Author: Robert Hyatt
Date: 15:46:11 10/04/99
Go up one level in this thread
On October 04, 1999 at 14:17:06, blass uri wrote: >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 I don't "know" that fritz doesn't do this either. That is why I clearly wrote "I don't believe...." which is quite different from saying "It absolutely does not..." >> >>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. > But who does _both_ and merges the scores together? That is the part that makes no sense from a tree-searching point of view. Because the two search spaces overlap a _lot_ and it is a lot of wasted effort... >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 Not necessarily... you can hold off such losses at times by giving up a bit of positional compensation.. a sort of 'positional horizon effect'. But each iteration takes you a ply deeper and you have to give up more to hold the loss beyond the horizon... and down, down, down goes the eval...
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