# Feedback and Questions

I've received a lot of interesting comments and questions from Sudoku fans over the last few years and this page
is where I try to answer them. Please feel free to drop me a note on the side of the page or try the Facebook
comment box. Or you can email me directly at andrew@str8ts.com.
**Post a Comment or Question here...**

## Friday 20-Dec-2013

## ... by: Ray, Tulsa, OK

There is only one strategy that uses solved cells and it is very interesting. Have a look at:

Avoidable Rectangles

## Monday 16-Dec-2013

## ... by: Amb, New Zealand

For example:

X-Wing (once understood) is much easier for a human to find than simple Colouring.

Unique Rectangles, WXYZ Wing are both easier than X-Cycles and XY-Chain.

Could the strategies that are more like trial and error be moved to a later sequence in the moves list?

ie: For tough

X-Wing

Y-Wing

Sword Fish

X-Y-Z Wing

Simple Colouring.

ie For Diabolical

Unique Rectangles

Extended Unique Rectangles

Hidden Unique Rectangles

WXYZ Wing

Others (eg X Cycles, XY-Chain, 3d Medusa etc)

These strategies are ones that can be found by learning what to look for and why, and are better than the other trial and error feeling ones. Quite often when I have stepped through a sudoku and turned off X-Cycles, XY-Chain, Jellyfish, I've ended up finding the WXYZ step was available and this seems to be a more elegant solution to the sudoku.

Amb

## Monday 16-Dec-2013

## ... by: Ron, US

## Friday 13-Dec-2013

## ... by: mwalimu, Illinois, USA

It seems to me the trickiest part of programming it would be validating that the shapes are contiguous (although even if they're not I suspect all the solving strategies would still work and the solver wouldn't care, even though it would be very haphazard and hard to follow for a real person to follow). Consider allowing one box to be non-contiguous, which would allow for something like a puzzle where one "box" consists of cells B2, B5, B8, E2, E5, E8, H2, H5, H8 while the other eight boxes have to be contiguous as usual.

## Wednesday 4-Dec-2013

## ... by: Jeyaraj, India

Load Sudoku: CLICK TO LOADThanks

## Wednesday 27-Nov-2013

## ... by: Chug, Boston, USA

I'm an avid consumer with a buglet to report. I loaded KenKen "#865, November 26" into the solver and solved it. However, the solution doesn't make sense but when I click Take Step it says, "That's it, all done". The board I entered clearly does not solve the KenKen. See http://www.rolke.org/kenken-bug-20131127.png

I got all the rows and columns to have 1..6 but they don't satisfy the cage constraints. Is this a bug?

-Chug

BTW, thanks for your efforts in this work. I have really improved my game on your account. -C

Refresh the page a couple of times and make sure you can see version number 2.08

## Monday 18-Nov-2013

## ... by: PeteTy, ohio, usa

http://www.sudokuwiki.org/Daily_Killer_Sudoku.asp

cant seem to find todays killer....

## Tuesday 12-Nov-2013

## ... by: Gerrit Altena, The Netherlands

Load Sudoku: CLICK TO LOADAlternating Inference Chain

AIC on 3 (Grouped Discontinuous Alternating Nice Loop, length 10):

-3[E6]+3[D4|E4]-3[A4]+1[A4]-1[G4]+5[G4]-5[B4]+3{B4|B9}-3[B6]+3[E6]

- Contradiction: When 3 is removed from E6 the chain implies it must be 3 - other candidates 1/4/6/7 can be removed

Although the end result is correct, the path is not:

after -5[B4] it should be: +7[B4]-7[B9]+3[B9]-3[B6]+3[E6]

The reason is that with +3[D4|E4] the 3 in B4 is already eliminated.

Great program though!

I have built into the algorithm for chains the rule that the same candidate can't be used twice - mainly to stop endless loops appearing. But no rule exists for this type of check. However, if there was a problem doing this then I would have found an example where it creates a mistake. In the zillions of tests I've done this situation hasn't created a false positive, so I conclude it is harmless. I think the way to look at it is that these candidates are being turned on "potentially" - they have no reality to them since we don't know if they will eventually be the correct solutions to those cells.

Do you think there is always an alternative chain that doesn't violate this rule?

## Monday 11-Nov-2013

## ... by: Amb, New Zealand

## Sunday 10-Nov-2013

## ... by: Florent Chayet, France

Just one small remark: either the auto clear is on, and then when a solution is added all the previously manually suppressed candidates are back, either it is off and to suppress the candidates after adding a solutio is quite a job. The dream would be that auto clear on results in suppressing the candidates due to the added solution without changing anything else on the grid.

Thanks again!