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cgordon
Joined: 04 May 2007
Posts: 769
Location: ontario, canada
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 Posted: Wed May 16, 2007 6:25 pm Post subject: 16 May - Very Hard |
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| No fish, wings and skyscrapers there. Methinks a typo - should have read Easy not VH. |
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Mesmin
Joined: 01 May 2007
Posts: 12
Location: Ontario
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| Posted: Wed May 16, 2007 7:03 pm Post subject: |
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The opening basic moves certainly get you pretty far into the puzzle, but I don't see how you can get past here without using an x-wing.
| Code: |
+-------------+---------+------------+
| 359 1 258 | 34 7 28 | 489 6 58 |
| 36 68 7 | 34 9 5 | 1 48 2 |
| 59 28 4 | 1 6 28 | 7 59 3 |
+-------------+---------+------------+
| 2 3 1 | 8 4 9 | 5 7 6 |
| 7 56 56 | 2 3 1 | 48 48 9 |
| 8 4 9 | 6 5 7 | 2 3 1 |
+-------------+---------+------------+
| 1 259 25 | 7 8 3 | 6 259 4 |
| 4 289 3 | 5 1 6 | 89 29 7 |
| 56 7 568 | 9 2 4 | 3 1 58 |
+-------------+---------+------------+
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Play this puzzle online at the Daily Sudoku site
The puzzle still needs some more work after using the x-wing and doing some cleanup. I saw an xy wing or finned x-wing to break it open.
Last edited by Mesmin on Wed May 16, 2007 7:19 pm; edited 1 time in total |
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George Woods
Joined: 28 Mar 2006
Posts: 304
Location: Dorset UK
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| Posted: Wed May 16, 2007 7:14 pm Post subject: once again doing it my way |
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Looking at col1 we see some well connected doublets, so take the first available i.e. 36 in box1
so set it to either 3 or 6 if 6 col1 and row 7 lead to an 8 in r9c9
if 3 then row 2 solves to give the 8 in box3 and via the 58 in box3 we once again find r9c9 is 8 so r9c9 = 8 solves the problem
similarly if we wish a variant - just set the original 36 to 6 - this gives the 8 in r9c9 BUT the consequent 8s in boxes3 and 7 are both in col2 so r2c1 must be 3
Either approach gives an easy solution thereafter |
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Mogulmeister
Joined: 03 May 2007
Posts: 1151
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| Posted: Wed May 16, 2007 7:17 pm Post subject: |
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cg,
Just a note of caution - it is very easy to get false positives in this game. I have blithely waltzed to a correct sudoku solution but later found I got there on wrong assumptions.
George has used an excellent forcing chain which is effective but uses trial and error as opposed to a logical construct like a fish, wing etc
I'm not saying that you are wrong but like Mesmin I found I couldn't do this without needing the x-wing. What route did you take - did you get to a grid similar to Mesmin's ? |
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David
Joined: 02 Jun 2006
Posts: 58
Location: Bedford, UK
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| Posted: Wed May 16, 2007 7:35 pm Post subject: |
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I reached the exact same grid as Mesmin, but fail to see how it can be solved from there using simple techniques.
However, there is a quick route without the X wing on the 8's. If you use simple colouring on 5's; r3c1-r3c8-r7c8-r9c9 allows the elimination of the 5 in r9c1 and the puzzle collapses from there. |
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Mogulmeister
Joined: 03 May 2007
Posts: 1151
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| Posted: Wed May 16, 2007 7:41 pm Post subject: |
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OK - so an alternative to the x-wing would be a 2 string kite on 5's which eliminates the 5 in r9c1. I spoke too soon!
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Mogulmeister
Joined: 03 May 2007
Posts: 1151
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| Posted: Wed May 16, 2007 7:47 pm Post subject: |
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| I see we reached the same conclusion minutes apart David...slightly different take on which 5's to use. |
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David
Joined: 02 Jun 2006
Posts: 58
Location: Bedford, UK
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| Posted: Wed May 16, 2007 8:44 pm Post subject: |
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| Well, it was a fun puzzle and it's nice to be able to use a range of techniques. |
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George Woods
Joined: 28 Mar 2006
Posts: 304
Location: Dorset UK
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| Posted: Wed May 16, 2007 9:42 pm Post subject: forcing chains etc.. |
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| Mogulmeister wrote: | cg,
George has used an excellent forcing chain which is effective but uses trial and error as opposed to a logical construct like a fish, wing etc
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Thanks for the complement -
My attitude to what you and others have called my forcing chains is that most of the advance techniques are based on a formalisation of a forcing chain i.e. this cell content could be either this or that and with either alternative the "same thing applies" e.g. none of the relevant other numbers on two columns (X wing), or that cell seeing both pincers cannot contain that number(XY wing). or indeed when I first did Suduko the trivial naked pair if this is 6 other is 7 or if this is 7 other is 6 so no 6 or 7 in the rest of the same "element"
So if I can see the impact of both possibilities in my mind I don't think of it being so much a forcing chain but maybe "slightly advanced logic" Here the skill is in knowing where best to look!
Indeed how many of us have not solved a cell by being very clever, and afterwards found a far easier route to the same result?
If one is writing a computer program to solve sudoku then I guess one must formalise the rules. But my enjoyment is being me, rather than competing with a computer |
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cgordon
Joined: 04 May 2007
Posts: 769
Location: ontario, canada
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| Posted: Wed May 16, 2007 9:45 pm Post subject: |
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Yes, Mr Mogulmeister is absolutely right - it is very easy to get false positives. I misread my scribblings and lucked out.
Correct me if I'm wrong but isn't there a perfect square on 8s in cols 3 & 9 and rows 1 & 9 - which eliminates the 8 in R1/c6. Plain sailing from there eh? |
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Marty R.
Joined: 12 Feb 2006
Posts: 5770
Location: Rochester, NY, USA
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| Posted: Thu May 17, 2007 12:24 am Post subject: |
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| Quote: | | Correct me if I'm wrong but isn't there a perfect square on 8s in cols 3 & 9 and rows 1 & 9 - which eliminates the 8 in R1/c6. |
Yes, simple coloring on 8 eliminates the 8 from r1c6.
I also solved the puzzle by eliminating the 5 from r9c1. However, I reached a position where I had only a pair of 59 left in row 7, so I used remote pairs to zap that 5.
Last edited by Marty R. on Thu May 17, 2007 3:56 am; edited 1 time in total |
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Mogulmeister
Joined: 03 May 2007
Posts: 1151
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| Posted: Thu May 17, 2007 12:57 am Post subject: |
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| Quote: | Here the skill is in knowing where best to look!
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Ain't that the truth !  |
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Mogulmeister
Joined: 03 May 2007
Posts: 1151
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| Posted: Thu May 17, 2007 1:16 am Post subject: |
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CG,
| Quote: | | Correct me if I'm wrong but isn't there a perfect square on 8s in cols 3 & 9 and rows 1 & 9 - which eliminates the 8 in R1/c6 |
What you described was an x-wing on 8's. |
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