Minimax and tic tac toe - is my algorithm correct?

Question

I implemented a minimax algorithm for TTT. When I make the AI player make the first move, it evaluates all minimax values of the possible moves as 0. That means it can choose any square on the grid as the first move. However, any Tic Tac Toe guide will tell you that choosing a corner or center square when making the first move is the better choice since there is a higher chance of winning.

Why is my algorithm not reflecting this?

EDIT: To clarify, what I'm trying to ask is: is this a limitation of the minimax algorithm or is my implementation incorrect?

Answer 1

Your algorithm does not have to reflect this: if you try out all starting positions, you would find that the corners and the center give you more path to win than other cells.

With tic-tac-toe's lack of complexity, the minimax can look ahead all the way through the end of the game, starting with the very first move. The number of available moves goes down quickly as you progress through the game, so the full search finishes pretty fast.

With more complex games (othello, checkers, chess), the so-called "opening books" become more important. The number of available moves in the beginning of the game is enormous, so a traditional approach is to pick a move from the "book" of openings, and stay with pre-calculated "book moves" for the first three to six plays. Moves outside the book are not considered, saving a lot of CPU on moves that remain essentially the same.