The New Sudoku Players' Forum

[brucec]

Hey, everybody, I read some of the mentioned sites on XWing and Swordfish but still do not get the concept.

Please tell me your personal strategy for properly using SF and XW. Are they common and useful?

[emm]

Have you read this - http://angusj.com/sudoku/hints.php It's hard to explain it better than the expert.

Neither Xwing nor swordfish are needed in Easy - Med - Hard puzzles. Xwing are only required for Very Hards, though you can find them in less difficult ones. Swordfish are very rare and highly prized. I still look for them in the harder puzzles - just on the offchance! I read somewhere that when you get to the point of needing them forcing chains are easier to use.

[9X9]

brucec - em invariably speaks wisely, lightening her words with sensitivity and wit where she can.

Shhh, in case she's listening but I carry a quiet torch for her.

[emm]

Wow! I'm touched. If I was going to Cheltenham I could've hitched a ride in the Audi. Damn!

[9X9]

em - we'll be listening to Classic FM together, even though you'll only be with me in spirit.

[9X9]

em - goto cheltladiescollege.org for a preview of where "you" and I will be.

[tso]

Forcing chains may be of no help when a swordfish is needed and vice versa.

The "fact" that swordfish are rare is misleading. It depends entirely upon the creation/selection methods. There are *trillions* of potential Sudokus that cannot be easily solved without identifying a Swordfish. If you only play newspaper puzzles and Pappocom, you will never see one. Simple Sudoku and Sadman Sudoku can make puzzles with Swordfish on demand.

X-wings are much easier to spot. Coloring can be done instead in all cases of x-wings, but NOT in all cases of Swordfish.

Here's one that has a Swordfish fairly early on, and an X-wing near the end:

Code: Select all

 . 5 . | . . 8 | . . .
 . 3 1 | 9 . 6 | 4 . .
 9 . . | . . . | 3 . .
-------+-------+------
 1 . . | . 8 . | . 4 3
 . . . | 3 . 9 | . . .
 5 8 . | . 2 . | . . 6
-------+-------+------
 . . 5 | . . . | . . 1
 . . 9 | 8 . 3 | 7 2 .
 . . . | 7 . . | . 8 .


[QBasicMac]

Here's one that has a Swordfish fairly early on, and an X-wing near the end

Well, I get to the point below fairly easily without finding either and can't make any more progress.

What is the next pencilmark to be erased and why.

Example:
In box 2, row 3, 1245 1457 12457 become 145 145 145 because these are the only occurrences of 1,4,and 5 in that box. (I know that's false. Just an example)

Example:
r3c4 and r7c4 are the only cells in column 4 that have pencilmark 2 (known false)
r3c6 and r7c6 are the only cells in column 6 that have pencilmark 2
This is the X-Wing you are looking for

Thanks,

Mac


Solution So Far

Code: Select all

-5- -38 ---
231 9-6 4-8
9-8 --- 3--
19- -8- -43
--- 3-9 8--
583 -2- --6
875 --- -31
--9 8-3 72-
3-- 7-- -8-


Pencilmarks So Far

Code: Select all

467       -         467       124       -         -         129       169       29       
-         -         -         -         57        -         -         57        -       
-         46        -         1245      1457      12457     -         1567      257     
-         -         267       56        -         57        25        -         -       
467       246       2467      -         1567      -         -         157       257     
-         -         -         14        -         147       19        179       -       
-         -         -         246       469       24        69        -         -       
46        146       -         -         1456      -         -         -         45       
-         1246      246       -         14569     145       569       -         459     


[Cec]

Hey, everybody, I read some of the mentioned sites on XWing and Swordfish but still do not get the concept.

Please tell me your personal strategy for properly using SF and XW. Are they common and useful?

Hi brucec, Excluding Swordfish which I'm still not clear on either, a thorough but easy to follow explanation of the X-Wing concept was given by Scrose (July 17 post) who responded to my thread "Can't solve hard puzzles" (July 14). Thanks to Scrose, I'm sure you'll be happy after looking at his explanation.

PS. I endorse 9X9 sentiments re 'em' and suspect more torches would be carried.

Bonsai Cec

[SteveF]

QBasicMac, there are locked candidates in row 7 which should allow you to remove pencil marks in box 8.

[QBasicMac]

row 7 box 8.

Thanks. I now see that since candidate 4 in row 7 only appears in box 8 then there can be no other 4's in box 8. Accordingly, I removed 4 from r8c5, r9c5 and r9c6.

Sorry to report that after doing that, I'm still stuck.

Mac

[angusj]

I'm still stuck.

Look for a swordfish on candidate 5s.

[QBasicMac]

Look for a swordfish on candidate 5s.

Well, I had already seen this pattern:
The only 5's on row 2 are in r2c5 and r2c8
The only 5's on row 8 are in r8c5 and r8c9
which I presume is a "swordfish"

But I couldn't see how that leads to any progress.

I can imagine r2c8 and r8c9 each equal to 5. In fact I see 5 cells with pencilmark 5 in column 5 and 3 in column 8.

What pencilmarks can I eliminate and why?

Sorry to be slow here.

Mac

[MCC]

The swordfish comprises:

r2c5, r2c8.
r5c5, r5c8, r5c9.
r8c5, r8c9.

The candidates that can be eliminated are:

r3c5, r3c8, r3c9, r9c5, r9c9.

This'll enable you to place a 5.

[SteveF]

Yes, that is the same swordfish I found, and is also what Simple Sudoku finds.

However on looking closer, I think there is a second swordfish in the 5's in columns 4, 6 and 7.

The cells involved are:

r3c4 r4c4
r3c6 r4c6 r9c6
r4c7 r9c7

It allows the placement of the same 5 as MCC gives.

Is this swordfish also valid?