Author: blass uri
Date: 11:17:06 10/04/99
Go up one level in this thread
On October 04, 1999 at 11:52:43, Robert Hyatt wrote: >On October 04, 1999 at 10:30:40, blass uri wrote: > >>On October 04, 1999 at 09:41:30, Robert Hyatt wrote: >> >>>On October 04, 1999 at 04:26:17, blass uri wrote: >>> >>>>On October 03, 1999 at 23:44:31, Robert Hyatt wrote: >>>> >>>>>On October 03, 1999 at 23:17:29, Ricardo Gibert wrote: >>>>> >>>>>>[snip] >>>>>>> >>>>>>>my webster's defines 'sacrifice' as 'voluntarily giving up something of >>>>>>>value'. I have a hard time saying 'I will sacrifice a ten-dollar bill if >>>>>>>you will give me a 20 dollar bill in return...' >>>>>>> >>>>>>>:) >>>>>> >>>>>>Ok, you got me. I neglected to explicitly state I was refering to the _chess_ >>>>>>version of the term. >>>>>> >>>>> >>>>> >>>>>then here is a 3-move sequence. Sacrifice or combination? >>>>> >>>>>RxB, NxR, RxN. >>>>> >>>>>RxB obviously dumps a rook for a knight. or if you look to the end of the >>>>>combination it wins two pieces for a rook which is a significant advantage. >>>>> >>>>>Sacrifice or combination? >>>>> >>>>>How is that different from QxP+, RxQ, RxR#?? >>>>> >>>>>Dumping a queen for a pawn? Or winning the king? >>>>> >>>>> >>>>>>> >>>>>>>But I don't object to the term being used.. I just think that for a computer, >>>>>>>the concept 'sacrifice' is wrong. It is just a perfectly computable >>>>>>>combinational tree search... >>>>>> >>>>>>You can give up a bishop to obtain a draw by perpetual check and because you >>>>>>never get the material back, it is a called a sacrifice. I know it seems trivial >>>>>>and is not what people generally have in mind when they use the term >>>>>>"sacrifice", but I do believe it's use in such cases is fairly universal. >>>>> >>>>> >>>>>in the case of a computer, it isn't 'sacrificing'. It _sees_ that it can >>>>>draw or that it can win. IE it isn't giving up _anything_. A human might >>>>>toss a bishop 'thinking' (but not sure) than he can force a perpetual. But >>>>>a computer either 'proves' that it can force it, or it won't ever go for the >>>>>move in the first place. >>>> >>>>Not truth. >>>> >>>>Some programs use also selective search. >>>>I believe that Fritz evaluates positions based on some average between >>>>The evaluation based on selective search and the evaluation based on brute force >>>>search. >>>> >>>>If the selective search show perpetual check and the brute force does not see it >>>>then Fritz (in a bad position) might 'think' that he have chances to do a >>>>perpetual check without proving it and play for it. >>>> >>> >>> >>>However, that is a _bug_ and not a _sacrifice_ because the program searched and >>>found the perpetual. Even though it was wrong. But the _search_ said draw, and >>>the tree it searched 'proved' to the program that it was a draw. Unfortunately, >>>if this is the way Fritz searches (I don't believe it does this personally, >>>because it would be so horribly inefficient to do both kinds of search, that >>>Fritz would not be nearly as tactically strong as it is today) then the sac is >>>the result of a bug, not because of a computer 'speculating'... >> >>I know that Fritz is speculating and it is not a bug. > >Sorry, but I don't believe that. It either searches and 'sees' something >or it searches and 'doesn't see' something. I know of no algorithm that can >just 'guess' at a result, and fold this into the alpha/beta search along with >a normal deep null-move search, and then somehow combine those two different >results. The fact that you do not know does not prove that it does not exists. I also do not know if it exists and only guess because I had no explanation to some strange behaviour of the evaluation function that I saw(not often). It is possible that this strange behaviour is a bug. > >IE CSTal doesn't 'speculate' in that form... it just has large positional >scores it tosses into the mix when it sees certain things going on on the >board, such as the king too exposed or whatever. And deep/fast searchers >generally are able to spot the fatal flaw in such speculation and pounce on >it with both feet. I have _never_ seen Fritz behave in this manner because if >it did, it would get crushed by programs that didn't behave like that... It is possible that usually the selective search does not lead to mate so the number of the selective search does not have big influence on the evaluation function. If you use 0.08*selective search score+0.92*brute force search score then you will see problems only when the brute force search score leads to mate. You also can use a different formula that is not linear. > > > > >> >>In a case the selective search show draw by perpetual check and the brute force >>search does not see it the evaluation is probably going not to be 0.00 but >>something between 0.00 and the evaluation of the brute force search. >> > > >Again, I don't believe that fritz is doing _two_ searches, one selective and >one non-selective. It might be adding some selectiveness on to the end of the >normal search, as that has been done as far back as the original greenblatt >program... However, Thorsten has reported seeing lots of 0.00 scores when they >are simply wrong. I have played fritz on the servers and had the opponent say >"I am seeing a draw" while Crafty was seeing +3.00, and in many cases, the 0.00 >was wrong... > > > > >>I do not remember cases of speculating perpetual but I remember cases of >>speculating when it saw a win for itself in some selective lines and decided >>to do a sacrifice(sometimes it may be right sacrifice and it also may be >>a wrong sacrifice). >> >>I guess that it does an everage between selective search and brute force >>because I saw some evaluations that I can explain only by this theory. >> >>I remember a case when the evaluation started to go down slowly from a big >>advantage for white 7-8 pawns towards no advantage and >>The sequence of evaluations was arithmetic sequence. > > >that happens. It simply means that the evaluation is grossly faulty, or that >the search is faulty... we all have that problem from time to time... I have >lost +5 games on ICC and won -5 games, against computer opponents.. I remember an evaluation that cannot be explained by the position It started from +8 or +5 (I am not sure about the exact number and got down by 0.31 every iteration(again I am not sure about the exact number) evaluation like +3 pawns could not be explained by a logical evaluation because if you see that you win the queen it should be at least +8 pawns and if you do not see it because of null move problems the evaluation should be close to 0. Uri
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