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I want to do alpha-beta pruning on this tree:

  1. Consider nodes J and K. K is the max. Therefore, node D has an alpha value of 20, node B has a beta value of 20.

  2. Move to Node E. Pass the beta value of 20 to node E. Node L has an alpha value of 30, therefore, at this point 30 (alpha) > 20 (beta) and we can prune the E to M branch.

  3. Now is my question. My original beta value at node B was 20, and the alpha value passed up to node A was 20. Then, in step 2, I changed the alpha value to be 30. Do i then change the beta value at node B to be 30, and the alpha value at node A to be 30? (and therefore pass 30 as the alpha value to node C)? Or do I keep the original value of 20 at node B, A and C?

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I couldn't understand your question clearly, however, it think you are making a slim mistake. let's look at the flowing code from "Russell " and do pruning step by step: enter image description here enter image description here
Assume your are In D and you have traversed its both children, Alpha at D becomes 20. We then return back to B, Beta becomes 20 (note that Alpha is -Inf in D). We go to E then L and then back to E. 30 is greater than Beta, so the rest is pruned. Not that Alpha remains -Inf in E, though its value here isn't important. we go back to B the value of Beta remains on changed 20. If we go to F and return back to B Beta remains unchanged and is still 20 (Note that Alpha is still -Inf). We return to A, Alpha becomes 20, And Beta remains +Inf.
So, Considering the code, the max node does not get the Alpha value from its children, rather it decides on its own by setting Alpha = max(Alpha, and returned value of its child) after visiting each child. the same is true for Beta.

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