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 andrew@str8ts.com.

Post a Comment or Question here...

Friday 28-Feb-2014

... by: Cathy, Edmonton, Alberta

Hi, how do I change the level to EASY - I am just starting and I like the Jigsaw idea but I don't want to start at 5 of 6 stars...

Hi, I'm doing a project on Sudoku solver myself and I found your site a very great resource! However I think I found a bug in your program that you should know.

I got this Sudoku from http://norvig.com/sudoku.html. It has no solution. Your solver cannot solve it (or prove that there is no solution), but it deduces a naked pair 5,6 in H5, J5. However, when I plug in both possibilities, it can quickly determine that there is contradiction.

So I stared at the puzzle to see if there is a logical deduction without guessing. And I found it - row 8 actually has no cell with number 1 as a candidate! I think your solver should be able to detect this right away, but you probably missed it because the focus of the solver is not on the "no solution" puzzle. However this should be nice and easy to add :)

Andrew Stuart writes:

That is an interesting situation in that puzzle. I do tack on checks for validity now and again when I see a hole in the validation. Looks like another hole. I can see that 1/5/6 want to fit in only two cells. Any puzzle that doesn’t have exactly one solution will not produce a credible solution path on my solver, for logical reasons.

Friday 28-Feb-2014

... by: Michael, UK

I use your killer solver on my iPad. It is brilliant! - however one small complaint - the number font when I enter the grid is too large to see in the cells - for example, if I enter 12 in a group of boxes this just displays as 1. Is there anyway of reducing the font size so that the totals display in full?

Andrew Stuart writes:

Ok fixed it. iPad was defaulting to quite a large padding, set that to zero. Made the font smaller. Also, I've changed the input box type to number so the right keyboard popups up on the ipad

Sunday 16-Feb-2014

... by: Dan, UK

hi there :) the solver is perfect, also handling of solver. but I miss one (for me important) thing: a simple reset button for resetting puzzle to 'first given', mostly defined by url. because if I am doing tests of techniques, or want to start over in same puzzle, I have to refresh page. a simple 'reset puzzle' button, which wuld do a reset of puzzle would be very nice :) thankx Dan

Andrew Stuart writes:

The solver is designed to tackle the puzzle at any given stage without memorizing any previous steps. Such memory retention would be extremely complex to implement on the server side with so many different users and on the client side when you could potentially have many solvers open. But in fact is doesn't need a memory because each state contains all the information necessary to proceed to the next step. Also, in Sudoku, it doesn’t need to distinguish between cells that are clues and those which are previously solved cells. I do allow this distinction in the small board with "enter clues or solutions" which gives you the two colors for large numbers. But as far as solving that is cosmetic.

So given that, a reset button is impossible to implement because there is nothing in the puzzle itself that says "this is the start". That is equivalent to asking "is this puzzle minimal" in the sense that all the clues are necessary. But some published puzzles are easier because they have extra clues and the minimal requirement is not a requirement for publishing.

But, we can brush all that aside by having the user decide the puzzle is at the start. So what you need is the "Save" and "Re-load" buttons which are above the small board. That does what you ask, but you need to remember to hit "save". I suppose if the puzzle information was in the URL a "restart" would be possible but I'd have to hide the button if the URL wasn't being used. I'll try that out.

You don't normally go this far out...did you unintentionally omit a solved value?

My trial and error algorithm takes a full 30 seconds to solve (Escargot "only" takes 9 seconds).

Andrew Stuart writes:

David Filmer has kindly provided me with some of his puzzles that he's created. I'm finding it a very long process to get decent unsolvables now and he's been working on very hard puzzles for some time with a lot of success. Check the name under the page title where it says "guest compiler" so you know it's one of his.

I recently found your site and I am impressed. This is a great resource. It provides clear descriptions of the logic being applied and a step by step mechanism for solving the puzzle. The page is well organized and was general easy for a first timer like me to use.

I do have a question though. Is there a FAQ? I graded the attached puzzle that I was having a bit of difficulty with and it graded it as "Tough". That makes me feel a little better knowing that it was a tough puzzle, but then I got to thinking what was the scale that this was on? How do I know that your not just placating me and making me feel good about myself. So I started looking for some help/information on your site regarding grading and the scale being used. I could not find anything. So I am wondering if you I missed it or if you might consider providing a page or a FAQ with the scale explained.

Thank you and keep up the great work!

Tom

Andrew Stuart writes:

Sorry for the late reply. Good point. There does need to be a quicker and more accessible explanation of the grading and scoring system. It's kind been buried. It is quite a complex issue but I do have a long answer : http://www.sudokuwiki.org/Sudoku_Creation_and_Grading.pdf

What I need to do is provide a cut down page and link to it directly from the solver. Added to job queue

Sunday 2-Feb-2014

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

Talking about Sudoku X puzzles-

Am I missing something or is it possible to have an X Wing in a Sudoku X puzzle when one or both of the columns/rows in the rule is/are replaced by diagonal(s) ???

I am thinking of something like this: picking 1 as the candidate value.

In column 2 have 1's in just A2 and D2. On the diagonal have 1's in just A9 and D6.

The reasoning would be that we have locked 1's pairs in column 2 and locked 1's pairs in the diagonal on rows A & D thus the reasoning remains the same resulting in no other 1's in row A or row D .

Thanks- Dale Kloss

Andrew Stuart writes:

You got it, that perfectly correct!

Saturday 1-Feb-2014

... by: Julien RENARD, France

This solver is probably the best one, because it is very helpful to learn step by step different ways to solve a Sudoko problem. I agree with Amb, for the use of 0 (zero) to skip a square to leave blank. Best regards. JR

Andrew Stuart writes:

Thanks for the feedback! Try the zero in the small board, I implemented it. Should work as discussed

Monday 13-Jan-2014

... by: Amb, New Zealand

When you are typing in a sudoku into the solver, it's easy to type numbers in and have it jump to the next square. But if its a square you want to leave blank, you either hit the arrow key or the tab key. I'd like to suggest that you could also have the 0 key register the cell as "Blank" and move on. (Of course if the user has the Auto Tab option off, it will just blank back out as before)

The advantage of using 0, is then you can type in your puzzle with one hand on a number pad!

Andrew Stuart writes:

Good call, not to difficult to add. Thanks for the suggestion. Refresh the solver a couple of times. Cheers Andrew

## Friday 28-Feb-2014

## ... by: Cathy, Edmonton, Alberta

Thank You,

Cathy

http://www.sudokuwiki.org/Daily_Jigsaw_Sudoku

?

The 30 day archive has puzzles on Monday which are the easiest.

Also, there is an easy jigsaw on

http://www.sudokuwiki.org/5_Daily_Puzzles_for_the_iPad

every day

## Friday 28-Feb-2014

## ... by: Nat Sothanaphan, Thailand

Load Sudoku: CLICK TO LOADI'm doing a project on Sudoku solver myself and I found your site a very great resource! However I think I found a bug in your program that you should know.

I got this Sudoku from http://norvig.com/sudoku.html. It has no solution. Your solver cannot solve it (or prove that there is no solution), but it deduces a naked pair 5,6 in H5, J5. However, when I plug in both possibilities, it can quickly determine that there is contradiction.

So I stared at the puzzle to see if there is a logical deduction without guessing. And I found it - row 8 actually has no cell with number 1 as a candidate! I think your solver should be able to detect this right away, but you probably missed it because the focus of the solver is not on the "no solution" puzzle. However this should be nice and easy to add :)

## Friday 28-Feb-2014

## ... by: Michael, UK

## Sunday 16-Feb-2014

## ... by: Dan, UK

the solver is perfect, also handling of solver. but I miss one (for me important) thing: a simple reset button for resetting puzzle to 'first given', mostly defined by url. because if I am doing tests of techniques, or want to start over in same puzzle, I have to refresh page. a simple 'reset puzzle' button, which wuld do a reset of puzzle would be very nice :)

thankx

Dan

So given that, a reset button is impossible to implement because there is nothing in the puzzle itself that says "this is the start". That is equivalent to asking "is this puzzle minimal" in the sense that all the clues are necessary. But some published puzzles are easier because they have extra clues and the minimal requirement is not a requirement for publishing.

But, we can brush all that aside by having the user decide the puzzle is at the start. So what you need is the "Save" and "Re-load" buttons which are above the small board. That does what you ask, but you need to remember to hit "save". I suppose if the puzzle information was in the URL a "restart" would be possible but I'd have to hide the button if the URL wasn't being used. I'll try that out.

## Saturday 15-Feb-2014

## ... by: Wayne, US

You don't normally go this far out...did you unintentionally omit a solved value?

My trial and error algorithm takes a full 30 seconds to solve (Escargot "only" takes 9 seconds).

## Friday 14-Feb-2014

## ... by: puzzler77, Texas, USA

Load Sudoku: CLICK TO LOADHere is a puzzle for anyone who loves Sudoku. I created it last summer.

puzzler77

## Tuesday 11-Feb-2014

## ... by: Tom, Eau Claire, WI, USA

Load Sudoku: CLICK TO LOADI do have a question though. Is there a FAQ? I graded the attached puzzle that I was having a bit of difficulty with and it graded it as "Tough". That makes me feel a little better knowing that it was a tough puzzle, but then I got to thinking what was the scale that this was on? How do I know that your not just placating me and making me feel good about myself. So I started looking for some help/information on your site regarding grading and the scale being used. I could not find anything. So I am wondering if you I missed it or if you might consider providing a page or a FAQ with the scale explained.

Thank you and keep up the great work!

Tom

http://www.sudokuwiki.org/Sudoku_Creation_and_Grading.pdf

What I need to do is provide a cut down page and link to it directly from the solver. Added to job queue

## Sunday 2-Feb-2014

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

Am I missing something or is it possible to have an X Wing in a Sudoku X puzzle when one or both of the columns/rows in the rule is/are replaced by diagonal(s) ???

I am thinking of something like this:

picking 1 as the candidate value.

In column 2 have 1's in just A2 and D2.

On the diagonal have 1's in just A9 and D6.

The reasoning would be that we have locked 1's pairs in column 2 and locked 1's pairs in the diagonal on rows A & D thus the reasoning remains the same resulting in no other 1's in row A or row D .

Thanks- Dale Kloss

## Saturday 1-Feb-2014

## ... by: Julien RENARD, France

Best regards.

JR

Try the zero in the small board, I implemented it. Should work as discussed

## Monday 13-Jan-2014

## ... by: Amb, New Zealand

The advantage of using 0, is then you can type in your puzzle with one hand on a number pad!

Andrew