Tic Tac Toe Simulation — Random Moves

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)

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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