Tic Tac Toe might be a futile children’s game but it can also teach us about artificial intelligence. Tic Tac Toe, or Naughts and Crosses, is a zero-sum game with perfect information. Both players know exactly what the other did and when nobody makes a mistake, the game will always end in a draw.

Tic Tac Toe is a simple game but also the much more complex game of chess is a zero-sum game with perfect information.

In this two-part post, I will build an unbeatable Tic Tac Toe Simulation. This first part deals with the mechanics of the game. The second post will present an algorithm for a perfect game.

## Drawing the Board

This first code snippet draws the Tic Tac Toe simulation board. The variable *xo* holds the identity of the pieces and the vector *board* holds the current game. Player X is denoted with -1 and player O with +1. The first part of the function draws the board and the naughts and crosses. The second part of the code check for three in a row and draws the corresponding line.

draw.board <- function(board) { # Draw the board xo <- c("X", " ", "O") # Symbols par(mar = rep(0,4)) plot.new() plot.window(xlim = c(0,30), ylim = c(0,30)) abline(h = c(10, 20), col="darkgrey", lwd = 4) abline(v = c(10, 20), col="darkgrey", lwd = 4) pieces <- xo[board + 2] text(rep(c(5, 15, 25), 3), c(rep(25, 3), rep(15,3), rep(5, 3)), pieces, cex = 6) # Identify location of any three in a row square <- t(matrix(board, nrow = 3)) hor <- abs(rowSums(square)) if (any(hor == 3)) hor <- (4 - which(hor == 3)) * 10 - 5 else hor <- 0 ver <- abs(colSums(square)) if (any(ver == 3)) ver <- which(ver == 3) * 10 - 5 else ver <- 0 diag1 <- sum(diag(square)) diag2 <- sum(diag(t(apply(square, 2, rev)))) # Draw winning lines if (hor > 0) lines(c(0, 30), rep(hor, 2), lwd=10, col="red") if (ver > 0) lines(rep(ver, 2), c(0, 30), lwd=10, col="red") if (abs(diag1) == 3) lines(c(2, 28), c(28, 2), lwd=10, col="red") if (abs(diag2) == 3) lines(c(2, 28), c(2, 28), lwd=10, col="red") }

## Random Tic Tac Toe

The second part of the code generates ten random games and creates and animated GIF-file. The code adds random moves until one of the players wins (winner <> 0) or the board is full (no zeroes in the *game* vector). The *eval.winner* function checks for three in a row and declares a winner when found.

There are 255,168 possible legal games in Tic Tac Toe, 46,080 of which end in a draw. This implies that these randomised games result in a draw 18% of the time.

eval.winner <- function(board) { # Identify winner square <- t(matrix(board, nrow = 3)) hor <- rowSums(square) ver <- colSums(square) diag1 <- sum(diag(square)) diag2 <- sum(diag(t(apply(square, 2, rev)))) if (3 %in% c(hor, ver, diag1, diag2)) return (1) else if (-3 %in% c(hor, ver, diag1, diag2)) return (2) else return(0) } # Random game library(animation) saveGIF ({ for (i in 1:10) { game <- rep(0, 9) # Empty board winner <- 0 # Define winner player <- -1 # First player draw.board(game) while (0 %in% game & winner == 0) { # Keep playing until win or full board empty <- which(game == 0) # Define empty squares move <- empty[sample(length(empty), 1)] # Random move game[move] <- player # Change board draw.board(game) winner <- eval.winner(game) # Evaulate game player <- player * -1 # Change player } draw.board(game) } }, interval = 0.25, movie.name = "ttt.gif", ani.width = 600, ani.height = 600)

View the latest version of this code on GitHub.

## Tic Tac Toe Simulation

In a future post, I will outline how to program the computer to play against itself, just like in the 1983 movie *War Games*.

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