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!

Monday 21-May-2012

... by: jandscavill, UK

Great site Andrew. Just want to say thank you. I am learning so much about Soduku by taking your 'fiendish-sudoku' challenges as far as I can get and then using the solver to find the next step when I get stuck. Another happy fan ...

Andrew Stuart writes:


Friday 18-May-2012

... by: 49erfan, U.S.

Just a quick note re: your web site. I think your web site is absolutely fantastic! I have used other sites to try to better understand how to solve Sudoku problems. None of the reference sites come close to yours. Not only has it helped me solve problems I coluldn't solve myself, I started understanding how to solve other ones much quicker on my own. Thank you for your effort in putting this site together for all of us who use it. And for free!!!

Andrew Stuart writes:


Friday 18-May-2012

... by: Derek, Calgary - Canada


First off..Love your site! Best place online to try some truly difficult sudokus.

I was curious if you had given any consiedration to expanding the uniqueness strategy solvers to include extended gordonian rectangles, like the Franciscan Rectangle. For instance, in today's Daily Sudoju (05/18), after the initial simple solving methods, you get a board that looks like this:

| 5 2 149 | 6 3 149 | 7 148 48 |
| 6 17 147 | 147 8 2 | 35 35 9 |
| 89 1789 3 | 1479 147 5 | 6 14 2 |
| 3 6 58 | 1459 14 149 | 18 2 7 |
| 1 79 79 | 8 2 6 | 45 45 3 |
| 2 4 58 | 157 17 3 | 18 9 6 |
| 4 18 2 | 3 6 18 | 9 7 5 |
| 7 5 6 | 2 9 48 | 34 348 1 |
| 89 3 19 | 14 5 7 | 2 6 48 |

At B7,B8,E7,E8,H7,H8 you can see an extended gordonian rectangle. We know that H8 cannot be a 3 or 4 since that would give us multiple solutions, therefore H8 is an 8. After that, the puzzle solves itself.

The solver, however, does not pick that up. Just curious about adding that stratgey because the pattern, at least IMO, it seems to be one that is fairly easy to spot.

Again, thanks for the awesome site!

Andrew Stuart writes:

Since I read about the methods used in the 17 clue proof I've been meaning to extend URs. (Gordonian is just another name for Unique Rectangle, no? I hadn't heard the term before). Well I'm implemented Type 1 which is your example. I've tested it on my 50k library and its pretty rare - just 62 occurences - even promoted above 3D Medusa, ie just after XY-Chains. But it is a very easy to spot strategy so I'll definitely expand the docs to talk about this one. I'd like to get through all the other UR variants first and see if it deserves its own spot in the list.

Tuesday 15-May-2012

... by: steve, sc

just wanted to let you know how much i appreciate your website. i enjoy the challenge of sudoku puzzles and you have helphed me get better at solving them. thank you for sharing and explaining your strategies. now if i could just understand a few more of them i would be a happy camper. never give up. thanks.

Andrew Stuart writes:

Thanks Steve!

Monday 14-May-2012

... by: Ray Granberry, USA

Enjoy your work very much! I have a new iPAD with latest updates and find it strange that the killer loads fine but the daily sudoku,puzzle only loads with funny red image blobs in each square where a puzzle predetermined answer should be. If I click on the iPad player it is not the same daily puzzle.

Andrew Stuart writes:

Hi Ray

The web page version hasn't been designed for the iPad at all and predates the iPad stuff we've been doing. The Killer is more recent and I've tried to be iPad compatible there.

But I am surprised the iPad puzzle is different - I'm sure there was a reason many moons ago, but I've switched it to the daily puzzle now.

By the way, we just waiting for Apple to approve a 5 puzzle player for the iPad. I think you might like it. There' be announcements on the site should you want to take a look

Sunday 13-May-2012

... by: Manfred Lösch, Germany

error in sudoku solver, version 1.84:
after NEW, the old simple colouring lines not cleard

Andrew Stuart writes:

Fixed. Thanks for the alert.

Saturday 12-May-2012

... by: tim, usa

Version 1,83
1. click "NEW"
2.input a puzzle on small board. Color of numbers on large board is blue.
3. click "SAVE". message => "board saved as cookie" click "OK"
4. click "Reload" => it use to change the color of the numbers to Red on the big board - now it DOESN"T -they remain blue. Also new numbers appear as if the "Take Step" had been clicked.

Andrew Stuart writes:

Fixed the colours. The problem is knowing when a number entered is a clue or a solved cell. I'm assuming if you are entering it in the small board its a clue - and if you refresh the page - bext time you "save/reload" it should be a red number.

Your last point - uncheck the new "Auto Clear" box if you dont want this behavior. It should save that setting.

Friday 11-May-2012

... by: Eric G., Canada

Hi Andrew,

I will buy it this week-end. One thing though ? It would seem to me that having a puzzle generator would be a great add-on to your solver. I like to do puzzles and then use your solver to see what I missed, how I could have done better, etc... Your puzzles are some of the best available. Why not offer that option . It would be a great combo.

Andrew Stuart writes:

Like the idea but it's not as simple as it sounds. Making a puzzle on the fly would be relatively straight forward and probably only take a few seconds, but making one to a specific grade is more problematic. Most people will want quite hard ones and they in the top 20% for diabolicals and < 1% for extremes, so could mean a lot of work on the user interface to display progress. However, there is an intermediary step we want to do - and that is to pull in from a feed the four daily puzzles from the site, one of each grade. We'd also like to make a player board as an alternative to the solver board and some kind of toggle between them.

If you do decide to buy now you'll get these upgrades for free when they happen

Friday 11-May-2012

... by: KIKS, Ohio

When I try to clear board all the numbers remain in all the blocks,

Andrew Stuart writes:

Does it say version 1.82? Refresh the page a few times if not

Tuesday 8-May-2012

... by: mike, uk

This is the first Sudoku solver I've come across that is a serious help to the mentally challenged Sudoku fanatic! Well done!

Andrew Stuart writes:

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Thank-you everyone for all your questions and contributions.