When unable to solve with reasoning/logic is it ok to guess?

Advanced methods and approaches for solving Sudoku puzzles

When unable to solve with reasoning/logic is it ok to guess?

I unable to solve this with reasoning and logic. Do I resort to guessing or am I missing something?

When it's a tough puzzle I usually put 1-9 in all the spaces I have left after the initial work through and go systematically through them until eventually something jumps out.
In this case I must be going blind as I can find no more.

This is as far as I have managed to get. Help!

* 2 * 9 * 8 * 3 *

* 8 * * * * * * 9

9 * 4 2 3 * * 8 1

4 * 5 * 8 1 9 * *

2 * 9 * * * * 1 8

7 1 8 6 9 2 4 5 3

8 * * * 5 9 1 * *

1 * * * * * * 9 *

* 9 * * * * * 6 *
chester

Posts: 5
Joined: 01 July 2005

Re: When unable to solve with reasoning/logic is it ok to gu

chester wrote:I unable to solve this with reasoning and logic. Do I resort to guessing or am I missing something?

You missed a "naked pair" in column 2.
angusj

Posts: 306
Joined: 12 June 2005

Just found the answer to my question

It appears that T and E is acceptable.
I can now use that strategy and complete the puzzle myself.
chester

Posts: 5
Joined: 01 July 2005

naked pair

Thank you for that info Angus.
chester

Posts: 5
Joined: 01 July 2005

It appears that T and E is acceptable.
I can now use that strategy and complete the puzzle myself.

Acceptable yes, but not necessary to complete this puzzle. (Or have you solved it already?)
Last edited by simes on Sun Dec 11, 2011 2:27 pm, edited 1 time in total.
simes

Posts: 324
Joined: 11 March 2005
Location: UK

I got a bit bogged down.

No I haven't solved it.
I looked for naked and hidden pairs, triples and quads but can't actually see anything that can move me on.
What am I missing I wonder?
chester

Posts: 5
Joined: 01 July 2005

Done!

Couldn't have done it without checking in to this forum. Thanks for all help.
chester

Posts: 5
Joined: 01 July 2005

Solving puzzles

best guess/trial and error is certainly allowed because some of the puzzles can not be solved with true logic alone. sometimes you come to a fork in the puzzle and you select a number based on the information available and it works and sometimes it doesn't. you can sometimes narrow your choices to 2-3 numbers but in the end its still a SWAG. it's exciting to find a puzzle that you can logically find a solution and feel smug that you were able to solve it but that's why people enjoy gambling.
Guest

Posts: 312
Joined: 25 November 2005

You are incorrect; all Su Doku puzzles can be solved through logic alone, even if that logic takes the form of an exhaustive search for solutions through all possible digit placements.
Hammerite

Posts: 44
Joined: 20 June 2005

Actually, Michael Mepham puts in his intructions that T&E is necessary to solve his more difficult puzzles - he calls it bifurcation. I suppose it depends on what you define as logic.

Anyway, it's up to the individual puzzler as to how s/he solves the puzzle in front of them.

Luna
lunababy_moonchild

Posts: 659
Joined: 23 March 2005

The term "logic" is surely not up for redefinition!!

If you have studied logic...

( ( A implies B ) and ( not (B) ) ) implies ( not (A) ). Therefore bifurcation is logic, if all branches are followed.

And I never dispute that people have the freedom to play whatever puzzles they want (after all, I would be a fool to do so)... it merely irritates me intensely that people have such a weak grasp of what "logic" means...

(sorry for the rant)
Hammerite

Posts: 44
Joined: 20 June 2005

Oh God, we're off again.
Karyobin

Posts: 396
Joined: 18 June 2005

Luna *getting old isn't it?*
lunababy_moonchild

Posts: 659
Joined: 23 March 2005

Yes, it's getting old.

But what are we to do when people continually point up and call it down?

A great person -- it may have been my wife -- said: "Things are how you speak of them." If the un-truth is written enough unquestioned, it *becomes* the (accepted) truth. Off the subject, several news articles have mentioned that Sudokus are popular in Japan because with their pictographic written language, crosswords are impossible. This is the opposite of the truth. In Japan, crosswords are more popular than any single other type of puzzle, including Number Place (Sudoku) and Nonograms. The Japanese written language includes both chinese charactors (Kanji) and a more alphabetic written language (Kana). Kanji puzzles are ALSO more popular than Sudoku. But these news papers and internet sites keep quoting each other, do not bother to do their own research, and eventually, this is what people take to be the truth. If you speak up now, it's too late -- they come back with two dozen quotes in the English speaking press "proving" their position.

There are some who speak of all these things the as if they were the same thing:

1) Guessing blindly at any point and trying to solve from there, taking dozens of steps, and maybe more guesses, until reaching contradiction or solution.

2) Making a "bifurcation" or chosing one of two possibilities when most or all other methods have been exhausted, knowing one will lead to a quick solution while the other will lead to contradiction. In some cases, this can be done in the head.

3) Looking for an short sequence of forcing moves -- as few as four -- in which any placement in one cell leads to the same result in another. In many cases, this is done in the head.

Puzzles that *require* this last technique are routinely -- and incorrectly -- labled *invalid* -- but this is NOT a hard technique -- certainly not as difficulty as finding an swordfish. One usually applies this when there is little left but cells with only two possibilities -- it can be often *very* easy to use *in your head* -- in fact, it can be so obvious as to be difficult to ignore.

It's as if *any* look-ahead is the same as looking in the back of the book for the answer! This may come from the mixing of two groups of people with very different goals -- solvers, who wish to have the most enjoyable, pleasing experience and programmers, who wish to find the most efficient algorithms so that they can eventually solve the largest, most complicated Sudokus possible. It may make sense for a programmer to avoid some techniques that may be difficult to implement that are child's play for humans. What's easy for me may be hard for a computer, what's easy for a computer, may be impossible for you.

I see terms including, but not limted to, "guessing", "trial and error", "not solvable by logic", "invalid", "arguably unfair" used interchageably and applied with little rhyme or reason, no way for the reader to infer much specific about the actual puzzle in question based on the terms used -- other than the solver had some trouble with it and will feel as if s/he has given up if forced to use *magic* to get the answer.

Of course, all these methods are logic. But that's not the only point. I believe that nearly ALL of us on both sides of the issue agree that if one can aviod having use trial and error in such a way that it requires making a duplicate copy of the grid or filling in a large number of numbers only to have to go back and erase them all -- well, any other way is better. Most people will put off bifurcation for as long as possible -- but our side will not ignore what becomes obvious to search for a false "higher" truth.

I also KNOW that before the recent Sudoku explosion in England, there were 100's of thousands of us solving these for decades. The 2nd and 3rd methods described above have always been standard tactics used in some more difficult problems, regardless if there might be another path to the solution. If Pappocom would have called them "ultra-difficult" and "mega-hard" instead of "arguably unfair" and "invalid" "we will not make them", I doubt this issue would exist. It doesn't seem to exist outside this forum.

One last thing -- some of us enjoy solving puzzles in which very little progress can be made by typical methods, that might require lots of experimenting. Unless and until several new tactics are discovered, the only way to solve some puzzles is by very organized, trial and error.

For example:

Code: Select all
` . 3 . | 7 . . | . . .  . . 1 | . . . | . . 2  5 . . | 6 9 . | . . . -------+-------+------ . . . | . . . | . . 4  . 7 . | 5 . 3 | . 6 .  8 . . | . . . | . . . -------+-------+------ . . . | . 2 4 | . . 8  9 . . | . . . | 7 . .  . . . | . . 1 | . 5 . `

Though not a normal Sudoku, take a look at puzzle number 8 in the most recent Internet Puzzle Solver's Test. Took me all day:

http://diogen.h1.ru/english/puzzle05-2.html
tso

Posts: 798
Joined: 22 June 2005

Even if it is old I don't see how "Therefore bifurcation is logic, if all branches are followed. " can be classed as logic, or perhaps I just haven't followed other, previous, arguments on the subject.

As you say : "( ( A implies B ) and ( not (B) ) ) implies ( not (A) )" So.... how does 'let's try this and see how far it goes and if it's not successful I'll try that and see how far that goes, therefore I'll get an answer eventually if I keep trying' fit in to logic?

Perhaps, if I may suggest a new slant and ask the users of this forum to stop stating that forms of logic used by other people are simply wrong and start stating things like 'IMO' (in my opinion) and 'I use this form of solving techinique' would be a better way of communicating and then we wouldn't get bogged down in the same old same old. Which seems to be turning into a chicken and egg scenario anyway.

Luna

"One last thing -- ........." Already said it was up to the individual how they played the game and I certainly didn't say that it was wrong and that solvers shouldn't do it, if that was implied in any of my posts then it was most definitely accidental .
lunababy_moonchild

Posts: 659
Joined: 23 March 2005

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