Author: Ricardo Gibert
Date: 14:47:20 02/22/04
Go up one level in this thread
On February 22, 2004 at 17:23:06, Bob Durrett wrote: >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. What he described applies to both minmax and alphabeta. For a friendly description description of alphabeta, check out the section "Alpha-beta and the bag example" at the following link: http://www.brucemo.com/compchess/programming/alphabeta.htm What constitutes a "take back" or "unmake" is disguised in his explanation, but you should be able to work that out.
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