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TPM
Joined: 13 Jun 2006
Posts: 4
Location: Gauteng, South Africa
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 Posted: Tue Sep 12, 2006 4:02 pm Post subject: September 12 Very hard |
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Just when I thougt I'm getting the feel for this thing, pops one out of hell(if there is such a place).
Did the basic elimination until I got and X-wing on 5's in rows 3&7 cols 2 & 7. So, far I have the following:
7 {2369} {569} {238} {245} {248} {345} 1 {46}
4 {23} {15} {1237} {257} 6 {35} 9 8
{136} {356} 8 {13} 9 {14} 7 {3456} 2
5 {78} 3 6 1 {27} {248} {248} 9
{69} {6789} {679} 4 {27} 3 1 {28} 5
2 1 4 9 8 5 6 7 3
8 {57} 2 {17} 3 {147} 9 {456} {46}
{1369} {369} {169} 5 {246} {2489} {2348} {2348} 7
{369} 4 {57} {278} {267} {2789} {2358} {238} 1
Asking for a hint gives me a 4 in r3c6. can't see why. Am I missing something? |
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blackwood
Joined: 12 Sep 2006
Posts: 4
Location: Lehi, Utah, USA
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| Posted: Tue Sep 12, 2006 5:51 pm Post subject: |
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There is a hidden set {5,6} in R3C8 and R7C8. This then leaves the 4 in R3C6 as the only 4 for R3. (A pinned square)
Blackwood |
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Marty R.
Joined: 12 Feb 2006
Posts: 5770
Location: Rochester, NY, USA
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| Posted: Tue Sep 12, 2006 9:29 pm Post subject: |
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Just to say the same thing as Blackwood did, except a little differently. In rows 4, 5, 8 and 9 in col. 8 there is a naked quad of 2348. When you make the eliminations from this quad, the 56 goes from a hidden to naked set.
I personally do not have the eye to spot hidden sets, but can generally spot the complementary naked sets. |
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George Woods
Joined: 28 Mar 2006
Posts: 304
Location: Dorset UK
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| Posted: Tue Sep 12, 2006 11:24 pm Post subject: |
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I solved this one by what seems to be like an XY wing. r7c6 cannot hold 4 'cos the 4 propagated by the 46 in col9 would make for a 4 in r3c6 which is of course a no no- so there is now a 17 pair in row 7 allowing progress- and a solution to be obtained.
Trying to understand the analysis above. I found a diificulty with the original contents of r8c8 and r9c8 (given as 2348 and 238) I make these two 23468 and 23568 so where have I missed the obvious? |
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Marty R.
Joined: 12 Feb 2006
Posts: 5770
Location: Rochester, NY, USA
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| Posted: Wed Sep 13, 2006 4:55 am Post subject: |
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| Just to give yet one more illustration of how many different ways there are to solve many puzzles, on this one there were two Finned X-Wings, an X-Wing and then strong links (aka fork, aka skyscraper) on "4" opened it up. |
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TPM
Joined: 13 Jun 2006
Posts: 4
Location: Gauteng, South Africa
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| Posted: Wed Sep 13, 2006 5:23 am Post subject: |
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Okay ta everybody. I should have seen the hidden 56 in 8. Just had one of those bad days. However, I've learned now about a quad. I have never used it before as I did not understand it.
George, you cannot have a 6 in r8c8 as the 6 in r7 can only be in col 8 or col 9, hence eliminating any other 6's in the last box. alternatively, looking at the bottom 3 boxes, you can see that the 6 in the first 2 boxes is in rows 8 & 9 hence forcing the 6 in the last box to be in row 7 only.
The 5 is eliminated by the X-wing on the 5's in rows 3&7 cols 2&8. Hope this helps. |
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George Woods
Joined: 28 Mar 2006
Posts: 304
Location: Dorset UK
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| Posted: Wed Sep 13, 2006 7:56 am Post subject: |
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| Marty R. wrote: | | Just to give yet one more illustration of how many different ways there are to solve many puzzles, on this one there were two Finned X-Wings, an X-Wing and then strong links (aka fork, aka skyscraper) on "4" opened it up. |
So how do you describe my method? (I spotted the "strong" link between r7c6 to r7c9 to r1c9 to r3c6 with the last one in my terminology a negative one i.e normally a 4 in one cell denies the 4 in the "strongly" linked cell but here a 4 in r1c9 forces the 4 in r3c6 - so in my terminology we have a chain but with one negative which makes for a nonsense) what I can't do (ignorance) is classify it a s a skyscraper or finned Xwing - never having taken a course in sudoku solving!
And whats more this leads to a solution directly - never putting more than a "double" candiate for a cell! and no need for the XWing |
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Marty R.
Joined: 12 Feb 2006
Posts: 5770
Location: Rochester, NY, USA
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| Posted: Wed Sep 13, 2006 3:50 pm Post subject: |
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| Quote: | | So how do you describe my method? |
I don't have an answer; maybe one of the more creative types can describe it. But whatever, if it's based on logic, that's the name of the game.
| Quote: | | what I can't do (ignorance) is classify it a s a skyscraper or finned Xwing - never having taken a course in sudoku solving! |
I've taken that course. It consisted of scouring web sites for basic and advanced techniques and printing them out to build up a reference base. Also on this forum, when someone used a technique that wasn't familiar to me, I asked about it and printed out the answer for my reference files.
I've now been into this nonsense long enough that I don't have to refer to that material very often, but it was a great aid in learning. The technique I mentioned above--Finned X-Wing--was one I didn't see on the web sites and learned from this forum. I suspect with my questions that I may have been considered a pest at times, but asking is the way I learn. |
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TKiel
Joined: 22 Feb 2006
Posts: 292
Location: Kalamazoo, MI
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| Posted: Wed Sep 13, 2006 10:09 pm Post subject: |
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| George Woods wrote: | | So how do you describe my method? |
It's known either as a 'turbot fish' or more generally as multi-colouring.
The grid probably looks something like this:
| Code: |
*--------------------------------------------------------------------*
| 7 23569 569 | 238 245 248 | 3458 1 46B |
| 4 235 15 | 12378 257 6 | 358 9 58 |
| 136 356 8 | 13 9 14a | 7 3456A 2 |
|----------------------+----------------------+----------------------|
| 5 78 3 | 6 1 27 | 248 248 9 |
| 69 6789 679 | 4 27 3 | 1 258 58 |
| 2 1 4 | 9 8 5 | 6 7 3 |
|----------------------+----------------------+----------------------|
| 8 567 2 | 17 3 147 | 9 456 46b |
| 1369 369 169 | 5 246 12489 | 2348 23468 7 |
| 369 4 5679 | 278 267 2789 | 2358 23568 1 |
*--------------------------------------------------------------------*
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The cells in row 3 are conjugate (strong links). The cells in column 9 are also. The cells with capitals both can't be 4 so one or both of the small lettered cells must be. Any cell that 'sees' both of a and b can't be a 4.
Last edited by TKiel on Thu Sep 14, 2006 12:10 am; edited 1 time in total |
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CT Yankee
Joined: 17 Jul 2006
Posts: 11
Location: New England
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| Posted: Wed Sep 13, 2006 10:59 pm Post subject: |
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| I solved this one using basic techniques, and then a Swordfish on the 5s. This is the very first time I have ever had to use a Swordfish! I've been aware of the technique for a long time, and I always look for it, but this is the very first time it ever really stood out as a way forward when I couldn't spot anything else. Most of the time I suspect I've been using other methods that supercede fishing expeditions - that's the only explanation I have for why I never seem to need a technique that everyone else considers indispensable. |
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TKiel
Joined: 22 Feb 2006
Posts: 292
Location: Kalamazoo, MI
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| Posted: Thu Sep 14, 2006 12:59 am Post subject: |
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CT Yankee,
The X-wing and the Swordfish are complementary and basically make the same exclusions. Many fish are accompanied by other fish. It makes sense when you think about it. If there are 5 rows & columns not solved for a particular number and 2 of those row & columns form an X-wing, the other cells must form a Swordfish.
As to why you've never had to use a Swordfish before I don't have an answer. Maybe you need to start doing easier puzzles.  |
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riderdude
Joined: 04 Jun 2006
Posts: 9
Location: Mass
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| Posted: Fri Sep 15, 2006 3:07 am Post subject: |
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| TPM wrote: | | George, you cannot have a 6 in r8c8 as the 6 in r7 can only be in col 8 or col 9 |
sorry, i'm with George. why can't there be a 6 in r2c7? |
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TKiel
Joined: 22 Feb 2006
Posts: 292
Location: Kalamazoo, MI
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| Posted: Fri Sep 15, 2006 10:38 am Post subject: |
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| riderdude wrote: | | sorry, i'm with George. why can't there be a 6 in r2c7? |
Not sure if the cell location is just a misprint, but r2c6 is a 6. |
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riderdude
Joined: 04 Jun 2006
Posts: 9
Location: Mass
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| Posted: Fri Sep 15, 2006 1:14 pm Post subject: |
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| TKiel wrote: | | Not sure if the cell location is just a misprint, but r2c6 is a 6. |
doh
yes, typo. i meant r7c2. if i can get rid of the 6 there, i can get rid of other 6s in box 9. then the 4 in r3c8 is apparent. i was at pretty much the same place as TPM when i got stuck, too.
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David Bryant
Joined: 29 Jul 2005
Posts: 559
Location: Denver, Colorado
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| Posted: Fri Sep 15, 2006 6:35 pm Post subject: Catching swordfish |
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| CTYankee wrote: | | This is the very first time I have ever had to use a Swordfish! ... |
There's a very cute example of a swordfish in the archives over at Ruud's excellent web site.
Oh -- it's on the "8"s, in columns. And it's a real swordfish -- the complement is a "jellyfish". There's also a "finned swordfish" (on a different digit) after you find the straightforward one. Better yet, finding those two finny friends doesn't solve the puzzle -- there's still a lot of work to do after you've reeled in your catch! dcb |
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Marty R.
Joined: 12 Feb 2006
Posts: 5770
Location: Rochester, NY, USA
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| Posted: Fri Sep 15, 2006 9:00 pm Post subject: |
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| Quote: | | And it's a real swordfish -- the complement is a "jellyfish". |
David, could you elaborate? I was under the impression that complementary fish added up to nine, whereas a swordfish + jellyfish = seven.  |
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David Bryant
Joined: 29 Jul 2005
Posts: 559
Location: Denver, Colorado
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| Posted: Fri Sep 15, 2006 9:27 pm Post subject: Fish & complements |
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| Marty R wrote: | | was under the impression that complementary fish added up to nine ... |
Hi, Marty!
The formula you need is
order(fish) + order(fishy complement) + # of digits "x" already resolved = 9
So if no "5"s (for instance) have yet been placed in the puzzle, and there's a "swordfish" on the "5"s, then the complementary fish will be of order 6. (It's a little more complicated than that ... for instance, the set of 9 columns and rows might be broken up into 3 distinct swordfish. Or there might be the one swordfish, one jellyfish, and one X-Wing. Or one swordfish and 3 X-Wings, etc.) But if 2 "5"s have already been placed in the puzzle, then the complement of the swordfish will be of order 4 (ie, a jellyfish) because 3 + 4 + 2 = 9.
In the "Nightmare" puzzle I was talking about two "8"s have been given as initial clues. So there are only 7 columns & rows that need an "8", and those 7 break up into a swordfish (on columns) and a jellyfish (on rows). Does that make sense? dcb |
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Marty R.
Joined: 12 Feb 2006
Posts: 5770
Location: Rochester, NY, USA
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| Posted: Sat Sep 16, 2006 5:03 am Post subject: |
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| Quote: | | Does that make sense? |
Yes, up to a point. I understand why I was wrong about the complements adding up to nine, but I'll have to read this a few more times to digest it fully.
Thanks David. |
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George Woods
Joined: 28 Mar 2006
Posts: 304
Location: Dorset UK
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| Posted: Sat Sep 16, 2006 11:28 am Post subject: |
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| riderdude wrote: | | TKiel wrote: | | Not sure if the cell location is just a misprint, but r2c6 is a 6. |
doh
yes, typo. i meant r7c2. if i can get rid of the 6 there, i can get rid of other 6s in box 9. then the 4 in r3c8 is apparent. i was at pretty much the same place as TPM when i got stuck, too.
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Look at the 57 attacks on box 7 -- your cell has to be 57 hence no 6!
I suppose this is described a s a "hidden pair" 57 in box 7 |
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keith
Joined: 19 Sep 2005
Posts: 3355
Location: near Detroit, Michigan, USA
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| Posted: Sat Sep 16, 2006 8:15 pm Post subject: Missing a fish? |
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David wrote:
| Quote: | order(fish) + order(fishy complement) + # of digits "x" already resolved = 9
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Except, in this case
| Code: | +-------------------+-------------------+-------------------+
| 7 23569 569 | 238 245 248 | 3458 1 46 |
| 4 235 15 | 12378 257 6 | 358 9 58 |
| 136 356 8 | 13 9 14 | 7 3456 2 |
+-------------------+-------------------+-------------------+
| 5 78 3 | 6 1 27 | 248 248 9 |
| 69 6789 679 | 4 27 3 | 1 258 58 |
| 2 1 4 | 9 8 5 | 6 7 3 |
+-------------------+-------------------+-------------------+
| 8 57 2 | 17 3 147 | 9 456 46 |
| 1369 369 169 | 5 246 2489 | 2348 2348 7 |
| 369 4 57 | 278 267 2789 | 2358 2358 1 |
+-------------------+-------------------+-------------------+
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Three (3) of the 5's are solved, and there is supposedly an X-wing (2) and a complementary swordfish (3). Adds up to 8.
Even more fishy, the eliminations of the swordfish and the X-wing are not the same.
Here is the X-wing:
Here is the Swordfish:
In fact, the X-wing is part of a second Swordfish. Here it is:
Keith |
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