dailysudoku.com

LloydB · Joined: 09 Jul 2007 Posts: 21 Location: Skagit Co., WA USA

I can't believe I'm the first to comment.

An X-wing on 9 followed by a 689 XY-wing solved it for me.

George Woods · Joined: 28 Mar 2006 Posts: 304 Location: Dorset UK

Yes - the same X Wing and a W wing in boxes 7,8,9

cgordon · Joined: 04 May 2007 Posts: 769 Location: ontario, canada

A funny one. There were so many basic eliminations, I thought it would end up being easy. But I had to use the x wing on 9; a skyscraper on 6 and a BUG + 1.

Deb · Joined: 30 Jun 2008 Posts: 8

I'm new to Sudoku. I find the "hard" ones easy but the "very hard" ones impossible...until today's. I completed it, but I'm not sure how; it wasn't a lucky guess, I used logic, but I just happened to start the logic thread at the right place. So, I was wondering if you could help formalize what I did.

After the easy stuff, I saw that the "69" pair in R5C1 and R9C1 and the "89" pair in R5C9 and R9C9 would eliminate any 9's in their path, which left me with boxes 7, 8 and 9 looking like this:

31.7.45.2
75.23.641
.24.1573.

R8C3 and R8C6 were a "89" pair so I just used the logic of "if R8C3 is an 8 how does it flow? Then, if R8C3 is a 9 how does it flow?" I realized that it would be impossible for R8C3 to be a 9, so the puzzle was solved.

Can you tell me what that configuration was in boxes 7, 8 and 9? And how I would recognize it again?

cgordon · Joined: 04 May 2007 Posts: 769 Location: ontario, canada

Hi Deb:

I believe your first statement relates to the x-wing mentioned in previous posts.
The grid below shows the unsolved candidates for 9. Look at C1 and C9 where there are only two 9’s in each column.
Either R5C1 or R5C9 has to be a 9 – so you can eliminate all other 9’s in R5 (marked 9X). Same thing in R9.

I think your next statement suggests a guess – though I have been known to be wrong. You should refer to the various techniques for solving the Very Hards, e.g. x, xy and xyz wings, skyscrapers, UR’s and many more. I don’t have up-to-date websites but I’m sure others could help.

And I would suggest copying the grid below (incl. the code symbols) for use when posting numbers and grids.

Good Luck.
Craig

Marty R. · Posted: Wed Jul 09, 2008 3:48 pm Post subject:

One W-Wing on 89 with pincer coloring was all I needed.

Johan · Joined: 25 Jun 2007 Posts: 206 Location: Bornem Belgium

I used the 26 UR* in R45C36 to solve this one, the digits marked + (= possible solutions to avoid a 26 DP) in the 26 UR pins <8> in R8C3.

R45C3=9 => R8C3=8

R4C6=8 => R8C6=9 => R8C3=8

R5C6=1 => R5C89=89 => R5C1=6 => R9C1=9 => R8C3=8

Deb · Joined: 30 Jun 2008 Posts: 8

I didn't realize one had to learn so many techniques. I thought it was all logic. Thanks for the pointers.

wapati · Posted: Wed Jul 09, 2008 8:15 pm Post subject: Re: Thanks

Deb · Joined: 30 Jun 2008 Posts: 8

wapati, thanks so much for the elegant demo. I see now that it really was a guess at the "89".

And now I know what a hinge is.

But I'm afraid the w-wing eluded me. (as did x-wing, xy-wing -- I see I have a lot to learn) You say it is marked, but I'm not seeing it. I'll google it and I'm sure I'll come up with an explanation.

I guess I like things simple so if a w-wing is all I need, that's what I focus on. I like the sudoku as a mental break. Thanks.

wapati · Posted: Wed Jul 09, 2008 9:25 pm Post subject:

Marty R. · Posted: Wed Jul 09, 2008 9:25 pm Post subject:

Deb, you can read about W-Wings in the fourth post in this thread:

http://www.dailysudoku.co.uk/sudoku/forums/viewtopic.php?t=2143

cgordon · Joined: 04 May 2007 Posts: 769 Location: ontario, canada

At the risk of alienating my peers (again) I would suggest the x wings, xy wings and xyz wings and holding the w's (I don't find them user friendly).

try http://www.sudocue.net/guide.php#XWing

Deb · Joined: 30 Jun 2008 Posts: 8

Medusa, Skyscraper, W-Wing! Who knew?!? Thanks all. I've bookmarked your links and I'll read (Ah, STUDY) them. Thanks for helping out a newbie.

nataraj · Posted: Wed Jul 09, 2008 10:33 pm Post subject:

Craig, I'm far from being alienated. But if it is necessary to have a discussion about good/bad/legitimate solving techniques, why not have it now?

wapati has given an excellent concise description of the w-wing.

The w-wing, speaking from my own experience, was an acquired taste. I ignored it when it was first proposed. Maybe I was lazy, maybe I did not grasp the simple underlying principle, maybe - I don't know - maybe I felt that the world did not need yet another "bla,bla"-wing. That has changed.

Knowing and using both the x-wing and the w-wing I'll make a strong point in favor of the w-wing: it is much faster to check a puzzle for w-wings than for x-wings.

x-wings could be hidden in ALL cells of the puzzle, sometimes using a cell with five candidates. Very hard to see and one has to go over all 81 cells repeatedly. "user friendly" ? not by my definition ...

w-wings, on the other hand, use bi-values only (easy to see), and only those which are identical (at least two of a kind must be present in the puzzle) and don't "see" each other.

That makes -what? 3, maybe 5 pairs of cells to check. And we know exactly what to look for: either a strong link in the first of the two candidates - or a strong link in the other candidate. Not exactly rocket science, is it?

Are w-wings necessary to solve puzzles in the "daily" section of dailysudoku.com? definitely not.
Could they be useful in solving sudokus published somewhere else? for sure.
Should we therefore agree on a taboo - banning w-wings or other techniques from our arsenal when solving the "daily" - I don't think so.

Don't get me wrong: I am not talking about limiting YOUR arsenal of methods or to force you into using any particular method. Feel free to solve these puzzles any way you like. I have my preferred methods, too, my "modus operandi". I am talking about NOT limiting the methods that we - as a community - might want to try and talk about in this forum... Yes, in the "daily" section.

keith · Posted: Thu Jul 10, 2008 2:39 am Post subject:

Nataraj proposes that W-wings are easy to recognize. I would add that M-wings and other variants of remote and complementary pairs are also relatively easy to recognize.

I will also add that for these techniques, the arms war belongs to us, the pencil & paper solvers. As has been pointed out over and over again, a W-wing can make toast of a multiple-step "Very Hard" puzzle. And, those that make and rate the puzzles, do not yet have W- and M- wings in their arsenals.

We may have this advantage for a while, for I think that these wings may be difficult to program.

Keith

keith · Posted: Thu Jul 10, 2008 3:06 am Post subject:

If you go back through the history of this list, you will see that it has always involved a discussion of other puzzles, and of techniques much more complicated than those required to solve the Daily Sudoku.

Keith

Marty R. · Posted: Thu Jul 10, 2008 5:20 am Post subject:

Craig, you can't alienate me. Laughing

But I love W-Wings. They're one of the new techniques I've learned over the last year, along with Medusa, pincer coloring and ERs, that have significantly expanded my puzzle-solving capabilities. (Although I have a long way to go).

cgordon · Joined: 04 May 2007 Posts: 769 Location: ontario, canada

Let me reiterate the situation.

A very nice person (Deb) wanted to learn the Very Hard techniques. And another very nice person - (Me) - directed her to the obvious basic techniques – x and xy wings. I am not denying that w-wings are a valid solution – I just believe that w-wings should not be recommended to novices.

Stay by me Deb – I will protect you from these elitists. Twisted Evil

keith · Posted: Thu Jul 10, 2008 11:49 pm Post subject: