Alpha-beta pruning for Minimax

Question

I have spent a whole day trying to implement minimax without really understanding it. Now, , I think I understand how minimax works, but not alpha-beta pruning.

Thi

Answer 1

To understand Alpha-Beta, consider the following situation. It's Whites turn, white is trying to maximize the score, black is trying to minimize the score.

White evaluates move A,B, and C and finds the best score is 20 with C. Now consider what happens when evaluating move D:

If white selects move D, we need to consider counter-moves by black. Early on, we find black can capture the white queen, and that subtree gets a MIN score of 5 due to the lost queen. However, we have not considered all of blacks counter-moves. Is it worth checking the rest? No.

We don't care if black can get a score lower than 5 because whites move "C" could keep the score to 20. Black will not choose a counter-move with a score higher than 5 because he is trying to MINimize the score and has already found move with a score of 5. For white, move C is preferred over move D as soon as the MIN for D (5 so far) goes below that of C (20 for sure). So we "prune" the rest of the tree there, pop back up a level and evaluate white moves E,F,G,H.... to the end.

Hope that helps.