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Subject: Re: Confusion on Null Move

Author: Robert Hyatt

Date: 19:59:52 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
>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.
>
>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,
>


one of the big 'time savers' is what happens _below_ a sacrifice for nothing.
Qxp pxq.  And now no matter what two moves white plays in a row, black's
position is still 'good' and everything below such moves gets trimmed away
instantly...

IE with a full-width search, there are _many_ captures that are losers.  Null
move reduces the depth below these moves and make dismissing them quick and
easy...

>KarinsDad :)



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