This video says (mark 0:45): "We're looking for 2 halves of an x-wing, where one set of corners line up and the other set of corners do NOT line up. Each corner has to be in its own block." I would like to add 2 gimmicks to this algorithm's explanation. Let's assume the skyscraper search is currently being performed row by row. 1. To speed up our search of the second pair of candidates, we should jump over to the first row of blocks below. (It makes no sense to look into rows, which belong to blocks of the first pair. Example: if found pair is in row 1, then further analysis should ignore rows 2 and 3.) 2. The eligible candidates for the second pair must also be in new blocks, which are below(!) those from the first pair. I mean: it's _NOT_ENOUGH_ to say: "the other set of corners have to NOT line up". Example: if our first pair belongs to blocks 1 and 3, then further search can surely ignore pairs of candidates which need blocks 5 and 8, because they are not below blocks 1 and 3. Do I think correct?
@dxsudokuchannel
2 жыл бұрын
Those are good additions. When I did video #27 I was trying to keep the terminology to a minimum. Here's a video on the Skyscraper which is my preferred way of thinking about them and searching for them: kzitem.info/news/bejne/yI97rmtniYiVo44 There is terminology for when you have only two possible location for a candidate within a house. It's called "conjugate pair" which is a terminology I really don't like. So I coined the term "Either-Or Link" which I think is more represented of the relationship between the two candidates within two different cells. So "first half of the X-Wing" part gets replaced by the terminology "two parallel Either-Or Links where on set of corners line up in one chute, and the other two corners are offset in the other chute. "Chute" is another term I left out of #27: kzitem.info/news/bejne/k29rm3lvg5WWq34 Also, for advancing to the next level of advanced puzzle solving techniques, make sure you watch this one: kzitem.info/news/bejne/xIytvGeDkYqpaJg Thanks for comments. I like that you are thinking about how to described in such detail. This is the correct way to learn these techniques!
@bostonpiano2000
4 жыл бұрын
very nice lesson. I like it
@dxsudokuchannel
4 жыл бұрын
Thank you!
@eugenetswong
4 жыл бұрын
Thanks for your use of the terminology, "kill zone". It helps me to understand it better. If it is possible to use both sky scrapers in then it makes sense to use both before making the final markings, right?
@dxsudokuchannel
3 жыл бұрын
The idea of a kill-zone may be my best contribution to this effort.
@RSMover
2 жыл бұрын
Question about skyscrapers in general: is there a reason you can’t eliminate the 8 in r9c8 in addition to the other 2 8s that were eliminated?
@dxsudokuchannel
2 жыл бұрын
This is a really good question. The reason the answer is no is because the way the logic works. It could be the case we have a value of 9 in cell R5C8 and we have a value of 1, 3, or 4 in cell R3C8. In other words, both R3C3 and R5C2 have a value of 8. In this case, then cell R9C8 must have a value of 8. However, what the Skyscraper does do is eliminate any possible 8 candidates sharing a house with both R3C3 and R5C2. Think of the Skyscraper this way: Either R3C3 has a value of 8, or, R5C2 has a value of 8, or, both have a value of 8. Here's another video on the Skyscraper you might find useful in understanding how the logic works: kzitem.info/news/bejne/yI97rmtniYiVo44 In these early videos I tried to keep them simple by not including too much esoteric terminology. But I prefer thinking of Skyscrapers in terms of Sudoku Links as well as pattern matching.
@lukejuras8024
11 ай бұрын
Question with the 2s at 13:09 - isn't there a second skyscraper in columns 2 and 4? Also, if there are two possible skyscrapers, can you apply the eliminations from both of them? The one I found removes 2s from r4c1, r4c3, and r6c6. the one you found also removes the 2 from r4c1 and r9c2. Can I remove 2s from all 4 boxes (one is eliminated with both) at once by applying both skyscrapers?
@dxsudokuchannel
11 ай бұрын
Yes there is. Another way to think about a skyscraper is that it is an X-Chain derivative. If you are not sure you found an skyscraper then think of it as a X-chain chaining sequence. If we assume cell 9 comma 2 is on, the this kills the possible 2 candidate in cell 8 comma 1 and the 2 in cell 9 comma 5. And if cell 9 comma 3 is off, the chaining sequence kicks in and then because of the strong link between 9 comma 3 and cell 6 comma 2, this means cell 6 comma 2 is on. When cell 6 comma 2 is on, the means cell 6 comma 6 is off. And when cell 6 comma 6 is off, this means cell 8 comma 6 is on. If 8 comma 6 is on, both cell 8 comma 1 and cell 9 comma 5 cannot have a 2 in them. So in both cases of cell 9 comma 2 being on and cell 9 comma 2 being off, it ends of killing the 2 in the two cells reference. The skyscraper pattern is just a useful eyeball way of seeing the chaining sequence. If you are interested in X-Chains, which are extremely common, watch this video: kzitem.info/news/bejne/xZ9nx4FrkWKZaY4
@zeldaprime
4 жыл бұрын
At 5:57 I don't see how this is correct, in Column 8 you say that because there can not be an 8 placed in R3C8 there must be an 8 in R5C8. BUT there is still an 8 candidate in R9C8, so why are you able to do this?
@dxsudokuchannel
4 жыл бұрын
This was video #27. I'm much better with video overlay graphics many videos later. You are right. I should have put a black box over the candidates so it was not confusing. The red 8 means that was the value chosen for the cell. If I ever update this video I will definitely make sure to erase out the possible candidates as I fill in the values. What I was trying to demonstrate was the sequence of logic cascading through the puzzle. Anyway, thanks for the good feedback.
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