Author: Tony Werten
Date: 00:29:25 09/08/03
Go up one level in this thread
On September 08, 2003 at 01:55:14, Russell Reagan wrote:
>I think I understand the differences and between fail hard alpha-beta and fail
>soft alpha-beta when described in higher level language (like English), but I'm
>a bit shaky on the implementation differences between the two.
>
>As I understand it, this is fail hard alpha-beta (from Bruce's site):
>
>int AlphaBeta(int depth, int alpha, int beta)
>{
> int val;
> if (depth == 0)
> return Evaluate();
> GenerateLegalMoves();
> while (MovesLeft()) {
> MakeNextMove();
> val = -AlphaBeta(depth - 1, -beta, -alpha);
> UnmakeMove();
To make it more clear, change the next for lines into
if (val>best_score) best_score=val;
*
if (best_score>= beta)
return best_score;
if (best_score> alpha)
alpha = best_score;
}
return best_score;
}
The only difference between fail soft and fail hard is now wether you initialize
best_Score with -oo or alpha.
(* well, almost... For a clean failhard you should lower best_score to beta, if
it is higher )
Tony
> if (val >= beta)
> return beta;
> if (val > alpha)
> alpha = val;
> }
> return alpha;
>}
>
>How would this function be rewritten to perform fail soft alpha-beta? My best
>guess is this (changes commented):
>
>int AlphaBeta (int depth, int alpha, int beta) {
> int val;
> if (depth == 0)
> return Evaluate();
> GenerateLegalMoves();
> while (MovesLeft()) {
> MakeMove();
> val = -AlphaBeta(depth - 1, -beta, -alpha);
> UndoMove();
> if (val >= beta)
> return val; // <-- Return val instead of beta
> if (val > alpha)
> alpha = val;
> }
> return val; // <-- Return val instead of alpha
>}
>
>Also, when called with an infinite window (ex. AlphaBeta(depth, -INFINITY,
>INFINITY)), both the fail soft and fail hard versions should give the same
>score, correct?
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