Author: Robert Hyatt
Date: 06:45:37 06/22/02
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On June 22, 2002 at 08:23:01, Stan Arts wrote: >Hello, > >Last time I wrote a question regarding strange behaviour of my simple fixed >depth alpha beta program. Indeed there was a bug in the program causing this >behaviour. Thanks for suggesting this because I had looked over my program >many times thinking there weren´t so I simply experimented a lot until I >located the error. Thanks! It now calculates "perfect" it seems! (=play as >should be expected upto X ply deep) Just like a real chessprogram... :) > >Anyway, now I noticed something else strange to the play of my program. When >it´s searching, on EVEN ply´s (for instance 6, or 8) it seems to play 100% >correct. But on ODD ply´s (like 5 or 7) sometimes it makes mistakes. (In the >calculationtree I count the 1st ply to be the possible moves by the computer, >of which one of these will actually be played after calculation. , so say a >5 ply deep search by my program is 3 moves by the computer, and 2 by the >opponent, and therefore, the deepest ply (5th) is that way a computermove) >Now I looked over the code very often, but can not find an error, but this >morning I suddenly thought of this: When doing a fixed depth search, without >extensions, would these evaluation errors on ODD ply´s come from the fact >that on this 5th ply it would for instance plunge it´s queen into a random >(protected) pawn because on the deepest move, ofcourse it can´t see a >recapture? And therefore mess up the evaluation because it thinks it can get >this material back? It seems to happen just rarely luckly, but I imagine in >some occasions with a queen centrally located for instance and on a shallow >search such as just 5 ply (not much change to the total position), this >material gain would show up in EVERY branch and therefore mess up the >evaluation? > >Would this be right and did anyone have the same problem? because then it´s >not a bug and I would sleep a little better at night... > >:) > >Stan There is no reason for odd plies to produce errors. Odd plies are harder to search than even plies. IE if you compare your effective branching factor for odd plies you will find it higher. The reason is that for odd plies, every branch at the frontier must be searched once you have searched the first root move. For even plies, you only search one move at the frontier before alpha/ beta cuts it off. Even ply searches (fixed depth like you are doing) double the nodes for the previous depth. Odd ply searches multiply the previous search by roughly 38.. But it should not produce errors if you don't have a bug.
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