Ttt 7 Valid Move


  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.


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
        execute something else


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.

View Validating Tic-tac-toe Input on and start learning to code for free.

Unlock your future in tech
Learn to code.

Learn about Flatiron School's Mission

With a new take on education that falls somewhere between self-taught prodigy and four-year computer science degree, the Flatiron School promises to turn students with little programming experience into developers.

In the six months since the Manhattan coding school was acquired by WeWork, it has spawned locations in Washington, D.C., Brooklyn, and London. Now, WeWork is opening a fourth Flatiron School location, this time in Houston.

Adam Enbar, Flatiron School's cofounder, believes now is the time to grow. "How the world is changing has impacted working and learning in very similar ways. We think education fundamentally is about one thing: enabling people to pursue a better life."

Learn. Love. Code.
Students come to Flatiron School to change their lives. Join our driven community of career-changers and master the skills you need to become a software engineer or a data scientist.
Find Us