squares 0 through 8).
• If the user's selection does not fall in the range of 0 through 8, their input is invalid. There are a few different ways to check to see if a number is included in a range. Look up the #between? method for starters.
• There are two conditions that need to be met in order for this method to return true––that the position is on the board and that the position is not taken. Think about how to construct a method that must check two conditions. Can you use two if statements? What about a boolean operator like &&?
• Think back to our lessons on the concept of truthiness. Both false and nil are considered to be "falsey". So, either a false or nil return value for an invalid move will suffice.

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