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!

Tuesday 18-Dec-2012

... by: Gerald, USA - California

Could you please take a look at the one of the examples from the strategy page: X-Wing Strategy. When I load X-Wing example1: Load Example into the solver, it looks nothing like the printed example. I entered the printed example into the solver and then compared it with the loaded example from the strategy page. It looks to me as if one is the inverse of the other.

Here's what I mean: X-Wing 1 - X-WIng 2
-gerald asp.

Andrew Stuart writes:
Very interesting report, thank you. Been while since I wrote that but I suspect I was rotating the puzzle to position the X-Wing in a certain direction for illustrative purposes but added the original textual strings into the page. It does, however, show that the strategies used (at least the early ones) are independent of the rotation of the puzzle. For the advanced strategies there are often many choices and it often picks the first example on a top-left to bottom-right 1 to 9 search order. I've put the correct strings in now.

Thanks for pointing it out and have a great Christmas

Friday 14-Dec-2012

... by: Parsi, San Francisco Bay Area

Thank you Andrew. Your site has saved me hours of head scratching, followed by random clues out of the solution the following day. Now I get the next clue from your solver when I get stuck and can then solve the rest of the puzzle. This has helped me learn strategies and improve my skills. Your solver is amazing - a great piece of work! Thanks again.

Monday 10-Dec-2012

... by: Pat OMahony, Dallas, Texas, USA

Sir, I have your book and follow this web site.
I just want to say (again) Thank You for all your efforts.
Season's Greetings.
Patrick John OMahony

Andrew Stuart writes:
Great to hear from you again, fantastic you are enjoying Sudoku and the site. Have a wonderful Christmas!

Sunday 2-Dec-2012

... by: Malcolm, London UK

I have usually managed to solve these puzzles using the elementary strategies combined with brute force trial and error. Now having found your site it all makes much more sense. Your explanations and examples are brilliant, some of them look like genius to me! Sorry that I am not clever enough to suggest any improvements but just wanted to say thanks for an amazing site that will save me hours of frustration. Thanks.

Andrew Stuart writes:
You are very welcome, many thanks for the kind words. Best of luck solving!

Saturday 1-Dec-2012

... by: zyg, uk

Load Sudoku:

Could you please explain the strategy you use to solve the first step when there is more than one candidate for a cell in a square.
Your answer come up as 9 in row 5 column 3.
But 9 is also possible in this square in rows 4 and 5 of column 1.
I also cannot follow how you allocate 3 to the next solution.
Any help really appreciated

Andrew Stuart writes:
The 9 in E3 is a 'hidden single' which means that it is the only 9 in one direction. There are other 9s in the box so we can't use the box to fix 9. There are many 9s in row E so again 9 is not fixable with the row. But if you look in the column you will see 9 appears only once. Therefore we must put 9 in that sole remaining spot. Same happens with the 3 - last remaining cell in the column.

Have a look on this page.

Friday 30-Nov-2012

... by: Raymond J. Filsell, UK

Great programme. I'm an oldie and would like to know if I can delete a number from the squares with possible solutions in , e.g. 1357 in a square as possible solutions to that square. Can I delete the 3 or 5 say. Thank you

Andrew Stuart writes:
Yes, do you see the line above the big board which starts "candidates can be [] edited or [] highlighted" ? Tick "Edited" and then click on cells. You will be able to add or delete candidates. Press enter to get out of the cell and "Take Step" to continue.

Tuesday 27-Nov-2012

... by: d, United States

For this post I'm using the terminology of the 1st edition of your book:
Thinking about MULTI-COLOURING-of -Simple- chains :
Is it possible to have MULTI-COLOURED- ALTERNAING- INFERENCE- CHAINS or does that simply reduce to 3D Medusa ?

Andrew Stuart writes:
Multi-colouring is strictly about how to get eliminations between two separate groups of chain sets. By separate I mean there must be no pairs connecting them at any point. I haven't worked out the implications of using multi-coloring that combined two 3D-medusa nets - it would be horribly complicated and you'd have to prove separate-ness for all numbers. It's theoretically possible and an interesting idea.

Wednesday 21-Nov-2012

... by: Dale Kloss, United States

Just went through Nishio Forcing Chains and understand them.
So you covered cases of:

Canidate ON/OFF (Digit Forcing Chains)
Canidate ON (Nishio Forcing Chains)

So what technique is (if any)
Canidate OFF

or am I missing a technique ?

Andrew Stuart writes:
I always think I've exhausted the possibilities only for someone to point out a new variant or leap of logic that makes something else possible, so I hesitate to say I don't think you can conclude anything from 'candidate OFF' alone. But here is why I think it's not necessary to consider it. OFF will only have a knock on effect if it is one side of a pair. OFF will force ON in bi-value and bi-location cases, eg when there are two candidates left in one cell. If there are 3 or more OFF doesn't say which of the others is ON. But if it is bi-value then why not start with the ON candidate - starting with the other OFF just adds one link to the chain and you're back on the previously mastered Digit/Nishio Forcing Chains.

Wednesday 14-Nov-2012

... by: maskerj, California

Great Solver. It would be great if you could go back by more than one move. And if you can save more than 1 game.

Andrew Stuart writes:
I'd like to extend those features as well but there is a limit on what I can store in the browser and using javascript. The apps we are building have much more capacity in these senses including full undo/redo. I'm also creating a membership/account system. Storing personal puzzle definitions would be a nice feature of that so I'll add that to the wish list.

Sunday 21-Oct-2012

... by: Hache, France

Load Sudoku:

Hi there,

Using the grader, the puzzle is rated moderate 79 but it contains an X-cycle which I have not yet got my head around. How does it rate at only moderate?



Andrew Stuart writes:
Some puzzles are relatively easy but also possess one bottleneck that requires a hard strategy to get through. My generator marks these sort of puzzles as undesirable to publish and I filter them out. The scoring is based on a) the number of rounds which is a measure of the number of opportunities to solve anything at any one time and b) the scores of each strategy used (plus some heuristic modifiers). So the grade not about which one hard strategy is used - in this case I think the X-Wing bumped it over from easy to moderate - but the overall accumulation of score parts.
More here if you are interested:
Sudoku Creation and Grading
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Thank-you everyone for all your questions and contributions.