So this is my question.

I am trying to understand this part of the book:

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At d) why do we stop looking at the other nodes in that branch? There could be a acceptable value next to the 2? I am just trying to understand alpha beta pruning for a while.


1 Answer 1


At $d)$, minimizer is comparing the $2$ he found with the currently best option for the maximizer higher in the tree (at node $A$, where maximizer will choose between $B$,$C$,...), which is $3$, at node $B$. He then knows that maximizer will not choose $C$.

The node $C$ will have value $2$ when minimizer sets it and would have value $2$ or less if minimizer explored it fully, so he doesn't need to waste his time searching other branches from $C$ if he already knows that maximizer will not choose anything lower than $3$.

I think what confuses you may be the following:
You are imagining a real-life gameplay scenario, but this is different in the sense that minimizer and maximizer must play their best move and will calculate it correctly.
So there is no way that minimizer could in this situation "fool" maximizer by setting the wrong value of $C$ (something higher than $3$) to force maximizer to choose it. If we assume that scenario a possibility, we must also implement the maximizer with "free-will" so he may not choose his best option but this is no longer an $\alpha$-$\beta$ pruning :)

  • $\begingroup$ My gripe is that, the second branch (one with ptuning) might have a value greater than 2? Anyway thanks for the answer, I guess the point is, that even if there is a number less than 2, it will not be chosen by the maximizer. $\endgroup$ Aug 24, 2018 at 18:49
  • $\begingroup$ Yes, exactly. And if there is a value greater than 2 (better for maximizer), minimizer must not choose it because it is not his best option (and he is the one choosing at node C). $\endgroup$ Aug 24, 2018 at 18:52

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