Author: Bruce Moreland
Date: 20:33:44 04/23/00
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On April 23, 2000 at 18:30:03, Michael Fuhrmann wrote: >On April 23, 2000 at 18:13:42, Robert Hyatt wrote: > >>On April 23, 2000 at 17:15:52, Michael Fuhrmann wrote: >> >>>Why would a program ever underpromote to a rook? Saw Crafty do this recently. >>>(In this particular case, it had no impact on the outcome of the game.) >> >> >>1. it is necessary at times. IE if you promote to queen, you stalemate your >>opponent. if you promote to rook, you can still win without stalemating him. >> > >Of course. Didn't think of that. > >>2. In the case of chess engines, it is pretty common to see this. The most >>common reason is that the =R is not a check, when the =Q is a check, or the >>rook allows fewer checks later in the tree. So by promoting to a rook, it >>avoids some tactic that it really can't avoid... IE this is a horizon effect >>situation.. > >Not sure I follow that. But I guess that was the case. Here's the relevant bit >cut-and-pasted from the log file, in case anyone's interested. The >underpromotion occurs on the a file. Neither =Q or =R leads to check. That's a case where underpromotion is the same as promotion to a queen, since in both cases the promoted piece comes off right away. It doesn't really matter what the program promotes to. You'd usually expect it to promote to a queen, since that's usually the first move tried, but sometimes it doesn't work quite like this. I've only seen one game messed up due to a spurious underpromotion. My program underpromoted to a B once, when it already had a B of the same color. If it had promoted to a Q, the opponent would have had to capture and it would have been BN vs N, with a few pawns scattered around, which should have been an easy win. As it was, the opponent just left the B there, and my program couldn't win with BBN vs NN, because there was nothing for the two bishops to gang up on and attack. bruce
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