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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!

Saturday 12-Nov-2016

... by: Roland, RŁschlikon, Switzerland

Load Sudoku:

Hi Andrew!
Daily Sudoku of 9. November 2016, tough:
in this tough Sudoku I tried to find out the clue 49/81 by the xy-chain method. I was not shure if I did it correct. So I watched what the solver does.
It starts a xy-chain at B2 with 2 and 9 in this cell and it ends at G3 with 2 and 9 in this cell too. the solver then markes in A3 and H2 the 2's yellow. Both see the 2's in B2 and G3, once on once off. So the 2's in A3 and H2 are eliminated. Okay, that's what I understand.
But look at the 9 in H2: it can also see both 9's in B2 and G3. So it has to be eliminated too. > We have no more number in cell H2! Impossible.

What is wrong?

I tried to start with this xy-chain at A3 with 1 off / 2 on.
That brought me to impossible eliminations too.
Maybe you can check this and tell me what is the reason?
Thank you very much and best regards,

Andrew Stuart writes:

Say the XY-chain starts with -9[B2] (9 off). The 9 is on in B6 which makes 3 off in B6. Now 3 off in B6 does not imply 3 on in G6 because there is an intervening 3 in E6. So the chain stops. No XY-Chain goes the same way as -2[B2]

You sorta see this with the new "chained" mode for candidates. Select that and then click on 2 in B2. All chains from that 2 will be shown. The 2 in H2 is never part of a chain. The 9 consistently is with most other 9s so that 2 is already suspicious.

Thursday 10-Nov-2016

... by: Anton, Canada

Hi Andrew,

Exocet strategy -- well done! If you remember, we exchanged emails in regards to exocets which started the ball rolling.

If you sent me an email after October 28th, I didn't receive it. As of late October, terminated its email service. If you go to website, you'll see a notice posted to all its members. (I should have known better. With a funny name like that, something bad was going to happen. Lost all my contacts and emails.)

Is the exocet strategy in beta format? The reason I'm asking is because Puzzle #207 is not an exocet but a double exocet. Your solver doesn't recognize certain kinds of DEs. The solver comes to the point where JC Van Hay has it and your solver says exocet. It's a "collapsed" DE. The first and the second X-Cycles collapses the DE to its bones (1,7,9). Now, if you uncheck Diabolical Strategies and 23 through to 37 -- so you have Basic, Tough Strategies and 22: Exocet checked, and start over, you have an "intact" DE (1,6,7,9). Again, your solver says exocet and not DE. See cenoman's posting which is an "intact" DE. The DE is in its basic form. Same with Puzzle #194. The first and second X-Cycles collapses the DE. Both #207 and #194 are solvable.

As I've stated in my last email, October 4th, about all your DE puzzles: "Many will be solved. All the double exocets for sureĒ.

Overall, well done Andrew. I'll soon be sending you an email (new address). I'm in transition.

Andrew Stuart writes:

Yes I wrote to you at the end of October to say thx again and explain what I'd done so far. Sorry to hear about your email, do contact me directly when you're back up again. Exocet is only partially implemented. I'm still working my way through all the elimination rules, variations and extensions. You should be able to compare what's in the solver with the totality of the compendium and your own understanding when I get the docs finished. I will stick to explaining what is in the solver and refer to an outside source when it's not. But certainly pleased with the effect so far

Wednesday 9-Nov-2016

... by: ray, Australia

The colour used to highlight the candidate 4 is too light. Could it
be made darker?


Andrew Stuart writes:

Happy to oblige, was a bit faint. Now half way between the orginal and the green used for the strong links.
Might need to refresh [CRTL]+[F5] to see

Saturday 29-Oct-2016

... by: oscarj, USA

Why did you change the way numbers were hilighted?

I enjoyed the usage that would let me highlight numbers in the large grid that would show all 1's or all 8's. Now when I click on a number all I get is that number in a small square?


Andrew Stuart writes:

The comments I've had over the long term mentioned that the granularity of the highlighting was too crude. You couldnít easily pick a single number unless you hunted around for a given. When you say " all 1's or all 8's" it actually did all "1's AND all 8's" if you hit a 1/8 cell. But you can still choose individually.
I also wanted it to toggle on/off which the new highlighter allows

Saturday 29-Oct-2016

... by: remmelt, Netherlands

Load Sudoku:

hi, if i solve sudoku the programs hangs in step 4: naked quads. i had it a few times. one example puzzle is below2

This is a text version of the board useful for posting on forums:
| 2469 2469 5 | 379 369 23679 | 236 8 1 |
| 7 1 26 | 4 36 8 | 9 5 236 |
| 3 269 8 | 5 1 269 | 4 7 26 |
| 69 8 369 | 1 7 39 | 5 2 4 |
| 149 49 379 | 6 2 5 | 17 39 8 |
| 12 5 27 | 39 8 4 | 17 6 39 |
| 269 269 1 | 8 5 369 | 236 4 7 |
| 8 7 4 | 2 369 1 | 36 39 5 |
| 5 3 269 | 79 4 679 | 8 1 269 |

Andrew Stuart writes:

Does the version number under the board say 2.05? Try pressing [CTRL]+[F5] to request all the files again. Sounds like you may not have the updated scripts

Thursday 27-Oct-2016

... by: AV, Canada

Thanks for a fantastic resource!
I'm using Safari on a Mac. As of today, (Oct. 27/16) I can no longer return to pure edit mode after using the highlighter; the 'possibles' remain coloured. The visual impact is hugely distracting, especially if 1 to 9 have been explored in highlighter mode. How do I revert back to a colour-free edit board?

As for the highlighter itself: once a cell is 'coloured', it stays. By the time I have explored from 1 - 9, all coloured cells are highlighted simultaneously (nothing can be removed).

I have shut the browser, cleared cache and done a reboot. Not happening. I've been so happy with the solver until today ... Please advise.

Andrew Stuart writes:

yes my update had an issue with the edit mode which I fixed this morning. I'm hoping you can see that the default is "edit" not highlight. Browsers are keeping people choices persistently now so it might not be obvious. When you highlight a color does it not revert back to background color when you click the same number again? Thatís one of the new ideas.

That edit mode does not clear the highlights is intended. I might add a clear button. You can clear by selecting "chained". I think a clear button might

Thursday 27-Oct-2016

... by: queensvelvet, australia

Just did the required update and having pressed 'take a step' the new set up locks you into a loop where it won't let you do anything to solve it yourself??? I do love this site and it provides me with hours of fun but I would like to try to solve it myself. Anything you can do?

Andrew Stuart writes:

Figured it out. Bug in update 2.05 in the radio button selection for edit/highlight/chaining.
You should be in edit mode by default now.
The correct cursor should display over the editing box in the cell
The finger is correct for clicking on a number to highlight/chain
Appreciate the alert!

Saturday 15-Oct-2016

... by: STSahasrabudhe, India

Load Sudoku:

All strategies (esp. the advance ones like Swordfish ) for eliminating (and thereby fixing value in cells) presuppose that candidates in all/specific cells are pencilled in.
Are there any 'rules of thumb' for zeroing on a candidate value or two, in r/o which a pattern, a chain or a strategy which will help solve Sudoku, can be looked for?
The reason for my query is that, when stuck in solving a Sudoku printed on paper, I find the job of adding candidates in each unresolved cell rather tedious and feel that there could be a shortcut.i

Andrew Stuart writes:

Many people use my solver to get to the first step which fills in all the candidates and then removes the ones from the clues. Then the print it. It is easier to solve if you are removing candidates as you go a long, rather than re-creating the spread at every step. It is easier because no candidate will ever be added in the solving process, always removal. I'd try that.

As to the advanced strategies, well, Sudoku is very deep and most of these were once considered so theoretical that only computers could use them. But they are still the easiest logical step if you donít want to guess. Most newspaper puzzles do not require them but on my site I am interested in the hard ones. Even now there are some puzzles which cant be solved logically.

Monday 15-Aug-2016

... by: Geomama, UK

Firstly, thank you for an outstanding teaching and learning tool for Sudoku.
I installed the Android app but had to uninstall it because the font is too tiny to see and cannot be adjusted. I did see some other comments to this effect but decided to give it a go anyway based on the photos. Alas, the numbers are nowhere near as big as the photos imply. Please can you make the font size adjustable or just make the numbers fill the boxes. I have low vision and could not even see the numbers but other users are having the same problem! The ability to make the numbers bold and high contrast would be very helpful - or just do this by default to improve everyone's experience!! Thanks for reading.

Andrew Stuart writes:

I've been aware that for some larger devices and screens the fonts donít seem right with the choices I've made. Thatís partly because there are so many devices and resolutions (unlike Apple devices) and because I only have access to a few actual android devices. Plus the emulators are very hard to make work in a development environment. But I had an idea reading your feedback - a font size slider could be the solution. I've added one in the Preferences screen. You have back out and view a solver puzzle board to see the effect but some back and forth will get a comfortable size I think. But I'd like to hear about your experience. Give it a day or so to appear as an update on the Play Store and try again

Sunday 24-Jul-2016

... by: Andrew, England

How are you doing? Thanks for the site.
I'm looking at your solution to a Kenken that I created. The step that I can't understand is as follows:
Killer Innies/Outies
DOG LEG (3): Removing 1/3 from C3
DOG LEG (3): Removing 1/3 from E2

How do you apply the indies/outliers to Kenken with -+/ and x involved? I can only find the reference to Sudoku.

Kind regards

Andrew Stuart writes:

Despite KenKen clues not always having + clues the rows, columns and boxes still have to add up to 21 (in the case of 6x6, 45 in 9x9) and therefore can in principle be decoded using the same technique.
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