Author: Steve Coladonato
Date: 06:31:29 05/04/00
Go up one level in this thread
On May 04, 2000 at 09:08:59, David Franklin wrote: >On May 04, 2000 at 08:13:51, Steve Coladonato wrote: > >>On May 03, 2000 at 18:26:09, Robert Hyatt wrote: >> >>>Alpha/beta finds the best move and only proves that the other moves are >>>worse, without proving how much worse they are. To do this requires a lot more >>>time. >> >>You have both given me essentially the same answer. I've never looked at the >>code for a chess engine so I don't know exactly what Alpha/Beta does. But the >>answers here are confusing to me. I was under the impression that the best move >>was determined by calculating the eval for the candidate moves. Your answers >>are implying that that is incorrect and something else is used to determine the >>best move, not the eval for the position. But if that is the case, is not what >>the program calculates somehow related to the eval? And if so, saving the >>result in an array would not incur that much more overhead so that the program >>would know what the top three moves are or rather the order of all candidate >>moves based on whatever it is calculating. >> > >The point is that if all you care about is the best move, you often don't >need to evaluate the other moves that carefully. Here's a hypothetical >example: > >I evaluate move A as +10, and start analysing move B. Very quickly I find >my opponent can make a move in reply to move B so that move B is worth only >+9. At this point, I *stop* looking at move B, as it can't possibly be as >good as move A. But I don't *know* the value of move B at all, just that it's ><= +9. There might be another response to move B that is devastating, so >that the true value is -999, but I don't spend time finding out. So at >the end of the search, all I know is the score for the best move. > >[Note that these "skip the full search" decisions happen all the way down >the search tree - the overall speedup is *huge*; it's not something you >want to avoid doing]. David, OK. So now you look at move C and its score is +12 (whatever that score is based on) so now move A is removed as the best move. What is used to determine this rough score that is being used to determine the best move? I'm also getting the impression that you are looking only a couple of ply deep to determine all this. Thanks. Steve
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