Author: Bruce Moreland
Date: 13:01:23 06/04/98
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On June 04, 1998 at 13:01:19, Komputer Korner wrote: >Alpha beta has nothing to do with plans nor strategies nor sacrifices >....etc. >It simply allows you to stop searching a subtree when you know that one >of the opponent's replies will knock you out anyway. You don't invade >Russia if you know that you will lose the war because of a certain >Russian defence called "Winter". You will not waste your time thinking >up attack strategies against other Russian defences when "Winter" will >kill you anyway. That is all Alpha beta is. Nothing more. If you want to >follow 2nd best move candidate strategies, alpha beta will save you >time even here in not having to search certain subtrees. So alpha beta >is an objective mathematical device not a subjective one. It is such a >simple concept that it should be called an arithmetical rule. Calling it >mathematical makes it seem too important. When someone starts talking about exploring a "second best move", the conversation can no longer be about alpha-beta. Alpha-beta doesn't concern itself with moves, it concerns itself with endpoints. Imagine you are doing a full min-max search to a fixed depth. You have this giant bushy tree. Min-max will look at all of the nodes at the tips, ask if this node is better than the best one that the side to move can force, and if it is, it will see if there is a way to force it. So you end up picking the best end point that you can force. The move path that gets you to this node isn't important to min-max. It doesn't care how complex the path is, it doesn't care how trappy the path is, it doesn't care how singular the path is, it just cares that there is at least one path it can force, assuming that all of the evaluations were accurate. Alpha-beta is just min-max with a bow tie. If you start talking about extending or limiting search in certain positions, based upon positional features, or based upon changes in the position that were caused by the last move executed, or any number of other ways of deciding to extend or limit search depth, the subject is no longer alpha-beta, it's something else. bruce
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