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Subject: Re: Confusion on Null Move

Author: KarinsDad

Date: 17:39:07 02/09/99

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On February 09, 1999 at 20:18:20, Eugene Nalimov wrote:

>On February 09, 1999 at 20:09:12, KarinsDad wrote:
>
>>I understand the concept of Null move, but I am confused as to why it works so
>>well.
>>
>>My confusion lies in the area of piece taking.
>>
>>In a lot of positions, both sides have moves which will allow a piece to be
>>taken by the other side. Often, in an equal game, this will not matter as the
>>side protecting the piece can gain back equal or greater material if the other
>>side takes the piece.
>>
>>Since chess programs (as a general rule) play a "reasonable" game of chess (i.e.
>>they do not hang pieces multiple ply down), then it would seem that at any given
>>position being checked, that it is the rare case that a 1 ply null move would
>>not drastically improve the score since regardless of the move being checked,
>>the best null move will be a piece take that improves the score.
>
>Programs plays reasonable moves, but most of the positions
>in the game trees are not reasonable. For example, in half
>of the nodes, program must check *every* possible move, so
>null move will help greatly after majority of that moves.
>
>Eugene
>

For someone who understand this well, your answer may be sufficient. But there
isn't enough detail for me to get the gist of the "why of it". Sorry.

For example, if as you state that in half of the nodes, the program must check
every possible move, then it would seem that those nodes were acquired by one
side or the other making a reasonable move (as opposed to a move which was
easily refuted with a one or a few move search response). When the null move is
applied to all of the responses to the reasonable move (giving the responder two
consecutive moves), it would seem that the reasonable move would be blown out of
the water quite often by at least one of the responses and you haven't
accomplished anything.

I would think that null move would be useful when you get a cutoff. If you can
refute my move and even if I make two moves in a row, you can still refute it,
then I have picked a really lousy move. But in this case, you already have a
cutoff, so why search deeper on it. I think I must be missing something here.

KarinsDad

>>I can understand that in certain positions, improving the score by a single
>>piece is not sufficient to alter the score enough to prevent a beta cutoff, but
>>to me (not having the data on hand since my program does not do this yet), it
>>would seem that this would be a rare case.
>>
>>Since null move is used by most everyone's program, this assumption would appear
>>to be false. I was just wondering if someone could explain to me why this is so
>>and also possibly the multiple cases (and possibly their frequency) of positions
>>that null move works well on (where it prevents additional searching).
>>
>>Or is it just a case that null move infrequently helps the search, but when it
>>does so, it cuts out major chunks of the search tree and not searching (and
>>calculating moves within) these major chunks makes up for doing multiple
>>evaluations (and an additional legal move generation) per node?
>>
>>Thanks in advance,
>>
>>KarinsDad :)



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