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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 .

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Many thanks to all the people who have helped improve the solvers and strategies with their feedback!

Wednesday 29-Jun-2016

... by: Carl, Northern Ontaro

Andrew, your solver cannot solve some of your diabolical puzzles which I have brought up. Does that mean there is no solution or might they be beyond the skill of the solver?

Andrew Stuart writes (30-Jun-2016):

The solver has been updated since the stock was made. I'm aware that there is now something tiny difference that is causing previously solvable puzzles (and therefore gradable) to become 'unsolvable' and I'm trying to track that down. What was the last puzzle with that problem? Can you remember?

Saturday 28-May-2016

... by: Pieter, Newtown

Hi Andrew,
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? :-)


Andrew Stuart writes (29-May-2016):

Thanks you for your best wishes! Been so worthwhile and a lot of fun

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:

Thursday 26-May-2016

... by: Dave, Indianapolis

When I click on a cell of the "Enter Clues" grid box on the left a small box pops up with "PK59A" in it. I'm guessing that's supposed to be suggested entries. However, the suggestions are always "PK59A". Doesn't seem to be relevant to me.

Andrew Stuart writes (27-May-2016):

That’s weird. What solver? Sudoku vanilla? What OS and browser are you using. Could be some debugging dropping I left behind but I cant reproduce. Certainly not a value suggestion

Saturday 23-Apr-2016

... by: Sean, Omaha

Thanks for this website. You saved my sanity and proved to my wife you don't have to "guess" to solve a legitimate puzzle. Pointing pairs. I won't soon forget it.

Andrew Stuart writes (23-Apr-2016):

Excellent! So glad you found it useful

Thursday 21-Apr-2016

... by: Eyad Khalil, Palestine

Hi Andrew:
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?

Andrew Stuart writes (23-Apr-2016):

In most cases yes, look for the "Load in Solver" button

Thursday 21-Apr-2016

... by: Bert, Netherlands

problems with '5 daily puzzles' on ipad.

last few days: unable to update kakuro puzzles:

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

No problems with the other puzzles.

Andrew Stuart writes (23-Apr-2016):

Yes, I see the problem. My tracker said I had puzzles up to 2019 but its not true, only certain grades. I will add some more today

Appreciate the alert!

Tuesday 19-Apr-2016

... by: Chris Green, Australia

I'm new here and I'm not sure if this is the place for comments about the Unsolvables.

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.

Andrew Stuart writes (23-Apr-2016):

Germaine to you comment you may be interested in
A New Metric for Difficult Sudoku Puzzles

which follows on from
Arto Inkala Sudoku

Thursday 14-Apr-2016

... by: UL, Hong Kong

Load Sudoku:

Hi Andrew,

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

Andrew Stuart writes (23-Apr-2016):

You puzzle has 25 solutions so it's not a valid puzzle. But, to explain that link ffragment, have a look at AICs with ALSs

Saturday 26-Mar-2016

... by: Uhm, Netherlands

Crazy Extreme (Windoku)

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:

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.

Andrew Stuart writes (27-Mar-2016):

Quite right, now I've appreciated the overlapping nature of the windows and boxes more work needs to be done. I was wondering why there are so many short cycles.

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:

Hi Andrew,

I 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

Andrew Stuart writes (26-Mar-2016):

You are quite correct. So I had a look in the code, and lo and behold, there was still a debugging line in there from my last testing and update. It had the tragic effect of focusing the AIC search for X-Cycles and other AICS to just one candidate. The candidate in a test puzzles I was trying to fix 20 days ago. Bit of an oops. Which is why I don’t work on mission critical system. Thanks bringing to my attention with such a clear example. Solver updated.
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