Author: Simon Read
Date: 05:12:47 01/05/98
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On January 04, 1998 at 20:45:00, Robert Hyatt wrote: >The moral? The pawn hole code definitely improved CB at shallow depths. > But >CB understood outposts and weak pawns and so forth, and going from 4-5 >plies >to 9-10 plies made that pawn-hold code simply a "double penalty" because >it >got a penalty for the hole, plus it got a penalty when it saw how to >utilize >that hole. Superficially, one would think a good position is a good position is a good position. However, it's only good if the subsequent search can take advantage of it. If I understand this correctly, your evaluation function can steer the program towards a position, thinking it's a good position. If the program searches (e.g.) 10 ply beyond that position (when it reaches it, of course) it can take advantage of it and win. If the search is only (e.g.) 5 ply, the search will in fact blow it completely and lose. So evaluation functions with shallow searchers should avoid delicate double-edged positions. This means that a search which is "safe" for a shallow searcher will actually hamper a deep searcher by having too great a margin of safety on some sorts of positions. My other comment on searching is the "side-effect" feature. Let's suppose that your evaluation function counts pawns AND NOTHING ELSE. As your search depth goes up, your program will discover that the way to keep its pawns is to preserve material, so that it's able to defend its pawns. Yet deeper searches will cause it to maintain a good control of the board generally to prevent the opponent getting into position to attack its pawns. Ultimately, of course, this sort of program will still crack up spectacularly, finding a deeeeeeeeeep combination which sacrifices all material in order to win all its opponent's pawns. However, in the meantime, it will (completely by accident) have synthesised a side-effect, discovering things like material and position. The above would make it very difficult to pin down exactly why a program behaves the way it does: it may appear to have a very good idea of positional control but only as a complicated side- effect of some other term in the evaluation function. Simon
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