## Stuck!! - Help appreciated

Advanced methods and approaches for solving Sudoku puzzles

### Stuck!! - Help appreciated

This is a grade V.Hard from the program. I've been stuck at this point for the past two evenings and would appreciate some help please.

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

Posts: 29
Joined: 05 May 2005

XWing of 5s, r1c1, r1c8, r4c1, r4c5.
Last edited by simes on Sun Dec 11, 2011 2:42 pm, edited 1 time in total.
simes

Posts: 324
Joined: 11 March 2005
Location: UK

### process of elimination

From what you are showing, r1c3 can be 1 or 9. Trying 9 leads to later error so must be 1.
Now r1c4 can be 5 or 9. Trying 9 leads to later error so must be 5.
Now r3c4 can be 2 or 6. Trying 2 leads to later error so must be 6.

With these 3 cells as 1, 5 and 6 respectively should be possible to progress all the way to solution.

I've included solution but you might like to try to progress yourself before using given solution.

631 549 278
452 178 963
879 623 415

218 936 754
795 814 326
364 257 891

143 762 589
926 485 137
587 391 642 Regards, Neil.
Guest

simes wrote:XWing of 5s, r1c1, r1c8, r4c1, r4c5.

That's not the X-wing... box1 already has a 5 so r1c1 is defently out of the question.

The X-Wing is r1c4, r1c8 and r4c4, r4c8

And this allos the 2 to be filled in in column 4.
Animator

Posts: 469
Joined: 08 April 2005

simes wrote:XWing of 5s, r1c1, r1c8, r4c1, r4c5.

Don't know what I was thinking of. rc1c isn't even empty. I must have been looking at the wrong puzzle! Sorry folks!
Last edited by simes on Sun Dec 11, 2011 2:42 pm, edited 1 time in total.
simes

Posts: 324
Joined: 11 March 2005
Location: UK

Many thanks all ... I must admit that I haven't been able to understand x-wings so far but then I haven't needed to until now. With the help you've given I'll try to work out the logic involved. Thanks again.
joolslee

Posts: 29
Joined: 05 May 2005

### logical approach

Logic involved is identify candidates for each vacant cell. Pick a cell that has only two candidates. Try one. If, that doesn't work try the other. Sometimes this will lead to picking a later cell that has only two candidates and repeating but method hasn't failed me yet. There is a good explanation of X-wings by simes in relation to the question - What are they? I've not needed to use that method yet. Regards, Neil.
Guest

Neil. I'd just like to point out that what you are advising is trial and error. In the Pappocom Puzzles this is not necessary - even for the Very Hard - and some people would prefer not to do it.

Luna
lunababy_moonchild

Posts: 659
Joined: 23 March 2005

Woo-hoo!!! Finally completed puzzle after 197 minutes - best of all I now understand what x-wings are about!! ... once the 2 was in place, the puzzle was finished within a couple of minutes ... I shall sleep easily tonight, thanks again :o)
joolslee

Posts: 29
Joined: 05 May 2005

hi im from australia & new to sudoku so far im still on the easy stages but am finding them very interesting im sure i will get better at this with heaps of practise
gaily39

Posts: 1
Joined: 26 May 2005

### Re: Stuck!! - Help appreciated

joolslee wrote:This is a grade V.Hard from the program. I've been stuck at this point for the past two evenings and would appreciate some help please.

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

The solution is:
6 3 1 5 4 9 2 7 8
4 5 2 1 7 8 9 6 3
8 7 9 6 2 3 4 1 5
2 1 8 9 3 6 7 5 4
7 9 5 8 1 4 3 2 6
3 6 4 2 5 7 8 9 1
1 4 3 7 6 2 5 8 9
9 2 6 4 8 5 1 3 7
5 8 7 3 9 1 6 4 2
Guest

Hey mathswizard,
Did you work this out yourself or just copy it from the entry from three days ago?? Even joolslee who you purport to be helping worked it out 24 hours ago!!! Keep-up.
Guest

### sorry

lunababy_moonchild et al,
This puzzle was one of my first attempts and I didn't realise that T&E was frowned upon. I have re-examined the discussion with respect to X-wing and I agree it is a far more elegant and efficient way to arrive at a solution. I'm now inspired to avoid T&E, (I thought I was helping!), but I can see that "reading the clues" is far more satisfying than just "grunting" my way through. Thanks for opening my eyes.

Regards, Neil.
Guest