Author: Robert Hyatt
Date: 19:37:48 10/04/99
Go up one level in this thread
On October 04, 1999 at 20:33:35, blass uri wrote:
>On October 04, 1999 at 18:46:11, Robert Hyatt wrote:
>
>>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...
>
>The relevant position is:
>
>7k/4K2p/7P/3p4/8/4Q3/1q6/8 w - - 0 1
>
>The first evaluation above 0 of Fritz5 is +5.16 pawns for white
>I do not believe that it can be explained by positional compensation.
>
>If it is because of a bug then Fritz3 and Fritz4 have the same bug(I do not know
>if Fritz5.32 shows similiar behaviour)
>
>Uri
are you sure this is the position? This seems to be a mate in 8 according
to my PII/300 notebook (crafty):
9 4.89 0.48 1. Ke6 Qa1 2. Kd7 Qa4+ 3. Kd6 Qb4+
4. Kxd5 Qb7+ 5. Kc4 Qa6+ 6. Kb4 Qd6+
7. Kb5 Qd5+ 8. Kb4 <HT>
9-> 7.46 0.48 1. Ke6 Qa1 2. Kd7 Qa4+ 3. Kd6 Qb4+
4. Kxd5 Qb7+ 5. Kc4 Qa6+ 6. Kb4 Qd6+
7. Kb5 Qd5+ 8. Kb4 <HT>
10 8.27 ++ 1. Ke6!!
10 12.34 Mat08 1. Ke6 Qa1 2. Qb6 Qe1+ 3. Kd7 Kg8 4.
Qf6 Qg3 5. Qe6+ Kh8 6. Qe8+ Qg8 7.
Qe5+ <HT>
10-> 18.59 Mat08 1. Ke6 Qa1 2. Qb6 Qe1+ 3. Kd7 Kg8 4.
Qf6 Qg3 5. Qe6+ Kh8 6. Qe8+ Qg8 7.
Qe5+ Qg7+ 8. Qxg7# <HT>
11 23.23 Mat08 1. Ke6 Qa1 2. Qb6 Qe1+ 3. Kd7 Kg8 4.
Qf6 Qg3 5. Qe6+ Kh8 6. Qe8+ Qg8 7.
Qe5+ Qg7+ 8. Qxg7#
11-> 36.33 Mat08 1. Ke6 Qa1 2. Qb6 Qe1+ 3. Kd7 Kg8 4.
Qf6 Qg3 5. Qe6+ Kh8 6. Qe8+ Qg8 7.
Qe5+ Qg7+ 8. Qxg7#
12 50.03 Mat08 1. Ke6 Qa1 2. Qb6 Qe1+ 3. Kd7 Kg8 4.
Qf6 Qg3 5. Qe6+ Kh8 6. Qe8+ Qg8 7.
Qe5+ Qg7+ 8. Qxg7#
This page took 0 seconds to execute
Last modified: Thu, 15 Apr 21 08:11:13 -0700
Current Computer Chess Club Forums at Talkchess. This site by Sean Mintz.