Solver App for Android
Strategies for Popular Number Puzzles
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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. I'm also directing Str8ts feedback here. Please feel free to drop me a note on the side of the page. 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!

Thursday 15-Sep-2011

... by: Becko, Germany


yetsterday (14.09.2011) i solved the daily Jigsaw, but when finished, 26 conflicts were reported. This morning i compared my solution, which have printed out, with the online solution and they are the same ???

with regards


Andrew Stuart writes:
Little late in replying but a big thanks for the bug alert. I really appreciate feedback where something its obviously not right, so I can fix it. I think I know what's going, and I'll try and update the player Sunday

Thursday 8-Sep-2011

... by: Alistair, Canada

Andrew, thanks for all the work you put into Sudoku. I am a rookie and have found your teaching very helpful.

recently I got ambitious and tried your diabolical competion from Aug 6

I managed to solve it however I have not yet learned diabolical strategies. I confirmed my solution to be correct on you Sudoku Solver. I stepped through this puzzle on your sudoku solver to see the strategies used and am sure I could not have used those strategies.

Is it possible to solve the competition Aug 6 using simple strategies??

Andrew Stuart writes:
No. There is a wide spectrum of difficulty withing diabolical but there will always be one or two bottlenecks that require several advanced techniques often in sequence. the solver tries to use the simpler strategies before the harder ones to exhaust the easier possibilities first. 'Tough' grades will not require anything after X-Cycles and XY-chains (if at all). Diabolicals will not require anything listed in 'extreme strategies'. There is more to it than that but generally thats the rule.

Thursday 1-Sep-2011

... by: Fred Peyerl, USA

the color did not print in color

Andrew Stuart writes:
There is a trick to printing background colours. Have a look at

Wednesday 24-Aug-2011

... by: Hache, France, Europe


I think this site is fantastic. By keying in my unsolved grids and following the solution through step by step I am learning the more complex solving techniques which I was struggling with before. One question: how are the overall scores for grading worked out. Yesterday I had a puzzle which had a score of 182, today I have one with a score of 174. Does that make today's puzzle slightly easier though they are both rated as tough?

Many thanks,


Andrew Stuart writes:
I wouldn't read too much into a score difference of under 50. I try to score based on a near optimal path and there are usually several if not many paths depending on what you spot first. And the scoring/grading spectrum is very wide since Sudoku is a very deep puzzle. Have a look at Sudoku_Creation_and_Grading.pdf if you'd like a lot of detail on scoring. Glad you like the site and thanks for the feedback

Tuesday 23-Aug-2011

... by: Ivar, Norway

Load Sudoku:

Hi Andrew.

The attached Sudoku is a Colour Sudoku, and while using your solver on it when I got stuck I noticed that your solver lacks a rather simple Colour/Line Reduction technique, which should be one of the top techniques, likely #8 on the list (under 7 Box/Line Reduction).

In the attached Sudoku the technique wouldn't help much, but it might on other puzzles. To elaborate; if you check line A, the 6's in A8 and A9, the only 6's in box 3, eliminates the 6 in the purple A4. Moving on to line D, there are 6's in the purple D1 and D4. All the other purple cells are either solved or lacking 6 as a candidate, so logically line D MUST have 6 in either D1 or D4; the 6's in D2 and D9 can be eliminated. This your current version of the solver doesn't do at the moment... Like I said, eliminating those 6's in this puzzle doesn't help much, but in other puzzles it might very well be a solving move...

Just thought you should know, it aught not be impossible to implement. ;)

Thanks for this fantastic solver!


Andrew Stuart writes:
My first feedback on the colour Sudoku! You're quite correct, I should be able to add that. Perfectly good strategy. In the job queue now :)

Monday 22-Aug-2011

... by: Chuck, Virginia

Hi Andrew,
I really like the option of being able to put "The Daily Sudoku" into the solver and printing it out with the candidates already in place. However, this option isn't available for the "30 Day Archive" puzzles. Would you consider adding that option to the archive puzzles as well?


Chuck Bruno

Andrew Stuart writes:
That's a good idea, I'll try and roll that out very soon

Update: Feature now available - see the bottom of the page

Thursday 18-Aug-2011

... by: Andreas von Delius, Germany

Load Sudoku:

Hello Stuart,
may be, this will interest you to develop your wonderful solvers. I tried this puzzle with your Sudoku solver. It shows a 9 in B9 by a Digit Forcing Chain. But it does not find before the off chain AIC, which you have in the same circle by ignoring B9, like it should:

Using this, you may take 4 and 9 from A8 and 8 and 9 from H1 and the puzzle becomes immediately quite easy.


Andrew Stuart writes:
This is a very interesting observation and I now I've investigated the code and tested it on my library, you are right - the solver was missing this trick. I've just now updated the solver back-end to capture this situation. I've also updated the article on AIC Nice loops with this example. The example can be view using the links.

Very nice addition, well spotted, and many thanks.

Monday 8-Aug-2011

... by: Dale Kloss, USA, Oregon, Portland

Have you stopped publishing Jigsaw puzzle packs????

Andrew Stuart writes:
I did stop for a while, but I am starting again to make some new ones. Hoping to release a pack for August but certainly September

Update: Jigsaw Puzzle Packs now available! yay

Monday 25-Jul-2011

... by: oui4, Canada

Sorry made a big misyake in my previous message few minutes ago. Here it is corrected : Hello! Found something with 2 aspects. While looking for all possible combinations in the small grid I realize some combinations weren't there when the box is L-shape and covering 2 boxes (9-square boxes). Like in no1218 july 25 for the sum of 7 where it show 1,2, 4 with skipping 3,1,3 and 2,3,2 and 1,5,1. The other side of it is when we use only the steps from the solver. In this no 1218, the 4th step make the same error not allowing for those 2 combinations. As I am concern, it is the same logical problem. Despite this, you are doing a great job and I am very happy to rely on your tools to better myself at solving sudokus.

Andrew Stuart writes:
Which puzzle/solver are you referring to? KenKen, KenDoku or Killer Sudoku? With Killer Sudoku there is a convention that a cage which dog-legs (such as the L you refer to) cannot contain duplicate numbers. This confines the Killer cages to a few less combinations and is deliberate. If you have a Killer which doesn't conform to this convention the solver will not work and produce erroneous results down the line. I do however need to create a page documenting this...on the job list. With KenKen this convention does not apply and you'll see such combinations available.

Monday 18-Jul-2011

... by: Riddick51PB, Lincoln, Nebraska USA

Have you thought about making your solver available as an Android Application (Paid App is fine). I own your book, altho have not read it yet. Your Sudoku Solver would be a great tool to help me learn advanced puzzles in my Motorola XOOM.

Andrew Stuart writes:
Yes, its definitely something I'd like to do. Waiting for the time. Have to learn yet another programming language : )
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Thank-you everyone for all your questions and contributions.