Author: Bruce Moreland
Date: 13:32:37 07/26/01
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On July 26, 2001 at 12:41:34, Uri Blass wrote: >In the relevant example not trading is a good alternative and there is no >problem of losing material. > >In theory there may be a position when the mistake was 4 plies before the >trading but practically it does not happen often. > >I agree that it is better to solve things by evaluation but it does not mean >that search does not help. Bob is right that there are lots of cases where search does not help enough. We have the concept of effective depth. An obvious example is doubled pawns. A program that does not evaluate bad doubled pawns will lose pawns beyond its search horizon. If it evaluates bad double pawns, it will stop losing these pawns. Essentially, it is able to avoid having its search depth rendered ineffective due to big sub-trees that are full of mistakes. A fast program with a limited eval function is vulnerable to what I call a "dead area" in its search. A dead area is a place where all of the evals in a sub-tree are wrong, and extra depth doesn't make the evals much better. There are lots of things that can cause vast dead areas, for instance improper handling of cases involving bishops of opposite color. Extra search does not help in these cases, so time spent searching is wasted. I don't understand why Bob is so hot on this one type of position though. Crafty didn't always evaluate this kind of pawn structure, and it's not like it was a bad program before it did. Perhaps this is an oblique continuation of the Deep Blue/GM argument. There are a lot of cases that you want to catch if you can, but I don't think that absence of checking for these cases is an a priori argument that a program sucks. If you told me I could fix any number of these minor cases (pawn structure, king safety, opposite bishops, insufficient material, etc., are more essential major cases) that I wanted to fix, and run the same speed I run now, or I could use my current stuff on a machine twice as fast, *against computers at least* I would take the faster machine every time. bruce
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