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Subject: Re: alpha-beta is silly?

Author: Don Dailey

Date: 16:48:17 06/03/98

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So here is a simple, high level description of Alpha/beta for you
non-chess programmers.  I'll leave out the nasty details.

Pretend we have a position with only two possible moves, lets
say they are e6 and Kb5 just for the sake of argument.

Now we look at e6, do a search and discover that we win
exactly 1 pawn.  Great.

So now we look at Kb5.  Let's say the opponent has 50 different
replies to this.  The very first one he looks at wins a queen
for him.  So what do we do now?  We KNOW we can play e6 and win a
pawn, or we can play Kb5 and lose a queen.  We now know looking
at the other 49 moves of his is silly, because we already know
we can avoid the whole mess by playing e6 and winning a pawn.

The only thing we might learn by searching the other 49 moves
is that he might win even more than a queen, if we were foolish
enough to play Kb5!

We "prunned" 49 moves without any side effects or taking any
chances.  Alpha beta prunning is a free speedup.

- Don




On June 02, 1998 at 05:03:32, Inmann Werner wrote:

>Alpha - Beta is silly?
>
>Yesterday, I tried the LCTII Test Suite, position LCTFIN01. I was
>amazed, that INMICHESS (my program) needed  depth 9 to solve  the
>position. (running pawn)
>
>
>The position: Kc2/Pf2/kc4/Pg4/Pf5/Ph6/pb7/pf7/pg7/ph6
>Best Moves: 1.f6  2.gxf6 3.f4 4.b5 5.g5 6.fxg5 7.fxg5 8.hxg5 9.g6
>....->Queen
>
>I did not understand, why it did not recognize the running pawn in depth
>5. The 3 beating moves afterwards should be made in the quiet search,
>and then there is the running pawn!
>Nothing at all! I debugged the program.
>Very fast it was in depth 5 and then in the quiet search. There it made
>the 3 beating moves up to ply 8 and the calculatin function brought back
>-844. (more then 8 pawns)!! Super!
>But then alpha-beta threw the move away, because it was blacks move, and
>black did not want to get this bad rating. This continued until the
>normal search was in depth 9, although the black moves are forced.
>But think! This all is normal alpha-beta with no programming failure!
>
>Conclusion: alpha-beta does no moves, where the player who moves get a
>bad rating, especially in qiet search, although if the moves are forced.
>It thinks, if ist my move, it only can get better. (Something like a
>Null-Move thinking). On one side, this makes alpha-beta fast. On the
>other side, trivial things get moved behind horizon.
>If you try to change this, the program gets worse, because in most
>positions, above is right.
>
>Is above conclusion correct?
>Has anybody thought deeper about this?
>
>Maybe, yesterday the first time, I understood alpha-beta.
>
>Besides: Fritz 4 also recognized the running Pawn in depth 9!
>
>Best Greetings
>
>Werner



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