Solver App for Android and iPhone
Strategies for Number Puzzles of all kinds
Latest Apps
Page:2015 1 2 :2014 2013 2012 2011 2010 2009 2008 2007 2006

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 .

Post a Comment or Question here...
Your Name or 'Handle'

Part of the World you're from

Email Address (optional) so I can reply directly if necessary (it will not be displayed here)

Any Sudoku you want to publish here for easy loading into the solver - 81 characters, use '.' or '0' for unknowns.

public - comment/question will be added to feedback column
private - email comment/question directly to Andrew Stuart, don't display here

Comment, question or feedback:

Enter these letters
Please enter the
letters you see:
Many thanks to all the people who have helped improve the solvers and strategies with their feedback!

Wednesday 18-Feb-2015

... by: Mark, Australia

Hello, Andrew.

Thank you for creating and maintaining this website. Being shown how to solve a given puzzle, step by step, is wonderfully educational! In just a few minutes, with your step-by-step solver, I have learned more than I'd learned in hours, poring over static examples at other websites. Thank you for freely sharing this functionality!



Andrew Stuart writes (19-Feb-2015):

Very glad you like, thanks for the feedback!

Saturday 14-Feb-2015

... by: FallsOffRocks, UK

I've read your comments on simple colouring several times now and cannot get a handle on the significant difference between your rule 4 and rule 5.

Surely Rule 4 is just a constrained subset of Rule 5, with the constraint being that the two coloured candidates that the candidate to be eliminated can see must be in the same unit as the candidate to be eliminated?

I cannot really see a separate justification for rule 4.

Andrew Stuart writes (15-Feb-2015):

This is a very good observation, and I think you have to be a programmer trying to build the algorithm to spot this, so congrats. I'm certain Rules 4 came a long first and then - much later - further eliminations were spotted that Rule 4 failed to get and led to rule 5. What didn’t happen was a consideration that this rules was a super-set of another. Anyway, I've taken a couple of examples with plenty of instances of Simple Colouring and 3D Medusa and satisfied myself that Rules 5 does get all rules 4 instances plus it's addition ones.

This one had 2x Rules 4 and 2x Rules 5 3D Medusas, but I'm patching the solver tonight, so you wont see them unless yr quick. It now shortens the solution path.

This puzzle has many Simple Colorings - only the last right at the end does the new rule get two 4s instead of one.

Basically, all I need to do is remove the code for Rule 4 and let Rule 5 do the job. That's awkward because I now have to rename Rule 5 to Rule 4 and Rule 6 to Rule 5. Can't have a gap in the sequence. I'll try to update the documentation on Simple Colouring and 3D Medusa at the same time.

I'd be delighted to credit you on the pages.

It's rare to find a simplification, it's normally the case I have to add a complication

Friday 13-Feb-2015

... by: FallsOffRocks, UK

Thanks so much for the site and all the work that must have gone into it.

I was validating my own solver and loaded up your Y-Wing example. My own Y-Wing algorithm clearly scans in a different sequence from yours, as it showed up a completely different Y-Wing, a very nasty one to spot without assistance. Just rotate the puzzle and your solver gets the nasty one first.

Now I have to start on the chaining methods :(

Andrew Stuart writes (14-Feb-2015):

Yes, there are usually several examples of strategy X at any one point, and it depends on the search order. I'm sure you are going top left to bottom right and 1 to 9, which seems natural, but from then on it really depends on your pattern identifier. For chains I create a list of all possible chains and rank them according to "best", which is a set of rules about how long the chains are, how many eliminations they make, if they give a single cell solution, if they contain awkward links like grouped cells or ALSs. So there is a lot in the air and no two solvers will match.

Ideally I'd like to give the user an option to scroll through all the instances of a given strategy but it would greatly complicate the UI.

Monday 9-Feb-2015

... by: 0t0, europe

Load Sudoku:

I've found a Sudoku with a step with 16 solved cells at once:
My first 3D Medusa Sudoku!

Andrew Stuart writes (12-Feb-2015):

A very pretty Medusa! Nice catch

Monday 2-Feb-2015

... by: dan, florida

I'm doing one of your puzzles, a moderate one. but the solver has to use "diabolical" strategies.
How can it be moderate if you have to use xychain or some other diabolical method to solve?

Andrew Stuart writes (12-Feb-2015):

A single instance of a hard strategy does not a hard puzzle make. These are two different spectrums. It is possible to make trivial puzzles with a single very hard step (a bottleneck) and I try and filter those out when selecting for publishing, as they don’t make good puzzles. They are useful as exemplars, eg see X-Wings

Do let me know what puzzle it was, if you can find it again, I'd be happy to take a look at it.

For a fuller idea of how grading works, take a look at this write up.

Monday 2-Feb-2015

... by: anonym, europe

Kakuro is my FAVOURITE of logic number puzzles. I'm missing Kakuro Solver.

Andrew Stuart writes (12-Feb-2015):

There is some demand for that, and I do make those puzzles, but not quite enough yet to make a solver. On my wish list as well

Friday 23-Jan-2015

... by: DeBo, Germany

This is one of the best (may be: The BEST) solvers I have any seen.

Andrew Stuart writes (28-Jan-2015):

Thanks Debo.
Will be ten years old in May. Had a lot of helpful comments aloing the way, and much encouragement

Monday 19-Jan-2015

... by: Arkis, USA

Perhaps I've missed it. How do you delete puzzles from the "favorites" list, or the dailty puzzels list ? Want to limit storrage size.

Andrew Stuart writes (21-Jan-2015):

If you are talking about the Sudoku Solver for Android, go to settings and change the choice for "how long to keep".
On favourites - long click will offer a delete

Best regards

Saturday 10-Jan-2015

... by: 0t0, europe

I've seen a bug on the Str8ts solver: When I click "Take Step", then appears a window: Oops - you've got an error on the board.

Andrew Stuart writes (11-Jan-2015):

Please use the "Email this board" button to share the puzzle in question. Can't help much otherwise. The solver has been stable for some time so I've not received any other bug reports.

Friday 2-Jan-2015

... by: seventeener, europe

kakuro solver???

Andrew Stuart writes (11-Jan-2015):

Kakuro is a nice puzzle, one I make. But there's not much strategy to Kakuro, it's pretty much combination elimination all the way. I've thought about it but I'd like to get more newspapers taking the puzzle before I put the time into it. Happy to be pointed at examples that are interesting though.
Page:2015 1 2 :2014 2013 2012 2011 2010 2009 2008 2007 2006