I give up. What's the next value and why?

qbasicmac

--- 13- --5
34- --5 2--
85- 9-2 ---
--- -5- 9--
--2 --- 4--
--3 -6- ---
--- -13 --6
-35 --- -1-
71- -28 ---

gaby · Posted: Wed Aug 24, 2005 11:33 am Post subject:

There is a matched quad (swordfish n=2) in there. The puzzle solves very quickly once you find it.

Hint: it's in the 6's...
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gaby · Posted: Wed Aug 24, 2005 11:59 am Post subject:

Oops, also forgot some other block interactions. If a number only appears in one row in a specific block, you can remove that number from all the other cells in that row. Likewise for columns. This removes 10 candidate numbers. Then you are looking at the matched quad above.
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qbasicmac · Posted: Wed Aug 24, 2005 3:07 pm Post subject:

Afraid I am not following.

Maybe you could just tell me if my current pencil marks are correct. If not, where the errors are.

If they are correct, what is the next pencilmark I can eliminate and why.

Example: You can eliminate 6 from r1c1 because .....

Thanks for you efforts

Mac

269 2679 679 4678 478 467 678 4789 4789
- 679 1679 678 78 67 1678 79 179
16 - 167 467 47 467 1367 347 1347
146 678 4678 23478 478 147 13678 23678 12378
1569 6789 16789 378 789 179 135678 35678 1378
1459 789 14789 2478 4789 1479 1578 2578 1278
249 289 489 457 479 479 57 24579 24789
246 2689 4689 467 479 4679 78 24789 2478
469 69 469 456 49 469 35 3459 349

gaby · Posted: Wed Aug 24, 2005 4:28 pm Post subject:

Going from the starting position that you showed, I can remove the following pencilmarks:

qbasicmac · Posted: Wed Aug 24, 2005 5:31 pm Post subject:

"the puzzle becomes very simple to solve"

ROFL

Well, I finally saw the X-Tree you pointed out. Dang, those are hard to spot. And I thought I looked everywhere.

Thereafter, it was possible to solve, but I wouldn't call it easy.

Thanks for your help! Now I can sleep.

By the way, not to complain, but you could have saved a lot of work if you had simply originally answered "The cells at r2c3, r2c4, r9c3, r9c4 form an X-Tree on digit 6. Let me know if you don't know what that means."

That was really all I was missing. I know the techniques, but can't see the patterns. Mental block.

As I said, No complaint. I would still be staring at that stupid puzzle next week without your help.

Mac

gaby · Posted: Wed Aug 24, 2005 5:42 pm Post subject:

Happy to help.

I just like to make sure I'm thinking clearly myself when I answer questions. It normally ends up with me writing much more than I need to. Whilst I was writing the response, I found three bugs in my code as well Smile

If I may ask, do you have a solver that you've written or are you doing these by hand?

Gaby
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qbasicmac · Posted: Thu Aug 25, 2005 12:36 am Post subject:

I have a non-solver.

I call it SuDoku Scratch Pad.

It is still under construction, but a more-or-less good version can be found at
http://www.sudoku.funurl.com/

It does the drudgery work of maintaining the pencil marks.

There are also three DOS-based utilities. One of them is a solver, but not of interest to you, probably. It simply does the most easy and then immediately begins guessing. It is fast, but uses no techniques beyond simple computing for each cell the legal values by eliminating those already in the box, row, or cell. When a cell has only one value, it is filled in. When stymied, then when a cell has two values, each are tried, etc. (Yawn!)

The source code in QBasic is available via that site also.

Mac