dailysudoku.com Forum Index dailysudoku.com
Discussion of Daily Sudoku puzzles
 
 FAQFAQ   SearchSearch   MemberlistMemberlist   UsergroupsUsergroups   RegisterRegister 
 ProfileProfile   Log in to check your private messagesLog in to check your private messages   Log inLog in 

Very Hard 13-01-2009

 
Post new topic   Reply to topic    dailysudoku.com Forum Index -> Daily Sudoku puzzles
View previous topic :: View next topic  
Author Message
leocar



Joined: 04 Mar 2010
Posts: 12
PostPosted: Thu Mar 04, 2010 5:46 am    Post subject: Very Hard 13-01-2009 Reply with quote
Hi, I'm just learning how to complete the very hard sudoku's and I'm confussed with the x-y wings. I have attached a code from 13-01-2009 VH.

I thought there was an x-y wing 567 (R6C1, R5C3, R4C8) removing 5 from R4C1 but that didn't work. Instead I used 679 (R2C1, R4C2, R5C3) removing 9 from the same cell to finish it. Is the second method correct or just luck? If it is correct, why is the first one incorrect? Thanks
Leo

Code:

+-------------+--------------+-----------+
| 2  5   3679 | 4679 367 347 | 8   37  1 |
| 79 679 3679 | 1    8   2   | 39  4   5 |
| 4  1   8    | 79   5   37  | 239 237 6 |
+-------------+--------------+-----------+
| 5  69  2    | 3    1   8   | 7   6   4 |
| 3  8   67   | 45   67  45  | 1   9   2 |
| 7  4   1    | 567  2   9   | 35  356 8 |
+-------------+--------------+-----------+
| 8  3   5    | 2    9   6   | 4   1   7 |
| 6  79  79   | 8    4   1   | 25  25  3 |
| 1  2   4    | 57   37  357 | 6   8   9 |
+-------------+--------------+-----------+
Back to top
View user's profile Send private message
Marty R.



Joined: 12 Feb 2006
Posts: 5770
Location: Rochester, NY, USA
PostPosted: Thu Mar 04, 2010 6:09 am    Post subject: Reply with quote
Leo, I edited your post to get rid of the long string so the whole thing could be seen on the screen without scrolling.

Your 679 XY-Wing isn't a Wing because the pivot (69) doesn't see the other two cells. The rule for an XY-Wing is to start with a cell we'll call XY. That cell must see both an XZ and YZ cell. Then any cell seeing both XZ and YZ cannot contain Z.

However, there is an XY-Wing involving 679 in r5c3, r4c2 and r8c3. R8c2 sees both the XZ (69) and YZ (79), thus 9 is removed from r8c2.

There can't be anything involving r6c1 since that cell is solved.

Welcome to the forum and feel free to ask all the questions you'd like. There are lots of folks happy to help out a new member.
Back to top
View user's profile Send private message
leocar



Joined: 04 Mar 2010
Posts: 12
PostPosted: Fri Mar 05, 2010 7:22 am    Post subject: Reply with quote
Thank you for that, for some reason I was using the cell I was trying to eliminate from as the pivot! It's much clearer now.

Cheers
Leo
Back to top
View user's profile Send private message
Display posts from previous:   
Post new topic   Reply to topic    dailysudoku.com Forum Index -> Daily Sudoku puzzles All times are GMT
Page 1 of 1

 
Jump to:  
You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot vote in polls in this forum


Powered by phpBB © 2001, 2005 phpBB Group