The New Sudoku Players' Forum

[Karyobin]

As a result of my being inspired to attempt a V. Hard without any pencil marks, I sat here this morning with a monumental hangover and had a go. I was genuinely surprised at how easy the first twenty-one moves were, but we all know that there are grades within grades. Then... *Bang*. Sudden stop. So I sat for twenty minutes, then gave up and inserted all the pencil marks. *Louder Bang*. No help there. So I really gave up and dubbed the puzzle into Sadman's program and asked it for the next move, and now we get the the crux of the matter...

Can anyone tell me why the next step is to place a 7 in r2c1?

* * * 7 3 9 * * *
* 1 * 5 * 2 * 9 *
* * 9 1 * 8 7 * *
3 * * 8 1 6 9 4 2
9 4 1 2 7 3 * 6 *
8 2 6 9 5 4 * * 7
* * 2 4 9 * 6 * *
6 9 * 3 2 * * 7 *
* * * 6 8 7 * * 9

(I'm expecting I've missed something obvious like a hidden triple, if that's not an oxymoron, but the Stella is still talking to me.)

[scrose]

Look for an x-wing and make some eliminations.
Then look for a triple in box 1 and make some eliminations.
Then you can place the 7 at r2c1.

Please reply if you need a hint about which candidate to examine for the x-wing.

[Karyobin]

Nope. I'm assuming the x-wing is in the 8's, but I've checked rows and columns and I can only think I've got a few dodgy pencil marks.

[angusj]

Nope. I'm assuming the x-wing is in the 8's

Code: Select all

.8.|...|.8.
...|...|...
...|...|...
-----------
...|...|...
...|...|...
...|...|...
-----------
.8.|...|.8.
...|...|...
...|...|...

[Karyobin]

Oh, for God's sake - nothing wrong with my pencil marks, just my eyes. Still, kind of proves my original thesis - would've been hard to complete that without a few pencil marks.

Cheers peeps.

[simes]

Karyobin,
Sadman's program gives...

X-Wing found for rows 1 and 7 in columns 2 and 8 for 8 - updating cell(s) (1,3)(1,7)(1,9)(7,9)
block 1: disjoint subset 245 in cells (1,1)(3,1)(1,3) - updating candidates for cell(s) (2,1)(1,2)(3,2)(2,3)
(2,1) = 7 : only possible value for this cell
etc

Or did it give you something different?

[Karyobin]

You mean it explains itself to you? Always knew I should explore these things a little more.

"Wise before your years you are young padawan, mmmmMMMM?"

How do you make it do that then?

{Five minutes later}

Of course, I could always just click on 'View solution log' couldn't I?

[stuartn]

How can this be Simes? - isn't there at 2 at R7C3?

Stuart
www.brightonandhove.org/sudoku/solver5c.xls

[stuartn]

With the 2 in place at R7C3, the logic goes like this....

The x-box reduces R1C3 to 45
If R1C2 was a 4, R1C3 would be a 5
Which would make R4C3 a 7
Which would mean R2C3 couldn't be a 7

Which would mean that R1C2 would be the only candidate for 7

Which is invalid - so it has to be 7 at R1C2.

Without triples.....;0)

Stuart

[simes]

How can this be Simes? - isn't there at 2 at R7C3?

Now you've confused me - who mentioned R7C3? (I didn't even mention R3C7, so it's not a row/column confusion.)

[stuartn]

In karyobins post there is a 2 at R7C3 which eliminates a triple including a 2 at R1C3. I'm sure you'll agree.

Stuart
www.brightonandhove.org/sudoku/solver5c.xls

[simes]

In karyobins post there is a 2 at R7C3 which eliminates a triple including a 2 at R1C3. I'm sure you'll agree.

Well, I agree there's a two at R7C3. Yep, I've just checked again, it's definitely there. To say this eliminates a triple including a 2 at R1C3 though? A 2 in column 3 eliminates another 2 in column 3? hmmm. Isn't that a teeny bit obvious? I must be misunderstanding, so I'm still confused. The only triple I can see is the one my solver found in a previous post.

<shrug>So there are at least two ways to proceed after the XWing. You've found a conclusion from a look-ahead approach, while my solver found the subset 245 in cells (1,1)(3,1)(1,3), and so eliminated candidates from cells (2,1)(1,2)(3,2)(2,3).

But in either case, I'm still missing the significance of:

How can this be Simes? - isn't there at 2 at R7C3?

[stuartn]

X-Wing found for rows 1 and 7 in columns 2 and 8 for 8 - updating cell(s) (1,3)(1,7)(1,9)(7,9)
block 1: disjoint subset 245 in cells (1,1)(3,1)(1,3) - updating candidates for cell(s) (2,1)(1,2)(3,2)(2,3)
(2,1) = 7 : only possible value for this cell
etc

You sent this. I've underlined the relevant entry. You also say this...

Well, I agree there's a two at R7C3

How can your ' disjoint subset 245' be possible in R1C3 when as you've agreed, there's a found 2 C3 already? Please enlighten?

[stuartn]

245 568 458
47 1 3478
245 356 9
3 57 57
9 4 1
8 2 6
157 3578 2
6 9 458
145 35 345

This may help make my point - (without resort to sarcasm). Cols 1 - 3.

It's not possible that you missed out the 2 at R7C3 when entering into your solver is it? - heaven forbid. ;0)

[simes]

Ahhh... I see.

OK, "disjoint subset" doesn't require that all the members of the subset appear as candidates for all the cells, just that the total number of cells with only subset members as candidates is the same as the number of subset members.

The subset consists of 2, 4 and 5, but the 2 doesn't appear as a candidate for (1,3)

Before applying the XWing, the candidates are

Code: Select all

{2,4,5} {5,6,8} {4,5,8}
{4,7}   {1}     {3,4,7,8}
{2,4,5} {3,5,6} {9}

After applying the XWing, the candidates are

Code: Select all

{2,4,5} {5,6,8} {4,5}
{4,7}   {1}     {3,4,7,8}
{2,4,5} {3,5,6} {9}

After applying the disjoint subset, they become

Code: Select all

{2,4,5} {6,8} {4,5}
{7}     {1}   {3,7,8}
{2,4,5} {3,6} {9}

It's not possible that you missed out the 2 at R7C3 when entering into your solver is it? - heaven forbid. ;0)

No, of course not, I used copy'n'paste.