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!

Monday 25-Feb-2013

... by: Xavier, France


I remark that the sum of the numbers inside an area is not checked. For example, here is [LINK] a killersudoku that is correct with respect to the sudoku rules but not with respect to the killer rules. The area starting in C3 should sum up to 17 but the solver propose a sum of 20 and seems to be happy with it.


Andrew Stuart writes:

This is an interesting issue. Partly the solver and the user interface assumes that if all your numbers add up to 405 then you haven't made a mistake in individual cages (at least two). The solver doesn't know if you have made a mistake until the end - when it has the solved cells. And if the puzzle finishes without an error, does it need to check? In this case it does. Are you certain the original puzzle has 17 in that cage? One other cage must also be wrong. I'm too lazy to add up all the numbers from a screen shot, but if you use the "Email this Board" button I can check.

Usually these sorts of problems are due to the puzzle maker not being aware/or using the Killer Cage Convention which this solver adheres to.

I cant see an example in this solution, so that probably not the answer

Friday 22-Feb-2013

... by: David Little, Liss, UK

Hi Andrew, I've enjoyed your website for a considerable time and just bought your android app for more mobile pleasure. I have an enhancement request. I generally like to do a sudoku without any candidates and simply have blank cells apart from the clues. Would it be possible to have another solving mode without candidates. When it that mode, it would make more sense for the number picker to select the digit you want rather than having to select every number you don't want (that becomes rather irritating when you solve puzzles without wanting candidates). Thanks for a great puzzle.

Andrew Stuart writes:

Thanks for this feedback. Yes, good idea. I'm working on an update now, so will include that.

Friday 15-Feb-2013

... by: Guenther, Switzerland


first of all, I'd like to thank you for this great site!!! It's really a great resource for sudoku.

I created this killersudoku:

When I ask the solver to take a step, it tries to remove potential digits from the Square of 34 on the right. It says

KILLER COMBO (Easy) on E8: cage of size 5 with clue of 34+ can only be 4/6/7/8/9, removing 1/2/3/5

I don't think, that this is correct. The cage spreads across two squares. Therefore the cage could contain a single digit twice, e.g. 9,8,7,5,5 would be a potential solution. Therefore it's incorrect to remove 5 as a potential solution.

Best regards,


Andrew Stuart writes:

My Killers and the Killer solver follow the convention that no cage can contain duplicate numbers. Have a look at
The Killer Cage Convention
Easy trap to fall into

Friday 8-Feb-2013

... by: Roy A Watts, Wichita Falls, TX, USA

Load Sudoku:


You may have already seen the likes of this puzzle. It is a combination of colour & "X." I created it with a regular sudoku solver and tested it in both the colour and "X" solvers.


Andrew Stuart writes:

There's lots of interesting variations. I don't know how to display that one - we're talking about a tone on top of a colour, so it would be hard to create the user interface. However my core engine is set up to combine variants. I've not tried color and X. Although many puzzles would be valid as a colour and an X and a normal Sudoku if you're making a variation it ought to only be solvable using the full variation. For example, the design of the boxes in Jigsaw Sudoku means one can use less clues than a normal Sudoku - it would give multiple solutions except in the Jigsaw solver. So with the extra constraints of colour AND X it ought to contain even less clues than either on their own. No always, but the interesting subset of possible puzzles should be unique to that variant, IMHO. I wonder what the minimum would be.

Friday 1-Feb-2013

... by: tim radder, los angeles

several times lately, after loading the puzzle, on the first or second step one square will be filled in without any explanation. In each case it was a box that I calculated could be any of four numbers. hitting the back button did not result in an explanation either. It's happened at least four times and makes the puzzle worthless. Before last week I never had this problem.

Andrew Stuart writes:

Are you talking about the website solver or something in an app? What browser? Can you send me the link to the puzzle (using the "Email this board" button). I'd like to take a look see whats going on.

Wednesday 30-Jan-2013

... by: did ez66, Spain

Just wanted to say, nice one.
Thanks. Must have been fun to do

Tuesday 29-Jan-2013

... by: Dan Sondgeroth, Austin, Tx

Unique Rectangles should be just before X-Wing or just after it. Unique Rectangles are so easy to see, they are easier to see than the Tough Strategies. I was doing the January 25, 2013 Daily Sudoku were the solver used Diabolical Strategy XY-Chains numerous times to solve the puzzle.

Andrew Stuart writes:

Certain types of UR are easy to spot I agree. I would have to split up the strategies and move some closer to the start. That sort of thing takes a lot of testing and I'd have to recalibrate the whole grading scene and lots of knock on effects - plus I know there is a more general way of searching for them which would catch more, still to be implemented.

Friday 18-Jan-2013

... by: Barry, Ontario, Canada

About the "5 Daily Puzzles"

I like it a lot, and highly recommend it. However,here are some improvements requested.

1) On iPad in portrait orientation the touch doesn't seem to work in the whole upper right cell. I have to carefully tap the lower left corner of that cell for it to be selected. All the other cells work easily.

2) I like to update the puzzles daily. Currently,, I have to do each of the puzzle types separately, by pressing NEW, then CONTINUE, then switch to the next puzzle type.. Could you put a button on the app which would change ALL the puzzles at once. Or even better, allow a setting option which I could use to AUTOMATICALLY update all COMPLETED or UNSTARTED puzzles on a DAILY basis.

Andrew Stuart writes:

Excellent feedback, many thanks. I'll take a look at the first point. I'm due to start work on an update and I'll include a way to download all puzzles, that’s a good idea. I had another idea which I'd like to bounce off you. Quite a lot of people fill in candidate notes on the harder puzzles. I was thinking of adding an option to fill in these based on the current spread of clues and solved cells. One removed them as the puzzle is solved rather than add them. How would you feel about such an option?

Wednesday 2-Jan-2013

... by: Ian Binnie, Sydney

I am unable to load any new Killer puzzles, as the app claims not to be able to access the internet. The normal sudoku can load new puzzles.
I have rebooted my i Pad and killed and restarted the app, but this does not help.

Andrew Stuart writes:

I just figured out the problem! I thought it was related to an old iPad issue but it’s the server. I added all the puzzles for 2013 but didn’t tell the pick up script the end date had moved, so it was still saying no more puzzles after Dec 31st. Very sorry about that, but it should work fine now. Have a go
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Thank-you everyone for all your questions and contributions.