The New Sudoku Players' Forum

[Roy McCoy]

I was particularly annoyed by this sudoku this week, because there is a woman in our office who could solve it in her head, and I couldn't solve it even after I'd penciled in all the possibilities.

Code: Select all

9 4 . | 5 . 8 | . . 1
. . 8 | 1 . . | 9 5 .
. 1 5 | 2 . 9 | 3 4 8
------+-------+------
. . 9 | 8 2 . | 7 1 .
2 . 1 | 7 . . | . 8 9
. 8 7 | 9 1 3 | . . .
------+-------+------
1 9 4 | 3 . . | 8 . .
8 5 6 | 4 9 2 | 1 . .
. . . | 6 8 1 | 5 9 4

Code: Select all

*-----------------------------------------------------------------------------*
 | 9      4      23     | 5      367    8      | 26     267    1      |
 | 367    27     8      | 1      3467   467    | 9      5      267    |
 | 67     1      5      | 2      67     9      | 3      4      8      |
 |-------------------------+-------------------------+-------------------------|
 | 45     36     9      | 8      2      456    | 7      1      356    |
 | 2      36     1      |                      |                      |
 |                      |                      |                      |
 |-------------------------+-------------------------+-------------------------|
 |                      |                      |                      |
 |                      |                      |                      |
 |                      |                      |                      |
 *-----------------------------------------------------------------------------*

Oops, now I see there's only one place for a 3 in row 5, so maybe that will break this one open for me. I'm sure I'll get stumped again and will want to ask for help, though, so right now I'll ask: (1) Is there an easy way to enter the code for a puzzle? What I was doing on the above was selecting the number of spaces of the digits I was going to type, then typing the digit(s) and right-arrowing to the next place. Maybe everybody does it in some way like that, but it seems awfully arduous. (2) Is it absolutely necessary to send the pencilmarks? I noticed that not every puzzle posted in this section has them. And (3) Is a link to a web page containing the puzzle sufficient?

Thanks,

Roy McCoy
Rotterdam, NL

[JasonLion]

Welcome!

There isn't any need to post pencil marks when asking about a specific puzzle unless you have been using advanced techniques to remove some of the default pencil marks.

This is a fairly simple puzzle, requiring only hidden singles in blocks and full house. Hidden singles in blocks can be spotted quite easily once you learn to do cross hatching. There are good cross hatching tutorials here and here. Once you have mastered cross hatching, you should be able to fill in the empty cells in this puzzle quickly, without using pencil marks or making any notes.

[Roy McCoy]

Thank you very much, Jason. I think you exaggerated in saying that only hidden singles are required in this puzzle, but not much and I do hope to solve it now without making notes. [Only I didn't; see below.]

The main reason I think you may have exaggerated is that it seemed I had to use a row/column interaction with block 8 to get the 8 in block 2. I'm not 100% sure I'd explored absolutely every hidden-single possibility at that point, but I think I may have.

I didn't immediately check further on your "only hidden singles" assertion after that because I spotted perhaps my first X-wing ever after placing the aforementioned 8, with 1s in blocks 1 and 7 enabling me to place the 1 in block 4. At some point shortly after that, however, I couldn't find any more hidden singles at this point:

Code: Select all

9 4 . | 5 . 8 | . 7 1
. . 8 | 1 . . | 9 5 .
. 1 5 | 2 . 9 | 3 4 8
------+-------+------
. 6 9 | 8 2 . | 7 1 3
2 3 1 | 7 . . | . 8 9
. 8 7 | 9 1 3 | . . .
------+-------+------
1 9 4 | 3 . . | 8 . .
8 5 6 | 4 9 2 | 1 3 7
. . . | . 8 1 | . 9 4

I thought I would nonetheless be able to solve it in my head using other methods, but I still couldn't. I finally solved it by a row/column action with the 5 in blocks 8 and 9, but didn't see this until after I'd marked all the possibilities up.

I don't know what's wrong with me and these things - a mixture of dumbness and mental laziness, I suppose. Thanks again, anyway. It was a good exercise in looking for hidden singles even if that method didn't suffice.

[JasonLion]

At that point there is a full house in column 4, but after that you need to use either hidden singles in a row/column or pointing pairs.

Here is a solving path for the original puzzle using only hidden single in blocks and full house:
Hidden Single (block): 3 in block 6
Set R4C9 to 3
Hidden Single (block): 3 in block 9
Set R8C8 to 3
Full House: row 8
Set R8C9 to 7
Hidden Single (block): 7 in block 3
Set R1C8 to 7
Hidden Single (block): 5 in block 6
Set R6C9 to 5
Hidden Single (block): 3 in block 4
Set R5C2 to 3
Hidden Single (block): 5 in block 4
Set R4C1 to 5
Hidden Single (block): 4 in block 4
Set R6C1 to 4
Full House: block 4
Set R4C2 to 6
Full House: row 4
Set R4C6 to 4
Hidden Single (block): 4 in block 6
Set R5C7 to 4
Hidden Single (block): 4 in block 2
Set R2C5 to 4
Hidden Single (block): 3 in block 2
Set R1C5 to 3
Hidden Single (block): 3 in block 1
Set R2C1 to 3
Hidden Single (block): 3 in block 7
Set R9C3 to 3
Full House: column 3
Set R1C3 to 2
Full House: row 1
Set R1C7 to 6
Full House: column 7
Set R6C7 to 2
Full House: block 6
Set R6C8 to 6
Full House: column 8
Set R7C8 to 2
Full House: block 9
Set R7C9 to 6
Full House: column 9
Set R2C9 to 2
Hidden Single (block): 2 in block 7
Set R9C2 to 2
Full House: row 9
Set R9C1 to 7
Full House: column 1
Set R3C1 to 6
Full House: column 2
Set R2C2 to 7
Full House: row 2
Set R2C6 to 6
Full House: row 3
Set R3C5 to 7
Hidden Single (block): 6 in block 5
Set R5C5 to 6
Full House: row 5
Set R5C6 to 5
Full House: column 6
Set R7C6 to 7
Full House: row 7
Set R7C5 to 5

[Roy McCoy]

I missed the full house - that's disturbing.
And I kept wondering where'd I'd gotten that 6 before!
But it will be interesting, following the entire hidden-single solution. Let's see...
Ah, I never got the 3 in block 6, and that's your first move.
I won't follow your solution, then, but will have yet another try myself...
But wait a minute, I see why I didn't get the 3. Aren't there two places for it, 5,8 and 4,9?
Why not 5,8?

[Roy McCoy]

Oh, sorry, never mind.
The original puzzle didn't have the 8 there, I filled that in myself.
So now I will try it.

[Roy McCoy]

Okay, finally.
I should have gotten the 3, that's what messed me up.
Thanks again.