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hello all from Mike Brooks in Richmond, UK, a keen Sudokuist but not an expert...I solved puzzle #268 on Sudoku Village rated very hard using X-wing but, when I back-checked things, Sudoku Solver opted to use Swordfish (although 2 squares didn't have the number 5) & snookered him/herself - does this make sense to anybody else?...rsvp, mb.

Mike, posting the actual puzzle here might help to get somebody to answer your question.

udosuk...thanks for your reply & my apologies; I thought people could access puzzle #268 from www.sudokuvillage.com but, having tried myself, I find they only go back a month & this puzzle was in July...I will attempt to post it:
12..6.37./...7....1/38.9..64./....7...3/..68.17../8...4..../..3..7.64/7......../.68.9..27
& hope you can follow my symbolism...I look forward to your comment(s), regards, mb.

Strange, I didn't find an X-Wing or a Swordfish in your puzzle. How about an XY-Wing?

Code: Select all

 *-----------*
 |12.|.6.|37.|
 |6..|73.|..1|
 |387|91.|64.|
 |---+---+---|
 |...|.7.|483|
 |436|8.1|7..|
 |87.|.4.|.16|
 |---+---+---|
 |..3|..7|.64|
 |7..|...|.3.|
 |568|.9.|127|
 *-----------*


You can't solve that puzzle with an x-wing or a swordfish as far as I can see. Here's my solution.
singles until you need a hidden pair 2 and 8 in row 2, then locked candidates on 2,4 and 8 in box 2 and on 4 in box 7, then an xyz-wing on 2,5 and 9 in column 3 and box 4, then locked candidates (type 2) on 9 in box 6, followed by locked candidates (type 2)on 9 in box and finally another xyz-wing on 5,8 and 9 in column 9 and box 3, leading to a cascade of singles.
Incidently this type of question is usually posted in the non pappocom section of the forum.

hello daj95376 & ab, thanks for your speedy responses & apologies for my subsequent delay...your IT skills & your Sudoku skills are far ahead of me: first, I wouldn't know how to cut & paste a grid onto this message & second, I barely understand X-wing & Swordfish let alone XY-wing or XYZ-wing...I posted my initial question on SPF because that's where SS led me to & I am still curious enough to try to find an answer...if you can understand my notation then, continuing from the original numbers & using sort-of-simple/straightforward methods including the matrix, the puzzle becomes:
12..6.37./6.473...1/38791.64./2.167.483/4368.17../87..4..16/913..7.64/7421.693./568.9.127
...I then used X-wing at A3 A4 F3 & F4 (with the number 5) & solved the puzzle but SS uses Swordfish with A7 F7 G3 G4 & G7 being the additional boxes (& the number 5 doesn't appear in either A7 or G3) & after removing a succession of 5s the pop-up-box reads "could not solve by steps etc" - ie snookered!...I hope all the above makes sense because if you understand my methodology then you might tell where I've gone wrong, or right...regards, mb.

mikebrooks wrote:12..6.37./6.473...1/38791.64./2.167.483/4368.17../87..4..16/913..7.64/7421.693./568.9.127
...I then used X-wing at A3 A4 F3 & F4 (with the number 5) & solved the puzzle but SS uses Swordfish with A7 F7 G3 G4 & G7 being the additional boxes (& the number 5 doesn't appear in either A7 or G3) & after removing a succession of 5s the pop-up-box reads "could not solve by steps etc" - ie snookered!...I hope all the above makes sense because if you understand my methodology then you might tell where I've gone wrong, or right...regards, mb.

This seems to be where you are stuck:

Code: Select all

 *-----------*
 |12.|.6.|37.|
 |6.4|73.|..1|
 |387|91.|64.|
 |---+---+---|
 |2.1|67.|483|
 |436|8.1|7..|
 |87.|.4.|.16|
 |---+---+---|
 |913|..7|.64|
 |742|1.6|93.|
 |568|.9.|127|
 *-----------*

 
 *--------------------------------------------------*
 | 1    2    59   | 45   6    458  | 3    7    589  |
 | 6    59   4    | 7    3    28   | 28   59   1    |
 | 3    8    7    | 9    1    25   | 6    4    25   |
 |----------------+----------------+----------------|
 | 2    59   1    | 6    7    59   | 4    8    3    |
 | 4    3    6    | 8    25   1    | 7    59   259  |
 | 8    7    59   | 235  4    359  | 25   1    6    |
 |----------------+----------------+----------------|
 | 9    1    3    | 25   258  7    | 58   6    4    |
 | 7    4    2    | 1    58   6    | 9    3    58   |
 | 5    6    8    | 34   9    34   | 1    2    7    |
 *--------------------------------------------------*


There is most definitely a swordfish on 5, c347/r167.
There is most definitely not an x-wing on 5. If I understand your notation correctly, G4 is the obstruction to your x-wing on 5.
For what its worth, an alternative to the swordfish or the xy-wing from here is the semi-remote naked pair between r5c5 and r3c6 on <25> that eliminates 5 from r4c6, solving the puzzle.

By skipping/missing the XY-Wing that ab and I mentioned, you managed to obtain 11 more eliminations through other means.

Code: Select all

12..6.37.6..73...138791.64.....7.4834368.17..87..4..16..3..7.647......3.568.9.127
12..6.37.6.473...138791.64.2.167.4834368.17..87..4..16913..7.647421.693.568.9.127
           *               * **                       **        *** **


As re'born indicated, there is now a Swordfish present. However, there's still another XY-Wing that you have to get past in order to complete the puzzle. Good luck!

I have moved this thread to theHelp with particular puzzles forum, as it seems to focus on a particular puzzle instead of a general discussion.

Anyway Mike, like re'born has said, you seemed to have spotted an incorrect X-wing on this puzzle:

Code: Select all

 *--------------------------------------------------*
 | 1    2   *59   |*45   6   @458  | 3    7   @589  |
 | 6    59   4    | 7    3    28   | 28   59   1    |
 | 3    8    7    | 9    1    25   | 6    4    25   |
 |----------------+----------------+----------------|
 | 2    59   1    | 6    7    59   | 4    8    3    |
 | 4    3    6    | 8    25   1    | 7    59   259  |
 | 8    7   *59   |*235  4   @359  |@25   1    6    |
 |----------------+----------------+----------------|
 | 9    1    3    |#25   258  7    | 58   6    4    |
 | 7    4    2    | 1    58   6    | 9    3    58   |
 | 5    6    8    | 34   9    34   | 1    2    7    |
 *--------------------------------------------------*

From here the 4 cells marked with * doesn't form an X-wing because r7c4 (or "G4" in your notation, marked with #) also contains a 5. If it hadn't had a 5 (or more drastically all the cells marked with @ hadn't had 5), then you could have been able to apply the X-wing.

Also, I suspect you haven't got the latest version of the Simple Sudoku software, which should be able to solve this puzzle by itself and doesn't get "snookered". If you click on the menu where it saids Help, then click About, you will get to see the version of your copy. If it isn't ver 4.2n then you have got an older version of the program. You can download the latest version from the following link:

http://www.angusj.com/sudoku