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 9-Dec-2010

... by: Hans Berkenkamp, Germany

if you open your MODERATE EXAMPLE and click the button TAKE STEP you come to the following explanation in the documnetation field:

INNIE - BLOCKS of 2 boxes... Box 2 = 1 at C4 (many inside:1 outside:2, inside value:89 outside:6)
INNIE - BLOCKS of 2 boxes... Box 7 = 6 at G6 (many inside:1 outside:2, inside value:84 outside:17)

excusing my bad english and low information about Killer Sudoku I like to understand your explannations in the documnetation field.:
I do not understand how you get to the INSIDE- and OUTSIDE VALUES.
Therefore it is unclear for me why C4 = 1 and G6 = 6.

Thanks for your reply in advance

Hans Berkenkamp

Andrew Stuart writes:

Have you seen this page?

Andrew Stuart

Monday 6-Dec-2010

... by: Grok, CA

I love the SudokuX solver but I would love even more if I could make this part work:

"Pressing 'Enter' on the keyboard after clicking on Take Step is a quick way to step through all strategies."

When I press enter after clicking on Take Step nothing happens. Is there some secret to this? (I'm using an iMac.)

Andrew Stuart writes:

If it doesn't work on a Mac then its because its a Windows specific key binding or default behaviour for that control. "The wonderful thing about standards is there are so many to choose from"

Thursday 4-Nov-2010

... by: Dave Allen, Kent, England

A Proposal. Many of us make mistakes in attempting to solve Sudokus, even with the guidance of your program. One then has the problem that it is very difficult to backtrack to the move that was wrong. (This was mentioned some time back in the Daily Telegraph correspondence column!) May I make the following suggestion.
With the impressive existing programs in your solver, could you add a backtracking feature, roughly as follows. Entries made in the recording grid would be recorded independently, in sequence. At any stage, this sequence of entries could be undone, one at a time, with consequent reconstruction of the possible candidates in the working grid.
One could then backtrack the solving process to a point where one or both of an illegitimate pair of entries was made, and recover from there.

Andrew Stuart writes:

Valid point. I'd like to introduce a full memory and backtracking. Its not super easy in javascript but I'll give it a go in the next release.

Thursday 4-Nov-2010

... by: Bill Jefferson, Sydney Nova Scotia Canada

Hello Andrew,
I am very impressed with your site.
Could you please tell me what the initals BH25 stand for?
I am trying to understand the X-Wing procedure.
Thank you,
Bill Jefferson

X-WING (Col->Row) 4 taken off G2, based on BH25
X-WING (Col->Row) 4 taken off G5, based on BH25

Andrew Stuart writes:

This is short hand for a rectangle of four cells in four corners. Rows 2 and 5 and columns B and H. You might also see BCH234 (eg) occasionally for SwordFishes which is a short and neat way of identifying a pattern of 9 cells in a 3 x 3 formation.

Thursday 30-Sep-2010

... by: John, Canada

What is Tabbing and how do I tab?

Andrew Stuart writes:

the [TAB] key on your keyboard. Use on the small board to get from one cell to the next while entering numbers.

Sunday 26-Sep-2010

... by: Kim, USA

What is the highest possible grade with your grader for soduku? 500?

Andrew Stuart writes:

I'd have to regrade quite a few puzzle from over the years to be definitive, but in my 2010 stock I see an extreme with a grade of 2682. My small stock of 'unsolvables' have no grade but are more difficult (as I don’t have a logical strategy etc)

Monday 13-Sep-2010

... by: Patrick John OMahony (pomahony2), Texas

I just wanted to say "Thank You" for having this site up and working.
It has helped me understand "Sudoku", how sudoku works and how I should work sudoku.
You should get some kind of "public service" award.
Again, ... Thank You very much
Pat OMahony, 112 University Village Drive, Richardson, TX. 75081

Andrew Stuart writes:

Thanks Patrick :)

Wednesday 4-Aug-2010

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

Any idea when you are going to put back the Uniqueness checks into the Jigsaw Solver?

Andrew Stuart writes:

Interesting question. I never included them. Basically, the 'deadly pattern' requires four cells in exactly two rows, two columns and two boxes. Boxes in Jigsaws are all over the place, so the opportunity for this formation is greatly reduced. I thought I could get away with not including them in the Jigsaw solver because it's less popular than the others and the impact would be minor. But you have been perceptive in asking for them. I'll add it to the job queue.

Sunday 1-Aug-2010

... by: Will Gibson, Canada

Your grader says:

Note: a number of heuristics, weightings and statistical methods are used to determine if a puzzle is fit for publication and the grade alone does not determine this.

I was wondering if you could elaborate on this.

Andrew Stuart writes:

Much of this is commercially sensitive, but as an example, a sudoku which is largely trivial except for a single bottleneck which requires one very tough strategy is 'marked' as being a bad puzzle. I try to filter these out before they go into a client stock list. Weightings are to do with the scores I assign to strategies to differentiate easy from harder strategies, etc.

Tuesday 27-Jul-2010

... by: Joe Defries, Abbotsford, BC, Canada

Andrew - kudoes once again on your superb Sudoku Solver (and on your book, 'The Logic of Sudoku')! It's been quite a while since I've emailed you (I was the one who had asked if you could attach a difficulty ranking to your puzzles - which you did - thanks again!). I wonder if you can help me here: I often use your Sudoku Solve to step through puzzles... can you also add a difficulty ranking for puzzles that users manually enter @ ?
Or am I missing out where to find the ranking for user-entered puzzles there?
Joe Defries

Andrew Stuart writes:

The "grader" button will act on anything that is in the board, whether you have loaded it from the example list, the entry board, the import button or the parameter url line. I believe those last three are all 'user-entered' options
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Thank-you everyone for all your questions and contributions.