Minmax Algorithm for tic-tac-toe game in Objective-C - iphone

Question

I am writing a minmax algorithm as the artificial intelligence for a tic-tac-toe game, I followed the similar instruction here, but the algorithm seems not intelligent enough, even though I tried to search deeper in the tree, can anyone help to analyze where goes wrong? thank you very much in advance!
- (int) miniMax:(int)depth : (UIImage*) player {
NSMutableArray *steps = [self generateMoves];
if (depth == 0 || [steps count] == 0) {
return [self evaluate];
}
int bestScore = player == myImg ? -1000000 : 1000000;
int currentScore = 0;
for (UIImageView *step in steps) {
step.image = player;
if (player == myImg) {
UIImage *opp = player == xImg ? oImg : xImg;
currentScore = [self miniMax:depth - 1 :opp];
if (currentScore > bestScore) {
bestScore = currentScore;
nextStep = step;
}
} else {
UIImage *opp = player == xImg ? oImg : xImg;
currentScore = [self miniMax:depth - 1 :opp];
if (currentScore < bestScore) {
bestScore = currentScore;
nextStep = step;
}
}
step.image = NULL;
}
return bestScore;
}
- (int) evaluate {
int score = 0;
score += [self evaluateLine:img0 :img1 :img2];
score += [self evaluateLine:img3 :img4 :img5];
score += [self evaluateLine:img6 :img7 :img8];
score += [self evaluateLine:img0 :img3 :img6];
score += [self evaluateLine:img1 :img4 :img7];
score += [self evaluateLine:img2 :img5 :img8];
score += [self evaluateLine:img2 :img4 :img6];
score += [self evaluateLine:img0 :img4 :img8];
return score;
}
- (int) evaluateLine:(UIImageView*)img1 :(UIImageView*)img2 :(UIImageView*)img3 {
int score = 0;
// first cell
if ([img1 image] == myImg) {
score = 1;
} else if ([img1 image] == oppImg){
score = -1;
}
// second cell
if ([img2 image] == myImg) {
if (score == 1) {
score = 10;
} else if (score == -1) {
return 0;
} else {
score = -1;
}
} else if ([img2 image] == oppImg){
if (score == -1) {
score = -10;
} else if (score == 1) {
return 0;
} else {
score = -1;
}
}
// third cell
if ([img3 image] == myImg) {
if (score > 0) {
score *= 10;
} else if (score < 0) {
return 0;
} else {
score = -1;
}
} else if ([img3 image] == oppImg){
if (score < 0) {
score *= 10;
} else if (score > 1) {
return 0;
} else {
score = -1;
}
}
return score;
}
What I use here is: if there exits the same image as the human player holds, the score plus 1. If there
are two or three player's image in a line or row or diagonal, the total score is 10 and 100 separately. If there exist both 'X' and 'O' in a same row, column or diagonal, the score is 0. Computer holds the negative score for these mentioned above.

Answer 1

Minimax assumes that the evaluation function is from the perspective of the first player. What you want is a function that always have a higher value if the first player is better.
It seems that your heuristic function adapts its value based on the current player (I am not an objective-C programmer). Change that function so that it does not know what myImg or oppImg is -- use xImg and oImg directly. Might just work.