The New Sudoku Players' Forum

[FJK]

I thought I was getting good at Sudoku, until I started working on some of the harder puzzles. I've used this form to try to learn solving techniques & even tried the angusj.com form for instructions. I have two requests for knowledge. The first is a better understaning of x-wing & swordfish & graphing. The articles or posts I have read just don't allow me to "get it" (the concept) or allows me to see how the conditions exist in the puzzle to indicate when these approaches should be used. Can anyone perhaps explain these concepts a little differently so that I can understand them & know when to apply them.

Secondly, the following is a puzzle with which I am stuck. I'm looking for what technique should be applied as the next step.

8-- 723 ---
73- 451 --9
--- 689 ---
374 968 --1
2-- 374 --8
6-- 512 374
--- 8-5 ---
4-- 237 -1-
--- 1-6 --2

The puzzle above is typical where I hit a dead end; few apparent conjugate pairs, no apparent hidden thriples or quads, and for a specific number, too many choices in a box, running both in rows & columns ( or what I call turning the corner).

Thanks in advance for any guidance you may provide.
FJK

[SteveF]

Given what you say about looking for triples, quads etc I assume you have made some eliminations,

If this is the case you should be able to see a hidden single in box 9.

If you are unable to see then I can forward the eliminations I have used.

[FJK]

Steve: Yes, you should forward me the eliminations you made in box 9. Obviously, I don't even see the triple & quad to get to a hidden 9. I still have 4 cells in box 9 with 9's.
Thanks for the reply, FJK

[SteveF]

OK, first up there is the naked pair in box 4 (8,9) which allows elimination of 9's in box 4.

Ditto for box 6, (2,5) which allows elimination of 5's in box 6.

Ditto for column 9, (5,6) which allows elimination of 5's & 6's in column 9.

Then look at where 9 can go in column 1, this should allow elimination of 9's in box 7.

After all this have a good look at row 8, there should be some good news about a candidate 9.

[NorwegianViking]

Hello FJK

What Steve writes is perfectly valid.

I am trying to get a grasp on some of the more advanced methodes, and I spotted a X-Wing in your puzzle.

Interestingly it reveals the same 9 as Steve, plus another 9.

Code: Select all

    |1,4,5,6,9 |    1,5,6,9 |    |       |     |  1,4,5,6 |    4,5,6 |  5,6 |
    |          |        2,6 |            |     |    2,6,8 |    2,6,8 |      |
1,5 |  1,2,4,5 |      1,2,5 |    |       |     |1,2,4,5,7 |  2,3,4,5 |3,5,7 |
    |          |            |    |       |     |      2,5 |      2,5 |      |
    |    1,5,9 |      1,5,9 |    |       |     |    5,6,9 |    5,6,9 |      |
    |      8,9 |        8,9 |    |       |     |          |          |      |
1,9 |  1,2,6,9 |1,2,3,6,7,9 |    |   4,9 |     |  4,6,7,9 |  3,4,6,9 |3,6,7 |
    |  5,6,8,9 |    5,6,8,9 |    |       |     |  5,6,8,9 |          |  5,6 |
5,9 |    5,8,9 |  3,5,7,8,9 |    |   4,9 |     |4,5,7,8,9 |3,4,5,8,9 |      |


X-Wing:
C1 and C5 both have two (and only two) positions to place a 9.
These positions share the same rows, R7 and R9. This is a X-Wing.

The rule is: Since the 9's HAVE to go in R7C1 + R9C5 OR in R9C1 + R7C5
we can exclude 9 from any other cell that intersects with these.

Therefore we can exclude 9 from:
R7C2, R7C3, R7C7, R7C8
R8C2, R8C3
R9C2, R9C3, R9C7, R9C8

That gives us 2 hidden singles, a 9 in both R8C7 and in R5C8.

You will probably be able to solve it from here.

Code: Select all

    |1,4,5,6,9 |    1,5,6,9 |    |       |     |  1,4,5,6 |    4,5,6 |  5,6 |
    |          |        2,6 |    |       |     |    2,6,8 |    2,6,8 |      |
1,5 |  1,2,4,5 |      1,2,5 |    |       |     |1,2,4,5,7 |  2,3,4,5 |3,5,7 |
    |          |            |    |       |     |      2,5 |      2,5 |      |
    |    1,5,9 |      1,5,9 |    |       |     |    5,6,9 |    5,6,9 |      |
    |      8,9 |        8,9 |    |       |     |          |          |      |
1,9 |    1,2,6 |  1,2,3,6,7 |    |   4,9 |     |    4,6,7 |    3,4,6 |3,6,7 |
    |    5,6,8 |      5,6,8 |    |       |     |  5,6,8,9 |          |  5,6 |
5,9 |      5,8 |    3,5,7,8 |    |   4,9 |     |  4,5,7,8 |  3,4,5,8 |      |


Further tips:
Then there is a naked pair in C8 and a naked triple in Box 7
From then on, just singles.

[FJK]

Got it !!
SteveF. Your last 2 clues unlocked the puzzle for me. It seems like when I am looking for "fancy" triples & quads, I loose sight of the obvious. As it was, I was finishing the puzzle & had an error. So, I had to re-input what I put in this post & start all over, but I got it.

Norwegian Viking: I'm going to go back through the problem & follow your x-wing routine & see if I can follow along. As it was, I guess I solved it independently of the x-wing concept, maybe??

I'm sorry I didn't post a reply sooner, because I know the time both of you took to reply. Much thanks. As it is, I seem to barely have time to do a puzzle, let alone look at this excellent form. I was waiting to put out this post for the last 2 weeks, and finally found some time on Friday when I took the day off.
Again, thank you.
Regards, FJK

[emm]

Therefore we can exclude 9 from:
R7C2, R7C3, R7C7, R7C8
R8C2, R8C3
R9C2, R9C3, R9C7, R9C8

Hi NV, this is just to clarify what an Xwing allows you to do.

The 9s in your Xwing are in row 7 & 9 - this doesn’t actually allow you to remove any 9s from r8 – just all the other 9s in row 7 & 9.

There is another Xwing of 9s going the other way – across the rows - in r1,6 & c2,3 which allows you to remove more candidate 9s, including the two you mentioned in r8.

[QBasicMac]

It seems like when I am looking for "fancy" triples & quads, I loose sight of the obvious.

LOL - Yeah, me, too.

Your puzzle solves, as you know, without any triples, x-wings, etc. Just regular stuff. It is hard to focus on "regular stuff" until it is all solved. When the regular stuff is completely gone, you can look for other things.

But whenever I come to the board stuck, it is always one last regular thing (such as locked candidates) that has been my blind spot.

Heh

Mac