# 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...**
## Saturday 28-May-2016

## ... by: Pieter, Newtown

Happy 11th Birthday to you again for the Solver!

I refer you to a comment in Feedback by Philipp, Germany on Saturday 30-Dec-2006 (2006 page 1). He says "If I try to solve this sudoku there is only found a hidden pair but then there is nothing else... Is it impossible to solve it by logic steps?"

I was surprised the Solver still didn't solve his puzzle even after 9 1/2 years further development, despite brute force finding the solution! I suppose there will always be unsolvables ... it just surprised me. :-)

An Unsolvable for your collection, though you probably already have a record of it!

Also a comment by steve brod, utah on Sunday 31-Dec-2006 ... referring to the 16x16 solver you wrote the year before. I've never seen it. Did/will it ever get done? :-)

Ciao,

Pieter

Not looked at 2006 for a while, that’s for sure. That sudoku reminds me of Escargot, high clue density. I would have liked to be able to offer a solver that solves them all by now, but still a case of chipping away bit by bit.

I can reveal the 16x16, very much an early prototype that hasn't seen any work on it since those days. I've shied away from it since 16x16 isn't a popular puzzle and I never made enough of them to grade the puzzle properly. Also quite a challenge on the UI. But as a bit of fun, here is the link:

http://www.sudokuwiki.org/sudokuhex.htm

## Thursday 26-May-2016

## ... by: Dave, Indianapolis

## Saturday 23-Apr-2016

## ... by: Sean, Omaha

## Thursday 21-Apr-2016

## ... by: Eyad Khalil, Palestine

Can the Daily Puzzles listed on your site be automatically imported into the Solver - without having to enter the 81 digits one at a time?

Thanks,

## Thursday 21-Apr-2016

## ... by: Bert, Netherlands

last few days: unable to update kakuro puzzles:

Could't reach the Internet to pick up the puzzles.

No problems with the other puzzles.

Appreciate the alert!

## Tuesday 19-Apr-2016

## ... by: Chris Green, Australia

I just solved Unsolvable #196, using a simple program I wrote (to save laborious work) plus a trial and error approach. I guess such an approach is not considered very valid since Andrew seems to be looking for more logic-based approaches.

Anyway, it took me 22 tries but when I eliminate the erroneous tries, it can be solved in 6 tries. (Of course, that's a case of 20/20 vision in hindsight!) I chose to start with Box 9 because it has the least number of possible candidates at the beginning.

The sequence I chose was: J8 =3, J2 =1, E1 = 1, B1=5, J4 =6 and finally, J3 = 5 for completion.

A New Metric for Difficult Sudoku Puzzles

which follows on from

Arto Inkala Sudoku

## Thursday 14-Apr-2016

## ... by: UL, Hong Kong

Load Sudoku: CLICK TO LOAD-1[G2]+4{G2?E2}

I don't understand this strong link in the puzzle, could you please explain it?

## Saturday 26-Mar-2016

## ... by: Uhm, Netherlands

Your solver does not find easy boxline reductions or pointing pairs, instead it finds x-cycles which does slowly the same reduction.

As you documented in:

http://www.sudokuwiki.org/Windoku_Strategy

A windoku is just a double sudoku, 1 constraint extra for each cell.

The first x-cycle it find is at 2 in column 5, which is just an boxline reduction from that cell. So you can exclude more which an easier strategie, those x-cycles are apparently hard for your solver.

Cheers

Edit 27 March: I have added in proper Pointing Pairs and Line/Box Reduction for 'windows'. I am recalibrating the grades as a result and will provide new examples in the solver example list.

## Friday 25-Mar-2016

## ... by: David Munson, Los Angeles, California, USA

Load Sudoku: CLICK TO LOADI have been noticing problems with the Sudoku solver. I'm working from Los Angeles, California, U.S.A. I cleared all my browsing data in Google Chrome, this includes "cached images and files". I have noticed that the Solver is no longer finding X-Cycles and it seems to also not be finding AICs.

Here is an example position; it is from your Jan 2016 Diabolical Puzzle Pack puzzle #14. From this position, after the Sudoku Solver gets through the basic 6 rules, the Solver finds an X-Wing that cancels two 9's.

Next after going thru the basic 6 rules, it finds an XYZ-Wing. But if XYZ-Wing is de-selected, then the solver should find an X-Cycle for 8's which cancels one 8. But it does not. Instead it finds an XY-Chain. Here is the X-Cycle for 8's: +8[B4]-8[B7]+8[C9]-8[H9]+8[H4]-8[B4]. This is Nice Loops Rule 3.

Also, after canceling the 8 on B4, then if we de-select all the rules from XYZ-Wing to Finned Sword-Fish, now the Solver should find an AIC. But the Solver proceeds on to Digit Forcing Chains. There are probably many AICs in this position, here's one of them: +3[B4]-3[B6]+8[B6]-8[B7]+8[C9]-8[H9]+6[H9]-6[H4]+8[H4]-8[G4]+3[G4]-3[B4]. Another Nice Loop Rule 3.

A month or so ago things were working fine. This happens for other puzzles too. I tried the above position in Windows Explorer as well as Google Chrome, same result. I still have Windows Vista Ultimate; don't know if that makes a difference.

Thanks for your time,

from Dave Munson

## Thursday 25-Feb-2016

## ... by: JB, France

Load Sudoku: CLICK TO LOADI tried to solve a sudoku with your solver, but it failed when applying the "Alternating inference chains" technique. The solver excluded digit 3 from J6, which is actually the correct value. As a result, the solver gets stuck later and finds no solution. Could there be a bug concerning this technique? I don't understand it myself, so I can't tell what the problem is...

But I can assure that this sudoku has a solution, as I solved it manually and verified it!

JB

Thanks for sharing

Edit: 29th Feb 2016: Fixed. Put in a check in the correct place and now this false positive does not occur.