Author: Robert Hyatt
Date: 10:11:51 07/17/00
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On July 17, 2000 at 12:14:16, Severi Salminen wrote: >Hi! > >Could someone explain or point to a site with info on Null moves and lazy >evaluation? I know null moves are about not making a move, but how is it used to >find better moves? And what is lazy evaluation? > >Thanks for any info. I don't need long explanations, just the facts. > >Severi 1. null move is a 'selective search' algorithm that is based on the idea of 'passing' (not moving). In simple terms, if I 'pass' and let you move two times in a row, and you _still_ can't hurt me, then your position is _really_ bad. In fact, in a chess engine, we 'pass' and then do a shallower-than-usual search, because two moves in a row ought to result in a big advantage. (IE I attack your queen then take it without your getting to move it away). The gain here is that the shallow search still proves your position is no good, but without the larger effort of a normal-depth search. 2. lazy evaluation is just a trick. If I am a queen down, then once I look at _all_ the evaluation terms that might give me a queen in compensation, I can stop and say "this position sucks". IE once you are sure your evaluation can't offset some material loss (or some big positional loss) then you skip out before trying all the small (but time-consuming) eval terms. Since trying them could not possibly affect the score enough to make it worthwhile doing so. This has been around almost forever..
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