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 31-Dec-2009

... by: Ben, USA

Load Sudoku:

I was wondering if there was any way to purchase this solver for when I am not near the internet...I live in the wilderness for parts of the year and only have my laptop without internet. Is there a way to purchase it?



Andrew Stuart writes:
The sudoku solver may be saved to your computer to use when you are not connected to the internet. However, without internet access, it will only perform the basic strategies 1 through 7, because the more difficult strategies execute on a server accessed via the internet.

To save the solver, in Internet Explorer, click File on the Menu Bar. (If the Menu Bar is not displayed, press the Alt key.) Click Save as. Change the type to Webpage complete and navigate to the folder where you want to save the solver. Click the Save button. The default name for the solver is Sudoku Solver by Andrew Stuart.

To use the solver while not connected to the internet, launch the solver from the saved location. Enter your puzzle the normal way, then press the "Take Step" button. If you get a No on step 7: Box/Line Reduction, do not press "Take Step" again. Save the puzzle by pressing the Save button. You must now solve at least one of the squares on your own, using advanced strategies, or just enter one of the candidates.

If you just enter a candidate, make a note of the number you chose. If you pick a wrong candidate and the solver later tells you that you have a error on the board, click Re-Load and try the other candidate.

If you just choose a candidate, to speed things up, try to choose a square with two candidates. Then, if you enter the wrong candidate, you have only one other candidate to try later.

More difficult puzzles may require entering candidates in two or three squares. When you get a second No to step 7, enter a candidate in another square and Take Step again.

Friday 18-Dec-2009

... by: AlanM, Australia

Just started using the solver. Great product, however I found the terminology a little confusing. You refer to 'Square', 'Cell' and 'Box'. If I understand correctly, 'Box' is 9 cells (3 x 3) and 'Square' is the same as 'Box'.
If so, why use both 'Square' and 'Box'.

Andrew Stuart writes:
I shouldn't be using 'square' anywhere - it’s probably down to a drift in the text as it was developed over several years. I'll keep an eye out and change those

Thanks for the alert

Tuesday 15-Dec-2009

... by: John Meyer (@dotnetzebra), Florida, USA

1. the check boxes for enabling & disabling the diabolical & extreme strategies aren't connected to the right check boxes (noticed in the killer solver, didn't check the others)

Andrew Stuart writes:
Thanks for the alert, fixed that. Appreciated

Tuesday 15-Dec-2009

... by: Joseph, US

There is something I am curious about that I really hope you can answer, although it's quite subjective and I suppose the answer will be a ballpark figure but I was hoping a sudoku expert such as yourself could take your best educated guess at.

If I know all of your basic, tough, and diabolical strategies, but don't go as far as any of your evil strategies that you list, what percentage of sudoku puzzles (in your opinion) do you think I could solve-80% of all puzzles that I would try? 85%? 90%? 95%? 99%?

What would you guess if you had to estimate? I know it's hard since there are literally trillions of puzzles, but easy, medium, tough, and many diabolical puzzles I can already solve with these current strategies, excluding your evil ones. Do you think the percentage of puzzles where you HAVE to use one or more evil strategies in order to solve the puzzle is a small percentage, perhaps 1%? 2%? 5%? 10%?

Just curious what your opinion is.

Fantastic sudoku site! I love it!!!

Andrew Stuart writes:
This is an interesting question and to explore it fully I've created a new article. Have a look at this:

The Relative Incidence of Sudoku Strategies

Friday 13-Nov-2009

... by: Dufflepod, UK

Solcing Sudoku in SQL server!

Thought the more I.T. savvy of you would find this interesting.

Andrew Stuart writes:
That's an awesome use of some difficult tech. My backend to this site is SQL server so I know stored procedures are not the easiest things to create. Hat's off to you :)

Wednesday 4-Nov-2009

... by: Jack, Oz

Google Chrome (as browser) and your solver doesn't work as neatly as Internet Explorer Browser and your solver. Steps in solution are occasionally out of whack, and solver comments under the puzzle are not deleted after the next step is taken. Could well be that Chrome is the problem, but not sure. Also getting some blue screen failures, but again possibly related to Chrome.

Andrew Stuart writes:
Reason I don’t use Chrome when using my solver is that the "Enter" key does not work on the "Take Step" button. In other browsers once you click on it its the default button for "Enter". So I have to click and it takes longer. However, I can't find any other your other problems. Results pane clears when new data is loaded. Steps are in sequence. Just installed it, version

Saturday 17-Oct-2009

... by: Bev, California

I've been doing Sudoku for about 2 years, and I love your solver. It has saved me a lot of frustration and taught me a lot. However, after more than a year of occasional attempts, I'm still stuck on Simple Coloring. When I put my puzzle into your solver, there's almost always more than one chain. I have a lot of trouble figuring out the path that eliminated each candidate. I'd love it if you displayed those eliminations one at a time; maybe then I could see it.

Andrew Stuart writes:
All chains, infact all strategies that have a tick beside them, do only report the first successful occurrence of that strategy. There shouldn't be several at once. I agree that what would be nice is to show with lines the connections between the cells. Currently I only highlight the cells being used. This is a bit of a technical challenge though, given the medium.

If by more than one chain - you mean lots of dead ends and branches, yes that is the nature of the beast.

Wednesday 14-Oct-2009

... by: Jerry, Virginia, USA

Thanks for the terrific solver. I have learned a great deal from it. There are a few techniques that I use that are not on your list. They can make puzzles that would require "evil" techniques play more like "diabolical". One is interaction between y-wing and other basic techniques. Besides the immediate implication of eliminating candidates in cells that see the ends of the y-wing, extended inferences from the two exclusive hypotheses can lead to elimination of candidates elsewhere in the puzzle. Less common, the same can be done with XYZ wing and its three exclusive hypotheses.

Another is non-unique configurations that involve more than four cells. The largest I have found to date is a ten-cell pattern in which candidates can be eliminated because otherwise the puzzle would have two solutions. These may be familiar to you. If you are interested, I would be glad to send examples.

Andrew Stuart writes:
I was aware there were interesting combinations and if you have examples I'd be interested to code them and add them to the solver. If they are original I'd certainly like to add a credit to yourself or a pointer to the source. I know it often the case these ideas just bubble up from the community in general. From an example I can generalise an algorithm. Usually with a new approach I'd test it against a huge library and see how it contributes to reducing the solve path (and therefore the difficulty of the puzzle). Be happy to share the results with you.

Sunday 11-Oct-2009

... by: martin marko, slovakia

Load Sudoku:

Your site has been a great help so far when I got stuck with any puzzle. Today I have found one that I was able to solve on my own relatively easily. But when inputed in your solver, I get an error. I double-checked starting grid several times, went through the puzzle using your solver step-by-step, but always end up getting an error. The key cell that I have different from the sovler is C3 - the solver reduces possibles to "6" at a point whereas I have "3" in the correct solution. I can not really follow why does the solver do that. I thought it worth sending to you for analysis.

Keep up the great site!


Andrew Stuart writes:
I don't get your error. I loaded the Sudoku and pressed "take step" until it finished. No hard strategies required. Solution count = 1 solution. I don't know how you generated the problem.

Friday 9-Oct-2009

... by: Craig, USA

Load Sudoku:

| 29 7 6 | 59 25 1 | 3 48 48 |
| 13 8 4 | 36 7 369 | 19 2 5 |
| 123 35 2359 | 23 4 8 | 7 6 19 |
| 5 13 38 | 7 6 2 | 89 14 49 |
| 6 19 7 | 59 8 4 | 2 15 3 |
| 4 2 89 | 1 3 59 | 6 58 7 |
| 28 6 35 | 4 9 35 | 18 7 128 |
| 2378 4 23 | 236 1 367 | 5 9 268 |
| 279 59 1 | 8 25 567 | 4 3 26 |

It doesn't find the x-wing in columns 3 & 4 on 2's?

Andrew Stuart writes:
I stepped through to the position below. I don't see how you eliminated the 2 in G3 to make the x-wing. My next step would have removed

SINGLES CHAIN (Type 1): Removing 2 from H1

Making it beyond your board above
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Thank-you everyone for all your questions and contributions.