abadfish/ttt-7-valid-move-q-000

Validating Input Tic Tac Toe

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 build 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 and not currently taken by a player.

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

Helper Methods

We already have a method, #position_taken? that handles the second part of our validation procedure. 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 handle 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?
		do some stuff
	else
		do some other stuff
	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. 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. You'll have to account for this in your method.

• The valid positions on the board, from the user's point of view, are 1-9. If the user inputs a number not included in that range, 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.

• Remember that the #gets method captures the user's input to the terminal and returns it to our Ruby program as a string. We can't ask Ruby to tell us whether a string that contains the number 5, "5", is between the numbers 1 and 9. You'll have to convert the position string into a number first. Check out the #to_i method.

• 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.