Author: Peter McKenzie
Date: 18:11:15 02/09/99
Go up one level in this thread
On February 09, 1999 at 20:39:07, KarinsDad wrote: >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 This is the key point, in a full width game tree there are alot of positions that are totally one sided. A good way to get a feel for this is to get a dump of an actual game tree into a text file and scroll through it with a text editor. Take a look at all the crazy lines in there! If you are serious about trying to write a decent chess program, you will almost certainly want a feature in your program that dumps the search tree to a file. Some bugs are almost impossible to find without this. Of course the trees can be very big, usually you'll want to limit them to 3 or 4 ply. >>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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