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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!

Tuesday 10-Aug-2021

... by: brent hudson, United Kingdom

Hi Andrew,

Firstly, what a brilliant programme! I have used it a lot over the last 2 years as I've tried to improve my own solving skills. It has helped me improve a lot.

However, on this board I have been able to solve a puzzle that the solver cannot, and I would expect it to be able to. I am wondering why it cannot.

Here's the board:

There is a Killer Sudoku board I would like you to look at

Click on this link:
https://www.sudokuwiki.org/killersudoku.htm?bd=121112211121133441112213122132112111332312322111311321221213311133221121114411121,300622000012002100000000001600040000000025003200191400000600000013000000000000130000151200210000000000000022130000210000000000180500000027001000000008000000000000

When it gets stuck, cells G6, H6, J5, and J6 sum to 13 (by summing columns 7, 8, and 9 and looking at the spill over). 13 in 4 cells in the same block has to contain 1. This means that J4 has to be 2 (the solver gets stuck with J4 being only 1 or 2).

If I set J4 to 2, the solver then solves the puzzle.

Why doesn't the solver see that J4 has to be 2?

I ask for two reasons:

1. The solver seems to solve much harder situations than this one.
2. If you do decide to update the solver again, maybe there's a piece of logic you could insert?

Again - thanks for doing the solver; it's a great tool!

Brent


Andrew Stuart writes:

Yes the Killer solver does lag behind the Sudoku in terms of solving power. Mainly because there are many more candidates and the chaining strategies can't bite. However I like your example below, gives me a clue to improve. Just have to figure out where exactly.

Sunday 8-Aug-2021

... by: Kelly, Belgium

Hi Andrew,

I have a really difficult (Double) Jigsaw Sudoku I want help solving with. The grid of the jigsaw is not one of your grids. How can i make my own grid and use your solver?

Thank you so much in advance!

Kind regards,

Kelly

Andrew Stuart writes:

Yeah I need to create a method for doing that!

Monday 5-Jul-2021

... by: Jurgen, Belgium

Hi

I'm trying to enter a puzzle in the small grid on the left but when I type 8 it gets replaced with a 9 ....

Andrew Stuart writes:

Can you [CRTL]+[F5] to refresh and tell me if the problem has gone away.
I was trying something to get ipad keyboards to map letters to numbers and I can't work out why it's effecting normal keyboard usage (I can't duplicate) so I've reversed the changes
Tell me if it's back to normal.

Monday 14-Jun-2021

... by: Steven, USA

There's a bug with Simple Colouring for SudokuX

In a simple colouring step, all of the 4s on diagonal 1 are taken away and there's no blue colored 4s, only yellow

Andrew Stuart writes:

Sorry for the very late reply, catching up

LOAD PUZZLE

Assuming it is Sudoku X my offline solver says this puzzle has no solutions
I ran through it until it found a contradiction and could not see the simple colouring step you describe although a few simple colourings were used.
I found a javascript bug when the 3D Medusa was used. That stopped Take Step button from working. Nice to fix that

Thursday 10-Jun-2021

... by: Philip Trueman, England

Naked pair elimination does not handle dog-legs in cages.

Load today's (10th June '21) Killer into the solver and turn off all optional techniques. Look at the cage that sums to 30. Mark the two cells in column 4 as a 5/9 naked pair. Hit Solve. The 5/9 elimination does not propagate to every remaining cell.

Andrew Stuart writes:

Good catch! Fixed now. Was the last cell in the dog leg.
Bug report most appreciated

Thursday 27-May-2021

... by: Christopher Eastwood, Washington State, US

I'd like to make a comment on the Exocet page, but every time I try, it says "No Article". Please help?

Andrew Stuart writes:

Doh, my bad. Since the recovery I seem not have uploaded a critical file for the comments system
Done now, have a go
Thanks!

Wednesday 14-Apr-2021

... by: David, UK

Load Sudoku:

Hi

This solve comes up with
XY-Chain
length=9, chain ends: A6 and E2
This proves 5 is the solution at one end of the chain or the other
-5[A6]+4[A6]-4[B6]+9[B6]-9[B4]+6[B4]-6[B7]+2[B7]-2[H7]+5[H7]-5[H5]+9[H5]-9[D5]+5[D5]-5[E4]+8[E4]-8[E2]+5[E2]
5 taken off A2

Can you explain how in the red route (5 in A6) that it can assert that 5 is in E4.

Sorry if I am missing something!
Fabulous site - thank you.

Andrew Stuart writes:

That's quite a magnificently long chain.
All the connecting cells have two values.
A 5 in A6 will cascade all the other cells to reach E2 forcing it to be 8.
A 5 in E2 will cascade all the other cells to reach A6 forcing it to be 4.
There are only two ways the chain can be settled, both imply a 5 in A6 or E2. Therefore no 5 in A2

Wednesday 14-Apr-2021

... by: Andreas, Germany

Good to hear that you could restore (most of) your website. I am missing the Kendoku Solver. There is a daily Kendoku in my local newspaper, but I am not able to solve it every time. So I was happy to find your website.
Found no useful alternatives on WWW.

Andrew Stuart writes:

Yes I had to take down the KenKen solvers since by program won't activate on the new server. I don't know why, the settings and permissions are identical to the Sudoku program. Really frustrating. Requires deeper knowledge of windows 10 than I have

Sunday 11-Apr-2021

... by: ATechGuy, United States

Andrew,

I was so saddened to hear your site had been hacked. As an IT MSP, we see it all the time and it's such negative energy. Your Apr 11 update made my day that you were able to recover your site. I so love your solver--it has taught me a lot about the puzzle. Sudoku is part of my life and I try to "wake up" my brain every morning by doing a few puzzles. I'm sure it also helped keep my mother's brain more active in her last years.

All the best,
-Ken

Andrew Stuart writes:

Thanks Ken!
Always appreciate a thumbs up from visitors

Saturday 10-Apr-2021

... by: Conway, England

You have a link for a pdf relating to creating and grading puzzles, unfortunately following the link says there�s not article found. Having spent 12 months of lockdown working on a sudoku engine I�m now at that stage where producing a puzzle is reasonably easy, but producing an interesting puzzle worth playing is a bit hit and miss. That PDF might be very helpful to me. No rush though, I think my sudoku bug is here to stay for a while. Interestingly I started my project with much the same view as you... there can�t be that much to this puzzle, haha, how wrong I was.

Andrew Stuart writes:

Thanks for telling me, hard to know what's missing after the big restore. I have to do it very piecemeal since there are changes (mins and visitors) all the time. The document is now back on the site.

Friday 2-Apr-2021

... by: Douglas Boffey, England

There seems to be a broken link, namely to /Gurths_Theorem

I know this is rather an esoteric technique, and not easy to explain.

Andrew Stuart writes:

Page now restored having got the pre-hack wiki back.
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