How Do I Solve This Puzzle Without Guessing?

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How Do I Solve This Puzzle Without Guessing?

Postby Larrysez » Tue Nov 27, 2012 11:11 pm

I cannot for the life of me see any way to solve this puzzle other than by just taking a wild guess about one of the remaining unsolved squares and seeing whether it works out or not. This happens once in a while. Is there some technique I could use?

Thanks much.

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Re: How Do I Solve This Puzzle Without Guessing?

Postby JasonLion » Tue Nov 27, 2012 11:46 pm

There is one locked candidate, and then singles from there.
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Re: How Do I Solve This Puzzle Without Guessing?

Postby Larrysez » Thu Nov 29, 2012 1:19 am

Well, I had to google "locked candidate," but now I see how it's done. Thanks for the tip.
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Re: How Do I Solve This Puzzle Without Guessing?

Postby dokusan » Thu Jan 03, 2013 12:48 am

Larrysez wrote:I cannot for the life of me see any way to solve this puzzle other than by just taking a wild guess about one of the remaining unsolved squares and seeing whether it works out or not. This happens once in a while. Is there some technique I could use?


The answer to your question about a technique you can use is yes, but I'm not going to elaborate on that now. (you might look at my other posts on the forum that come later).
For this particular puzzle above, you don't need "possibility matrices", "locked candidates" or any other convoluted *!#$.

This is row8
48_ | 593 | _6_

This is colH
-
6
4
___
9
-
2
___
1
8
5


row8 needs a 7 somewhere.
7 cannot go beside 8, because 7 at C3 blocks.
Therefore the 7 must come to the left or the right of 6.
Erase your pencil marks there and put "m7,6,m7"
Your two m7's indicate that 7 must come to the left or right of 6, and nowhere else in that box.
The presence of two m7's here means there is an "implied 7" in that box now.
In particular, your implied 7 is off the middle column.

Can we utilize this fact?
Yes.

colH needs a 7. Where could that go? It cannot go above the six because it is blocked by your m7s.
Therefore it must go in the other available slot. (It must go there).
colH is now fleshed out. 3 is the only remaining digit. It must go in the top.

3
6
4
___
9
7
2
___
1
8
5
Last edited by JasonLion-Admin on Thu Jan 03, 2013 2:33 am, edited 1 time in total.
Reason: Politeness
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Re: How Do I Solve This Puzzle Without Guessing?

Postby JasonLion » Thu Jan 03, 2013 2:39 am

dokusan, the technique you describe is precisely the "locked candidate" technique, expect you have introduced some additional notation which is not needed (and for me at least only serves to confuse the issue).
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Re: How Do I Solve This Puzzle Without Guessing?

Postby dokusan » Thu Jan 03, 2013 2:46 am

JasonLion,

I googled "Locked candidate", and this led me to a convoluted essay about possibility matrices.
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Re: How Do I Solve This Puzzle Without Guessing?

Postby JasonLion » Thu Jan 03, 2013 3:04 am

There are many descriptions of the locked candidate technique on the Internet. Not all of them are well written, nor are all of them written with beginners in mind. Sudopedia, while hardly perfect, does provide a reasonably simple/straightforward description of the technique (only to follow it with a far more complex example from Killer Sudoku, which is best skipped until you are more familiar with regular Sudoku).
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