Author: Jeff Anderson
Date: 19:41:41 02/18/00
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Thanks very much for the creative and lucid explanation. It helped me understand this a lot. Jeff On February 18, 2000 at 19:17:08, Bruce Moreland wrote: >On February 18, 2000 at 12:09:35, Jeff Anderson wrote: > >>Hi, I am still trying to learn about the Alpha-Beta algorithm. I have one more >>question. Thanks to all the people that helped me before: >> >>With the Alpha-Beta algorithm does the program generate the entire tree and then >>score the remaining positions when there has been a cut-off, or when there is a >>cut-off does it not generate unnecessary positions at all? > >It completely avoids some unnecessary crap. > >Here is a game for you. The rules are as follows. There are several bags, each >of which contains some money. You can paw through all of the bags, and at the >end you get to pick one bag. You'll get to keep the lowest-denomination coin or >currency in the bag. Obviously you want the bag that has no low-denomination >money. Even one penny will wreck the bag as far as you are concerned, since >you'll end up getting that if you pick the bag, even if there is a fifty dollar >gold piece in there. > >You look through the first bag. It contains a dollar, a fifty-cent piece, and a >quarter. So you know that you'll score a quarter if you take this bag. > >You look at the first coin in the second bag. It's a nickel. Do you really >need to look through the rest of the coins in that bag? You don't, because you >know that the *most* you are going to get is a penny, wherease you know that you >can get at least a quarter if you pick the first bag. > >So you discard this bag, and it's absolutely safe to do so. If there was a ten >dollar bill in the bag you wouldn't get it, you'd get the nickel. If there was >a penny in the bag, you would have gotten that, but you don't have to care, >since getting a nickel is bad enough, if you can get a quarter some other way. >So that bag sucks. > >You can look through the rest of the bags the same way. You look at coins until >you find a quarter or something worth less, at which point you can discard the >bag. > >You'll notice that if you have a quick way of selecting small coins first, you >can save yourself a lot of time, whereas if you consistently pull the big stuff >out first, you'll have to go through a lot of crap. > >Alpha beta works the same way, you can discard huge chunks of tree, but it works >best if you have a good way of guessing the best move for your opponent. > >bruce
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