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!

Friday 24-Nov-2006

... by: Fer van Nieuwenhuizen, The Netherlands

Dear Michael,
Stepping through my Double Mirror, Ex. 1, I encountered the following:
POINTING PAIR: Between Box=2 and ROW=2: 3 taken off B1
POINTING PAIR: Between Box=2 and ROW=2: 3 taken off B2
POINTING PAIR: Between Box=2 and ROW=2: 3 taken off B3
But the diagram tells me it’s Row B.

POINTING PAIR: Between Box=6 and COL=7: 9 taken off J7
Now the ‘7’ is a column.
Rather confusing, isn’t it?

Always wandered why you use the A, B, C... notation for rows and numbers for the columns. Spreadsheets use it the other way around. Ever considered to conform to the IT-world? ;-)

Fer van Nieuwenhuizen

Andrew Stuart writes:
I can't seem to reproduce those pointing pairs in that example - not sure what has changed. Nevertheless, on the topic of coordinates I much prefer the letter/number system as its clear what is a row and a column - the only other widespread system is rXcY - both of which I provide on the solver. However, I didn't invent either - although I did insist on not using the letter 'I' for the ninth row. So perhaps there was a chance somewhere back in the mists of time when it could have been spreadsheet like - but there is nothing so say which was is correct. I think the textual decriptions that my algorithms return should say ROW=B rather than ROW=2 - that's something I should clear up.

Friday 24-Nov-2006

... by: piers phipps, uk cornwall

Your solver has been incredibly helpful in learning the advanced solutions. I like to use it to save time in filling in all the numbers. Then I print off and have a reasonable sized puzzle to work on. However there seems to be a bug in bringing up the printable version. It can make rude noises for as many as 8 tries before letting me have the separate sheet. Am I alone in having this problem?

Regards and thanks

Andrew Stuart writes:
The printable pages are popups - but if your already have a window open with the same name it will use that window. If that window is behind a bunch of stuff on your screen - you may not notice it appear. Do you think something like that is happening?

Friday 24-Nov-2006

... by: Martha, Ohio USA

I just discovered this site late last night. My reaction was WOW finally some help with jigsaws (I'm rather new to them). I muddled my way through Shape 1/Example 1 with some help with the steps. (I'm so excited that I may finally understand LOL with a few more examples as well as other strategies.)

A problem though:

This morning I started working Shape 2/Example 1. I thought I would like to work with pencil and paper before going through the steps. However ... the printable version is NOT Shape 2. Help!

Thanks so much.

Andrew Stuart writes:
Hi Martha

That was a big bug, thanks for reporting it. I've just fixed it - you should see the right shape in the printable version now. Don't forget you can show candidates or not by ticking/unticking the "Show Candidates" box - it effects the printable version as well.

At least one example from each set of 4 for each shape contains one or more required "Law of Leftover" strategies. I'll be highlighting which one soon.

Thursday 23-Nov-2006

... by: Mike Tobias, UK

Hi Andrew,

As has been said before, this is an excellent trainer, showing in detail the different solving techniques. I think I may have found a bug though.

I fed Extreme Jigsaw #1 into the solver using the small box on the left, having chosen the appropriate design. About half a dozen cells wouldn't accept my entry, and reverted to the number from the example already there, as soon as I clicked off the cell.

I then discovered the 'Import Puzzle' button and everything worked fine that way.

Is this a bug, or have i missed something obvious? I'm using Firefox on Windows XP Pro +SP1.

Regards, Mike

Andrew Stuart writes:
Yes, a bug, many thanks for reporting it. I think I've fixed it now. Rather more complicated that the sudoku one. Most appreciated.

The extreme jigsaws now have a link from the archive so you can load them without typing.

Monday 20-Nov-2006

... by: Mike de Jong, UK

What's the difference between Singles Chain Types 1 & 2? I can't find a reference to it in the strategies. I did ask this question back in September but you seem to have missed it.

Andrew Stuart writes:
Sorry for taking so long to reply. The difference is that in Type 1 we are eliminating a candidate outside the chain. Type 2 we can eliminate all of one colour - half the candidates - in the chain because of a contradiction

Saturday 18-Nov-2006

... by: Terje Lerstein, Norway

Thanks for a very nice solver. Probably the best I have found. Use it almost every day. There are Sudoku's in almost every paper these days. I usually put in the one from my local paper. I fill in the numbers. Here I miss a function that can let you move from square to square by using let's say the up/ down arrows. Would be quicker. Next step is using the solution count to check for mistakes and make sure it is a real sudoku with one solution. Here I would like some kind of statement, telling the degree of difficulty. Anyway I can see that ? Last I do step one in the solver to get all the possibilities. Not necessary to spend time filling in the open squares. Then I print it out and go to work.
Regarding the sudoku's preloaded in the solver. Do they change from time to time ? Seems to be the same at each level all the time.
Once again . Thanks for a very useful solver.

Terje Lerstein, Norway

Saturday 18-Nov-2006

... by: Karsten Nyblad, Denmark

There is a bug in the Guardians strategy such that it deletes candidates that are part of the solution. Further, the text in the result window does not match what is show on the board.

From the examples chose diabolical.
From strategies chose Guardians ans Nishio only.
Start solving the sudoku. Second time the Guardians strategy is used, it tells you:
GUARDIAN (Double), 9 taken off E9 - 2 Guardian Cells: F5 E9
GUARDIAN (Double), 9 taken off E9 - 2 Guardian Cells: F5 E9
However, 9 is taken of E5 and F9. Unfortunately, in the solution F9 is 9.

Andrew Stuart writes:
I've finally got to take a good look at this example and yes there is a bugette. In my search for guardians I allowed units with more than three candidates to be used. This I think is wrong - guardians only works with sets of two and three - the threes being the imperfect trios.

I'll be updating the solver tonight. Unfortunately guardians+nishio alone don’t solve this diabolical, but one guardian is still found. I'm testing guardians+nishio alone against Ruud's 50k set of hard ones in case I've got another bug.

Great feedback, just the sort needed to icon out the solver

Friday 17-Nov-2006

... by: rt, peru

i use tiger osx, i forget which version number. however, everytime i use your program and begin to edit my custom board, my safari crashes. this has happened about 5 times now. just so you know. im not leaving an email because i dont want to at the moment. i hope you can resolve the problem for us mac users.

Sunday 12-Nov-2006

... by: pt, North America

Congratulations on a great Sudoku site!

Solver UI suggestion: rather than always having to single-step through the solver, it would be great to have an additional option: to solve all the "first tier" strategies at once. In other words, one click would have the solver not pause until the first time it looks at strategy 8, X-Wing.

To speed up slow drawing when importing a Sudoku (using Firefox), I uncheck "show candidates" first, then click it again after importing. Perhaps your script could implement that approach to speed up redraw automatically.

Cheers and thanks again!

Andrew Stuart writes:
pt - you didn't give an email address so I can only reply to you here.

There is a problem with the drawing in Firefox, I've noticed that. I think what is happening is that in IE all the draw functions are done in memory and when they are finished the final product is displayed. This is clever and efficient. In Firefox they are being draw in sequence and then adjusted as new drawing functions are requested. What I'd like to do is spend a day seeing if I can't draw everything with visibility = false and then turn it on. But it sounds like a lot of work and I'm fighting a variant of javascript.

I see you point and I like your idea but I also have to consider whether these speed up tricks wouldn't make the user interface any more daunting than it already is. I have a tendency to assume everyone knows it as well as I do.

I'm working on an off-line version where I may well introduce some of these tricks - I'll have more room for options and preferences

Saturday 11-Nov-2006

... by: Richard Broughton, UK

Load Sudoku:


found this gem that my solver gives up on - and unfortunately so does yours (although your solver gets one further candidate reduction than mine)

Brute force will solve it - and your solution count indicates that there's only one solution.

I'd be interested to see if there is any strategy that can be applied to bring a logical conclusion to this problem

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Thank-you everyone for all your questions and contributions.