Author: KarinsDad
Date: 12:09:35 05/14/99
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On May 14, 1999 at 14:25:05, J. Wesley Cleveland wrote: >On May 14, 1999 at 13:14:36, KarinsDad wrote: > >> > >You seem to misunderstand alpha-beta. Alpha-beta *always* finds the same "best" >move as exhaustive search does at any depth. What it does not tell you is how >much better the "best" move is. Alpha Beta finds the best move "under the same depth and constraints", but does not find the best move. That is why chess programs are not perfect at this point in development (since they are restricted from performing exhaustive searches). If after a quiescence alpha beta search, you determine that you have lost a knight for a pawn, most programs do not search much beyond that to find the forced checkmate 4 moves later and hence do not find the sacrifice. The GM, on the other hand, may find this. What this effectively means is that the alpha beta routines "prune" out lines which do not look promising, but in actuality are very promising. For example, let's take the opening. 1. d4 d5 2. c4 c6 3. Nc3 Most alpha beta programs would prune out the move c4 at ply 3 since there appears (via the evaluation routine) for there to be a lot of other moves at that point which develop pieces and are stronger. This does not mean that a program may not find this on a re-search or something, however, you have to get to ply 11 or so to start to appreciate the advantages of c4 and you will never get there with a standard alpha beta search. This is the MAIN reason that programs without opening books are so weak. The GMs have spent years analyzing to some depth various openings and have found moves such as 2. c4 which are strong, but a program would not find these moves in any reasonable amount of time since it prunes via alpha beta. So, in the example of exhaustive 6 ply search, there are many lines dropped via alpha beta at ply 1, 2, 3, 4, and 5 so that the evaluation routine does not even get to compare those nodes at ply 6. If a line is dropped at ply 4, how can you evaluate it's grandchildren at ply 6 or ply 9 or ply 12 and find that it is in fact actually stronger, not weaker? Do you still think that I do not understand Alpha Beta? Min Max and Alpha Beta are the reasons that other techniques such as null move and razoring, etc. have been developed. If we were doing exhaustive search to ply 14 and alpha beta search to ply 20 after that, no human in the world could beat the program and these other techniques would not be needed. KarinsDad :)
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