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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. Please feel free to drop me a note on the side of the page or try the Facebook comment box. 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 17-Jul-2014

... by: Kai Bachmann, Austria

Load Sudoku:

This Sudoku uses three times the XY-Chains strategy (solving steps 11, 12 and 13) with the same cells C3, C4, C5, F4 and F5, changing only the starting/ending point of the chain, and eliminates candidates 4 in cell C7, 1 in cells A5 and B5, and 2 in cell F6.
A nice example to show how a same chain, varying its starting/ending point, can exlude many different candidates.

Andrew Stuart writes (24-Jul-2014):

An excellent example, well found

Monday 7-Jul-2014

... by: Mike, USA

Was looking at X-cycles>Part 2>Nice Loops Rule 2> last paragraph. "We have two strong links joined at J1; therefore, J1 is 1. One way to make sense of this logically is to trace round the alternative. If J1 was not a 1 G3 and J8 would have to be 1s. That would remove the candidate 1 from E3 and oblige E8 to be a 1. But hang on - that forces two 1s in column 8. A contradiction so the 1 must exist in J1."

I think is should be it forces two 2s in row E. I"m not seeing the two 1's in column 8. Please explain it to me if I'm wrong. BTW love what you did here, it so cool.

Andrew Stuart writes (8-Jul-2014):

If J1 != 1 then it makes J8 = 1 (only two 1s in row J)
If J1 != 1 then G3 = 1 -> E3 != 1 -> E8 = 1 (only two 1s in row E)
So we get a 1 in both E8 and J8. Doesnít matter there is a third 1 in the column (A8), itís a contradiction that we get two positives in the column. That clinches it

Thursday 3-Jul-2014

... by: Kyle, Antarctic

I would like to report some serious problems about the SudokuX solver.Some of the advanced strategies dismiss strong links within diagonals.The sudoku X puzzle I display here proved to be a extreme one.Jsudoku created the puzzle,who could only use easy strategies like Xcycles,XYX chains and ALS.

Load puzzle

After several pointing and box line reduction,the next two steps Jsudoku taken were grouped Xcycles.The first one is:
+9C9 -9G9H9 +9G7H8 -9B2 +9B5 -9C5 +9C9...
This is a grouped Xcycle rule 1 ,9 in A5,E9 should be removed.It seems the solver failed to recognize groups in diagonal (or just dismiss diagonals)However,the solver jumped directly to NFC to remove 2 inC6.In fact,there is a AIC:
+2C6 -8C6 +8C9 -8A9 +8J1 -4J1 +4G3 -4C3 +2C3 -2C6
2 inC6should be removed in this way,with nice loop rule 3.The only reason I could come up with is that the strong link -8A9 +8J1 was dismissed.
Then,after some extreme strategies,the solver ran out.However,some easy steps can still be taken.For example:
+5H8 -5B8 +5C7 -5C1 +5A1 -5H8
5 in H8 should be removed.This is a very easy Xcycle or simple coloring.Again,the solver failed to recognize.In addition,I could hardly see any strong links in diagonals was used in the solver.I really like your solver and have used for years.The sudoku solver is quite good,but SudokuX solver definitely needs improment.

Andrew Stuart writes (9-Jul-2014):

Guilty as charged! But in my defence I get very few feedbacks on the X version of the solver and you're the first to discover the grouped cells problem on diagonals.

[edit] Finished the update. This example is now found and the puzzle completes, although its a beast.

Good detailed example, most appreciated!

Saturday 21-Jun-2014

... by: Dick, USA

Used to be the candidates were printed on the bottom of each cell. Now they are scattered throughout the cell which makes it hard went entering the correct number. Is there a way to have the candidates printed on the bottom of each cell.

Andrew Stuart writes (26-Jun-2014):

[Print 1] *should* display the candidates in the top of each cell, in a row. [Print 2] puts them centrally in a 3x3 grid. Not sure how you ever saw them in the bottom. Talking about this solver :- ?

However, if you have your browser to be zoomed in/out, especially the text, this will distort the layout. Reset zoom levels?

Tuesday 10-Jun-2014

... by: Geoff, australia

Firstly, thankyou for your wonderful site. Your instructions have allowed me to solve most of the published puzzles, but I want to practice the tough strategies, 7 to 11. How can I find Sudoku puzzles that have these strategies in them, or is there somewhere I can generate them? Thank you again.

Andrew Stuart writes (26-Jun-2014):

Glad you like the site. Check the daily puzzles
and pick the diabolical ones. The archive calendar has at least eight. Also, the puzzle packs
you want the diabolical and extremes

Wednesday 14-May-2014

... by: nono, france

Hi Andrew,
well, the future e-book !
thanks for the new compilation .
for this year ?

Andrew Stuart writes (15-May-2014):

Hopefully yes!

Thursday 8-May-2014

... by: Bruce K, Portland, OR

Here's another interesting one. This is one that is defeating me, so I tried feeding t into your solver.

Two problems: First, it seems to cause a failure to solve the four-color problem while entering it. Then the solver runs out of strategies,
Not sure it's a bad puzzle or a solver limitation.

Andrew Stuart writes (15-May-2014):

Well it had to happen, a pattern that my crude color mapping algorithm cant cope with. I've added a 6th color and it loads correctly now.

Try this link

My Killer solver is lagging behind the normal Sudoku solver. There is a significant subset (~10%) of killers which is draws a blank on. Still a lot of new ideas that have to be dug up

Friday 2-May-2014

... by: gunter, germany


used your Sudoku for years, until it disappeared and I'm glad that you are back.

My Problem: fill in numbers in small board (curser does not jump to next field), after filling in all numbers -> "saved as Cookie", but nothing is happening in the big board, regardless which button you press. It's dead.

May be there is an answer.

GŁnther Reese

Andrew Stuart writes (2-May-2014):

Hi GŁnther
Site is up and fine as far as I can see. Looks like you may have inadvertently turned off javascript? Want to have a look in your browser settings. Always a good tip to try a site in another browser to see if itís a local problem

Friday 2-May-2014

... by: qoheleth, Viorginia, USA

Andrew: I have been using the Solver every time I found I' ve made a mistake. I don't use it to solve everyday puzzles. The work you have done is outstanding and greatly appreciated.
I have been wondering if there is some way to print out the many strategies as one file out rather than page by page.

Andrew Stuart writes (2-May-2014):

Many thanks for the kind words, appreciated. Sorry, the documentation isnít set up for a global dump/print. I am working on an e-book which will contain the same content (but different examples) and some expanded general Sudoku articles. Will be more on the site when I get nearer the end

Tuesday 29-Apr-2014

... by: PeteTy, Ohio,USA

Just looked at recent updates "killer cage combinations"
The big list is more readable with the background shading for the cage groups
It may be helpful to add LOCKED candidates to the big list
or perhaps a short list of cages with locked canditates

8(3) must have a 1
22(3) must have a 9
12(4) must have 1 and 2
13(4) must have a 1

This helps a great deal on the harder puzzles and is not a very obvious point

Andrew Stuart writes (2-May-2014):

I think thatís would be a nice touch to the data on that page. I'm looking at my generator for that list, seeing what I can do to bring that out
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