Author: Robert Hyatt
Date: 09:50:44 02/05/04
Go up one level in this thread
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. > >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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