The New Sudoku Players' Forum

[asajar]

I solved this puzzle to this point

Code: Select all

*--------------------------------------------------*
 | 7    2    6    | 3    4    9    | 8    5    1    |
 | 9    1    3    | 8    5    7    | 6    2    4    |
 | 5    4    8    | 6    2    1    | 9    7    3    |
 |----------------+----------------+----------------|
 | 3    5    2    | 4    7    8    | 1    9    6    |
 | 8    7    4    | 1    9    6    | 5    3    2    |
 | 1    6    9    | 5    3    2    | 47   48   78   |
 |----------------+----------------+----------------|
 | 24   9    15   | 27   6    3    | 47   18   58   |
 | 6    3    157  | 9    8    45   | 2    14   57   |
 | 24   8    57   | 27   1    45   | 3    6    9    |
 *--------------------------------------------------*


I thought I had a row swordfish on 5's formed by G3,G9,I3,I6,H6,H9--It seems to me to follow the rule of 3 rows of no more than 3 5's and a total of 3 columns. I thought I could then eliminate the 5 at H3. The solution shows that H3 is a 5. Simple Sudoku says this puzzle is asymetrical but it came out of Mensa Absolutely Nasty Sudoku Level 3. Andrew Stuart's solution at this point is a Y wing which I do see now but still I don't understand why my swordfish thought does not work. I have been working on my swordfish patterns and thought I was thinking logically. Thanks to you experts for your assistance for a beginner.

[eleven]

The 5 at H3 (r8c3) is part of the swordfish. Seen as row swordfish you could eliminate 5's in rows A to E and columns 3,6,9. But no 5 there.

Probably the easiest way to solve from this grid is to see the BUG (only one cell with 3 candidates). H3 must be 5, because (different to 1 and 7) you have two more 5's in the same row, column and box.

[asajar]

Thanks so much for your help. Somewhere along the line I missed that any number within the swordfish pattern are disqualified. I am now trying to understand the Bug you quoted. Is this a fault in the design of the puzzle?

[Bigtone53]

asajar,

This link should assist with the BUG technique.

[udosuk]

What you need to solve this puzzle is not a swordfish, but an xy-wing.

Code: Select all

+----------------+----------------+----------------+
| 7    2    6    | 3    4    9    | 8    5    1    |
| 9    1    3    | 8    5    7    | 6    2    4    |
| 5    4    8    | 6    2    1    | 9    7    3    |
+----------------+----------------+----------------+
| 3    5    2    | 4    7    8    | 1    9    6    |
| 8    7    4    | 1    9    6    | 5    3    2    |
| 1    6    9    | 5    3    2    | 47   48   78   |
+----------------+----------------+----------------+
| 24   9    15   | 27   6    3    |*47   18   58   |
| 6    3    157  | 9    8   *45   | 2   -14  *57   |
| 24   8    57   | 27   1    45   | 3    6    9    |
+----------------+----------------+----------------+

xy-wing:

r8c9=5 or 7
If r8c9=5 => r8c6=4
If r8c9=7 => r7c7=4
=> One of r7c7 & r8c6 must be 4

Therefore r8c8, seeing both r7c7+r8c6, can't be 4, must be 1.

I wouldn't use a uniqueness-based technique such as BUG when such a simple move is available.

[asajar]

Thanks to all for your help. It is great that this site is available and you all are here.

[eleven]

I wouldn't use a uniqueness-based technique such as BUG when such a simple move is available.

Uniqueness methods for me are a great enrichment of the solving techniques. E.g. UR's are very easy to spot (easier than most x- or xy-wings) and often effective.

In this case with one glance you can see the only 3-candidate cell, with a second you have the right candidate.
On the other side you probably need a dozen glances to find the right cells for the xy-wing (IF one is in it ).

So i think the only reason to prefer the xy-wing here is a suspiciousness to the puzzle creator, which i dont have.