12.1 Introduction

It is finally time to put everything you know all together. You will no longer use programming elements in isolation; writing programs rarely involves using only if statements, loops, or objects. Typically, all programming elements are interleaved. For clarity, we described these elements in isolation; now we integrate them. The challenge is to create a task that requires integration that is not too simple to be uninteresting but not so complex that it requires 500 pages of text.

After much thought, we decided to develop the simple, well-known game of tic-tac-toe. The idea is to initially develop a program that facilitates two people playing against each other, and then to enhance the program so that a person can play against a computer. As correct play guarantees at least a tie, we provide sufficient algorithm detail to guarantee that the computer will never lose. We provide sufficient code segments to illustrate the development of the program but leave it to you to complete it. Good luck; you are ready for it.

Using what you have learned so far, it is time to put all of your knowledge together into one fun program. The game of tic-tac-toe is a classic, and the rules can be easily looked up if you are not familiar with them. First you will learn how to build a Ruby program that will allow two users to play the game. After this program has been established, we will extend the program to allow a user to play against the computer.

Our tic-tac-toe game will not have a nice user interface. User interfaces are a more advanced topic that is beyond the scope of this book. We want you to focus on problem solving in this course and not on figuring out how to create a pretty user interface.

Our board will be drawn with simple X’s and O’s and will be invoked from the command line of your operating system.

We will first talk about how to approach the problem, and then how to implement the solution in an incremental fashion.