The secret of solving Sudoku

To solve easy sudoku puzzles, all you have to do is to look for unique numbers amongst the candidates for any row, column, or box. However, that technique will not enable you to solve the harder puzzles. For them, you need to apply the rule of pairs, trios, or quartets of candidate numbers. This rule is not easy to state succinctly; but I shall first try to do so for pairs of candidate numbers in any two cells in a row, column or box, as follows:

If a pair of numbers are the only two identical candidate numbers in a pair of cells in any row, column, or box, then those same numbers can be eliminated as candidates from other cells in the same row, column, or box.

Similarly, if there are are only three identical candidate numbers in a trio of cells in any row, column, or box, then those same three numbers can be eliminated from the other cells in that row, column, or box.

Similarly, for only four identical candidate numbers in a quartet of cells.

I hope that is clearer than mud.

Rocker

Why not just download this? Click button, receive solution.

http://sudoku.sourceforge.net/

Master Splinter,

Thanks for posting that link; Sudopedia looks like a good reference for Sudoku junkies.

Rocker

I didn't record the URL, but there's a pdf file by Michael Mepham that's quite comprehensive. Google [sudoku "michael mepham" pdf] should find it for downloading and future reference.

I'm currently testing a graphical assistant using MSPaint - basically only a memory aid using the dots method. Not sure when or if I'll ever write it up.

Joe

I bought a copy of Sudoku from Pappocom. It is a great little program with lots of options.

Didn't cost much from memory ($US15?) but if you Google you can find a downlodd and a crack, but I'd rather pay the guy.

Highly recommended if you like doing Sudoku puzzles, I try to do at least 2 in the morning to wake up the brain:doh:

You don't need to pay to get a Sudoku puzzle generator. You can get a constant supply from http://sudoku.com.au/ , where you can choose the level of difficulty and also check whether your entries are correct so far, if you want to cheat.

Rocker

I like the super ones where they give you some shaded areas and the sum of the digits in those areas. They take some time:D

Here is another good URL for free sudoku, and explanations of advanced techniques: http://www.sudokuoftheday.com/pages/about.php

I am still struggling to grasp X-wings:(

Rocker

just don't play it in court :no:

Quote:

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Originally Posted by Ashore

just don't play it in court :no:

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A THREE-month drugs trial has been aborted at a cost of "millions" after the jury's forewoman admitted playing the number game Sudoku for at least half of the trial. The District Court trial, which cannot be identified for legal reasons, was aborted late yesterday, on the 66th day of evidence, after Judge Peter Zahra was told one of the accused men had noticed the jury playing the game. The forewoman was brought into the court and asked by Judge Zahra to estimate for how much of the trial she had been playing the game. "I don't know, at least half of it I'd say, I usually have it at hand," she said, explaining that "maybe four or five" others on the jury played Sudoku or other games. She added: "Yes, and we discuss it sometimes, or the word games out of the newspaper sometimes are written down as well".

The judge asked: "Do jurors bring those games into court. And they're playing those games during the course of the evidence?" "Yes," the forewoman replied. The woman said she had begun playing Sudoku "quite early on" in the trial, "probably when the surveillance evidence was on" which was estimated to be a couple of weeks after the trial began. More than 100 witnesses had been called so far. The forewoman went on to say that the puzzles would be copied into their notebooks and other jurors would "read it over (their) shoulder or look next to them". The judge said the activities of the jury were "breathtaking", especially given regular directions that the jury ensure the "integrity of the trial".

Legal sources estimated the cost to the taxpayer would run into "millions". One said it was generally estimated that the cost of a District Court trial per day was well in excess of $20,000. On that figure, this trial would have cost close to $2 million. In making his decision to discharge the jury, Judge Zahra said it was clear both the Crown and defence cases had been affected. "(The juror) appears obviously quite frank and it is not only obviously the Sudoku puzzle, but it appears a number of them come armed with the view to completing a word puzzle during the course of the day and then, during the breaks, discuss what their conclusions are or what they have concluded from the puzzle itself," he said. "It appears that this has occurred very much for a significant period of the trial. The only word I can use is breathtaking - despite my repeated directions to them that they are judges and that they must also have an obligation to preserve the integrity of the trial process, to quite openly defy that". Extraordinarily, the case had almost collapsed once before after a sheriff's officer was accused of making comments within earshot of the jury as one of the men standing trial was giving evidence - but Judge Zahra rejected an application to discharge the jury as it could not be proven anything was said or overheard.

The case returns to court in two weeks.

Yep cost us heaps , and they reckon the crims are stupid , worst case is that some of these crims involved may get of because of this :((

Well hang on, there are on trial to find out whether they are crims on not.

For those who are addicted to the sudoku.com.au website, how about a show of hands on your typical, repeatable average times for the different levels.

eg I would expect to do the Easy in 2'30" to 3'30" and the hard in 8'30" to 12'00.

Settings used are all boxes ticked except "allow incorrect moves" and "possibilities in grid"

regards

Alasdair,

Surely, it is far easier to do the puzzle if you have unchecked 'Allow incorrect moves'; that setting makes it unnecessary to employ advanced tecniques such as forcing chains, when you are deadlocked. I usually take 30 minutes or so to do a hard Sudoku, but I don't cheat by letting the computer do all the hard work.

Rocker

Blokes. Sheesh cant help yourselves! Have to analyze everything down to sub-atomic level.

I love doing my sudokus, particularly the difficult ones. I reckon they take about 20 minutes. I don't enjoy doing them on computers, I much prefer the in one in the Herald Sun (particularly Thursdays). Now I am gonna be looking at it wondering if its a Swordfish pattern,or a Nishio or if there are any disjoint subsets in there. I dont need to know all that guff, I just need to know that the elimination strategies I have developed for me work me. Sheesh!

And for the competitive among us, this mornings (19/6) Herald Sun Sudoku - 15:45 :wink:

Namtrak,

I also prefer to do Sudoku on paper rather than on a computer screen, so I print out this blank grid, to provide plenty of room for pencilmarks.

Rocker

That would have to be the hardest Sudoku I have ever seen!!!:p

and the easiest

Quote:

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Originally Posted by namtrak

Blokes. Sheesh cant help yourselves! Have to analyze everything down to sub-atomic level.

I love doing my sudokus, particularly the difficult ones. I reckon they take about 20 minutes. I don't enjoy doing them on computers, I much prefer the in one in the Herald Sun (particularly Thursdays). Now I am gonna be looking at it wondering if its a Swordfish pattern,or a Nishio or if there are any disjoint subsets in there. I dont need to know all that guff, I just need to know that the elimination strategies I have developed for me work me. Sheesh!

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I don't think you will be able to do this fiendish one without using forcing chains: http://www.sudokuoftheday.com/pages/...?day=1&level=4 . I managed it in about 30 minutes, but had to use that technique.

Rocker

12 :)

I gotta get a life

Quote:

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Originally Posted by Rocker

Alasdair,

Surely, it is far easier to do the puzzle if you have unchecked 'Allow incorrect moves'; that setting makes it unnecessary to employ advanced tecniques such as forcing chains, when you are deadlocked. I usually take 30 minutes or so to do a hard Sudoku, but I don't cheat by letting the computer do all the hard work.

Rocker

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Yeah Dave, point taken!

However, like Namtrak says, I do them for fun, and not as some major challenge. Keeping the "Allow incorrect moves" unticked does not mean that I use it to solve by trial and error. It is mainly to pick up the "idiot factor" mismoves that crop up.

I also only do up to "HARD" usually, where it is very seldom that you need to go with forcing chains, as the logic methods of elimination are usually sufficient, 8 /10 times.

I don't take it seriously enough to put up with the frustration of proceeding through a further dozen moves, only to find a mistake. If that happens, I would usually laugh it off. As it is, if I get the dreaded flash of red, and it is not "finger trouble" I mentally write the result off, and finish the rest as an exercise only.

regards

Did anyone do today's 'diabolical' sudoku from http://www.sudokuoftheday.com/pages/about.php ? I got a valid solution, but it differs somewhat from the solution given on that site for the puzzle. I though Sudokus were supposed to have a unique solution.

Rocker

Quote:

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Originally Posted by Rocker

Did anyone do today's 'diabolical' sudoku from http://www.sudokuoftheday.com/pages/about.php ? I got a valid solution, but it differs somewhat from the solution given on that site for the puzzle. I though Sudokus were supposed to have a unique solution.

Rocker

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I don't think that's necessarily true, although I'd be more inclined to believe it if it was actually stated. I've found at least one where a last empty pair could be interchanged. More than that would indicate something haywire in either the problem statement or my "solution," I think.

Joe

Quote:

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Originally Posted by joe greiner

I don't think that's necessarily true, although I'd be more inclined to believe it if it was actually stated. I've found at least one where a last empty pair could be interchanged. More than that would indicate something haywire in either the problem statement or my "solution," I think.

Joe

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Joe,

It is actually stated in Sudopedia: "A well-formed Sudoku has a single solution that can be found by reasoning, regardless of the complexity of that reasoning."

My solution to the diabolical sudoku for July 7th is mostly the same as the solution given, but 13 cells differ; there are three digits transposed in rows 4, 6, and 7, and four digits transposed in row 5. In the columns, there are two digits transposed in columns 2 and 9, and three transposed in columns 4, 5, and 6.

Rocker

Quote:

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Originally Posted by Rocker

Joe,

It is actually stated in Sudopedia: "A well-formed Sudoku has a single solution that can be found by reasoning, regardless of the complexity of that reasoning."

My solution to the diabolical sudoku for July 7th is mostly the same as the solution given, but 13 cells differ; there are three digits transposed in rows 4, 6, and 7, and four digits transposed in row 5. In the columns, there are two digits transposed in columns 2 and 9, and three transposed in columns 4, 5, and 6.

Rocker

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I mis-stated the anomaly I found. It was actually two empty pairs IIRC.

Consider a fragment:

x x x A x x B x x x
x x x Z x x C x x x
x x x B x x A x x x

A and B can be swapped willy-nilly from two remaining candidates without violating the requirements, as far as I can tell, but I might have to re-examine it if I can find it. The get-out-of-jail-free card is "well-formed." The puzzle-writing algorithm might have a hiccup; or not, of course.

I haven't visited the site in your link; too many other things on my Works-In-(non)Progress list.:(

Joe

Quote:

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Originally Posted by Rocker

Did anyone do today's 'diabolical' sudoku from http://www.sudokuoftheday.com/pages/about.php ? I got a valid solution, but it differs somewhat from the solution given on that site for the puzzle. I though Sudokus were supposed to have a unique solution.

Rocker

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Hi Dave,

Within some limits, this is not completely true.

I have a whole book of puzzles, and almost every one has 2 solutions----------- usually a 4-square pair reversal.

regards