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I go through periods of being taken by sudoku. It is unfortunate, then, that i have a sudoku program on my computer at work. I have worked through 50 sudoku puzzles in this program in the last year or so. I have noticed that a large number of them eventually come down to me having to guess a solution, and then test it to see if it is correct. (A few even seem to have two correct solutions - symmetry is a bitch.) This really bugs me. I also do sudoku in the newspaper, and out of a book that i have, and i don't run into this problem anywhere else.
The thing that really irritates me is that it is possible to 'guess' wrong - in other words, there is a solution to these puzzles, but i can't figure out how to get at it from a logical standpoint. (Well, i suppose guessing and then proving that guess to be wrong is sort of logical, but i'd rather figure out how to eliminate the wrong answer up front.)
An example of such a puzzle is:
_9_ 762 314
637 154 982
4__ __9 675
_59 618 4_7
_46 927 15_
17_ 543 _96
_83 495 _61
9__ _76 _4_
_64 __1 __9
At this point, if i guess 5 in the upper right corner, i can fill in the rest with a correct solution. If i guess 8 in the upper right corner, i run into an eventual contradiction.
Does anyone know what i'm missing that might allow me to eliminate the incorrect solution?
The thing that really irritates me is that it is possible to 'guess' wrong - in other words, there is a solution to these puzzles, but i can't figure out how to get at it from a logical standpoint. (Well, i suppose guessing and then proving that guess to be wrong is sort of logical, but i'd rather figure out how to eliminate the wrong answer up front.)
An example of such a puzzle is:
_9_ 762 314
637 154 982
4__ __9 675
_59 618 4_7
_46 927 15_
17_ 543 _96
_83 495 _61
9__ _76 _4_
_64 __1 __9
At this point, if i guess 5 in the upper right corner, i can fill in the rest with a correct solution. If i guess 8 in the upper right corner, i run into an eventual contradiction.
Does anyone know what i'm missing that might allow me to eliminate the incorrect solution?
no subject
Date: 2009-03-05 11:44 pm (UTC)no subject
Date: 2009-03-06 12:46 am (UTC)no subject
Date: 2009-03-06 01:20 am (UTC)As far as puzzles with a unique solution but requiring guessing, I agree with you. I prefer to solve sudoku puzzles without having to hypothesize and backtrack. It is still logical, but it does seem like there should be a way to do it without. I usually give up at the point I have to guess, but I can solve most puzzles.
I also prefer solving on paper, where I can use whatever notation I want.
For this puzzle, I haven't found a way yet. I'm working on it. I thought I had an idea, but it didn't work out.
no subject
Date: 2009-03-06 01:52 am (UTC)On puzzles I get truly stuck on, I've used Sudoku Solver to verify that I've gotten as far as I could on logic alone. I did your puzzle by hand, then entered your puzzle in there and it got as far as I did, then told me the next step required guessing.
I'd recommend you find a new program. :-/
no subject
Date: 2009-03-06 03:33 am (UTC)I do a lot of Sudoku using a freeware program called Simple Sudoku (http://www.angusj.com/sudoku/). It randomly generates puzzles of five difficulties and each always has a unique solution. You can also use it to solve puzzles and have it give a hint — the next step toward a solution. This program the above puzzle requires a technique it calls "colors" which is just a fancy name for trial-and-error.
no subject
Date: 2009-03-06 06:48 am (UTC)Nice, because i could do that with my trials, but, didn't see how to generate the contradiction directly. The colors technique does that.
no subject
Date: 2009-03-06 07:07 pm (UTC)How do you come to that conclusion? I mean, not so much for this puzzle, but in the general case?
How do I know when I need to resort to trial and error, and that I haven't just overlooked something?
no subject
Date: 2009-03-06 08:19 pm (UTC)no subject
Date: 2009-03-06 08:44 pm (UTC)no subject
Date: 2009-03-06 09:41 pm (UTC)no subject
Date: 2009-03-06 04:10 am (UTC)I recommend http://www.scanraid.com/sudoku.htm. Putting in what you have and seeing how that works it out can teach a few new techniques. Without actually analyzing the puzzles but noting the amount solved, XY-Wings or Unique Rectangles could be useful.
no subject
Date: 2009-03-06 06:46 am (UTC)no subject
Date: 2009-03-06 04:23 am (UTC)_59 618 4_7
_46 927 15_
17_ 543 _96
_83 495 _61
and blow it up to only include the possible values you get
you can see there's a contradiction in the 28? and the 27? in the 7th column if you set try to set the 28? on the left to 8.
no subject
Date: 2009-03-06 06:49 am (UTC)no subject
Date: 2009-03-06 04:49 pm (UTC)no subject
Date: 2009-03-06 04:33 am (UTC)no subject
Date: 2009-03-06 06:49 am (UTC)no subject
Date: 2009-03-06 07:04 am (UTC)But indeed, heuristics help reduce most cases you'll see.
no subject
Date: 2009-03-07 04:47 am (UTC)