Tic-tac-toe is a popular game that everyone has played once. But playing and explaining the game to a computer are different things. So, this article is a kind of explanation for non-programmers about how we reason about automating routines or creating programs.

Let’s start with the central part of the game, the board. Our board is a 3x3 table where we place our moves. If we want the computer to understand what is happening on the board, we must identify the possible positions. In this case, we use an MS Excel-like representation to make it more intuitive.

Now is easier to see when a user wins. One way to know if a player has three positions in a row is to use a simple sum operation to calculate the state of the winning positions on the board. In other words, if Player X adds 1 point and Player O subtracts 1 to the row. Player X wins if it sums a total of 3, and Player O wins if it computes -3. With this, we create these winning conditions for our game as follows.

Player X = 1
Player O = -1

Winning options

Winning rows

p1 + p2 + p3 = [+3 / -3]
p4 + p5 + p6 = [+3 / -3]
p7 + p8 + p9 = [+3 / -3]

Winning columns

p1 + p4 + p7 = [+3 / -3]
p2 + p5 + p8 = [+3 / -3]
p3 + p6 + p9 = [+3 / -3]

Winning diagonal

p1 + p5 + p9 = [+3 / -3]
p7 + p5 + p3 = [+3 / -3]

The loop

Now comes the game loop, when every player has their turn and marks a place on the board. So the game loop sounds like this:

1. A player selects a position available on the board
2. The game checks if there is a winner
  - If so, celebrates and the game ends.
3. The game checks if the board has more additional spaces left.
  - If so, changes the current user and start step 1 again.
  - If not, game ends with no winner.

So, that’s it. In another post we will create our tic-tac-toe game in a spreadsheet, and when we do, you will be officially a programmer.

See you next time.