Author: Bob Durrett
Date: 14:23:06 02/22/04
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On February 22, 2004 at 16:50:26, David Mitchell wrote: >On February 22, 2004 at 15:27:44, Bob Durrett wrote: > >>In http://www.talkchess.com/forums/1/message.html?350711, Christophe Theron >>made the fascinating and thought-provoking observation that: >> >> >>"Conceptually, an alpha-beta search is doing several thousands takebacks per >>second." >> >> >>I do not doubt that this is true. However, I've never heard the alpha/beta >>described quite that way. >> >>Could somebody please relate this to the more conventional concept of >>alpha/beta? >> >>Bob D. > >Very simple, really. Before any position is evaluated, the move that leads to >that position is "made", on a data struct inside the program. The internal >"board", if you will. > >But is that position the best? The program can't tell, without comparing it to >thousands, perhaps many thousands, of other positions. Each position is >preceeded by the "move" that leads to it. Which has to be "made" and then >"unmade", and then the next one "made", and "unmade", etc.. > >The important thing to understand A/B, to me, is that _first_ you make the moves >to a depth you choose, and only _then_ (in only from a quiet position), is the >position evaluated thoroughly and scored. Then, A/B takes those scores and the >moves that made them, and works back _up_ toward the root position. Best moves, >and scores are _lifted_ up from the tips, back toward the root, (starting >position), and scores for your opponent's moves are negated (so if they're good >for your opponent, they're bad for you), and vice-versa. > >All involving many moves being "made" and "unmade", every second. > >It's quite an amazing algorithim, actually. It's no wonder virtually every chess >program uses it so extensively. > >dave But alpha/beta was sold as a very sophisticated thing. Does it really just boil down to a bunch of takebacks? Isn't there something more than that to alpha/beta? Are we saying that alpha/beta is the same old thing we have all been using for centuries, but just adapted for computers? Still confused. Bob D. Bob D.
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