Author: Robert Hyatt
Date: 10:24:01 02/05/04
Go up one level in this thread
On February 05, 2004 at 12:54:08, Slater Wold wrote: >On February 05, 2004 at 12:50:44, Robert Hyatt wrote: > >>On February 05, 2004 at 12:46:04, Slater Wold wrote: >> >>>On February 05, 2004 at 12:22:44, Bob Durrett wrote: >>> >>>>Referenced by: >>>> >>>>http://www.talkchess.com/forums/1/message.html?345569 >>>> >>>>> An alpha cutoff is what happens when you search the second move, >>>>>> and you prove that if you play that move, your opponent has a move >>>>>> he can play that will produce a score less than your "lower bound" >>>>>> you established for the first move. There is no need to search >>>>>> further. >>>>>> >>>>>> For example, after that +1 on the first move, you try the second >>>>>> move and after trying the first move the opponent has in reply to >>>>>> that move, you discover you _lose_ a pawn. The score is -1.0... >>>>>> There is no need to search other opponent moves to produce a >>>>>> score even lower than -1.00, because you already know this move >>>>>> is at _least_ -1.00 and possibly worse, while the first move is >>>>>> +1.00. You stop searching this move and move on to your third >>>>>> choice... >>> >>>I haven't looked at many programs, other than TSCP, source, but I have a few >>>question about this also... >>> >>>When do most engines call the qsearch? After selecting a move believed to be >>>correct, of at the end of each search tree? >> >>When remaining depth = zero. IE if you want to do a normal 3 ply search, and >>you have no search extensions, I make a move at the root, subtract 1 from depth, >>and recursively call search. There I make a move for the opponent, subtract 1 >>from depth and if it is > 0 (2-1 > 0 so true here) I again call search, >>otherwise I call the q-search. At ply=3 I make a move for the program again, >>subtract one from depth and now call the q-search since depth==0 after the last >>subtraction. >> >>I did 3 full plies of looking at everything, but ply-4 and beyond are captures >>only. >> >>> >>>The reason I ask is, say you get a cutoff, because the 2nd move produced -1.0 >>>(as in above), but after 4 checks and a capture, you regain that pawn & better >>>position. >> >>Alpha/Beta is called a "depth-first" search strategy, because you _never_ stop >>searching before you reach a tip position, therefore in the above you regain the >>material before reaching the tip. > > >So the tip, is where captures and checks stop? tip or more commonly "leaf"/"leaves". Yes > > >>>How do you prevent from not making a move that caused a cutoff, that actually >>>leads to a better position? >> >>depth-first. See above.
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