How to keep track of current player minimax algorithm

Question

I am trying to build a tic-tac-toe game using the minimax algorithm. It is not functioning correctly yet (meaning it is generating moves that are not optimal), and I think this is because I am not accounting for the moves of the opposite player. I'm just not quite sure how to incorporate this into my code. For context, I am working from http://neverstopbuilding.com/minimax.

Here is my code. The helper methods are all functional on their own but i did not include them here.

// this variable stores the optimum next move. 
var choice; 
// this stands for 'computer mark', the computer goes second and plays as 'x'
var cmark = 'X'; 
// mark of human player. Currently not integrated into the algorithm. 
var pmark = 'O' 
// 'game' is an array which starts as [0,1,2,3,4,5,6,7,8], each number corresponding 
//to a space on the tic tac toe board. 
function minimax(game){
    // this is the last state of the recursion, it checks if the game has ended
    // score() returns +10 if the computer wins, -10 for loss, 0 for tie 
    if (gameOver(game)){
        return score(game);     
    }
    // this array stores all the possible scores so we can later search for the highest. 
    var scores = []; 
    //this array stores the moves that correspond to the scores array
    var moves = []; 
    // loops through every open move. 
    //HOW DO I MAKE IT SO IT ALTERNATES BETWEEN HUMAN AND COMPUTER MOVES  
    for (var i = 0; i<game.length; i++){
      if (open(game[i])){
        //game[i] is a number corresponding to a space on the board. 
        moves.push(game[i]); 
        //create a new variable representing the game state if this move is chosen
        var possibleGame = game; 
        possibleGame[i] = cmark; 
        //run minimax on this game state,thus obtaining scores for all possible outcomes.  
        scores.push(minimax(possibleGame)); 
      }
    }
//this is another place where I need to add something for opposite player? 
//find the maximum score from the scores list. this will be returned. 
var maxScore = Math.max(...scores);
var maxScoreIndex = scores.indexOf(maxScore); 
//find the move with the same index as the maximum score. this will be stored as 'choice' 
choice = moves[maxScoreIndex]; 
return maxScore; 
}

What do you think `var possibleGame = game; possibleGame[i] = cmark;` does? That will update `game` too... — nnnnnn, May 30 '16 at 02:15
**It is not functioning correctly yet** is super vague. You've barely explained what the function *should do* and you havent mentioned at all *how it fails to do that*. have a look at [How do I ask a good question?](http://stackoverflow.com/help/how-to-ask) — Wesley Smith, May 30 '16 at 02:27

score 0 · Answer 1 · answered May 30 '16 at 14:33

Just keep track of the current user, after the for-loop has ended, you choose a move. So right before you return a choice at the end of the minmax function, you change the current user (which you make a global variable, or at least outside of the minmax function's scope.

It's important that you find the minimal value for a move made by the opponent instead of the maximal value that you correctly find for the player. This principle is derived from the idea that your opponent is a perfect player, meaning that he will always choose the best move available for him.

So in conclusion: create a 'global' variable keeping the current player. When it's the homeplayers turn, return the maximum score's move. When it's the opponent's turn, return the minimum score's move.

How to keep track of current player minimax algorithm

1 Answers1