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Subject: Re: Confusion on Null Move

Author: Eugene Nalimov

Date: 17:18:20 02/09/99

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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
>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.


>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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