Solving by guessing

Hi Graysmith,

IMO, it is possible to solve ALL sudokus without guessing - if you know, understand and can use ALL of the solving techniques currently known (I don't).

Obviously, the sudoku compilers/creators try their hardest to require new solving techniques.

I almost never resort to guessing. If I can't solve one without guessing, I start over and usually can get it.

IMO it depends on your definition of "guessing" and "solving". This may seem pedantic but has been the cause of many arguments previously.

Solving tends to be two categories:-
1 - Find a solution
2 - Prove that yours is the only solution

Defining guessing is trickier. If you enter numbers randomly you are unlikely to arrive at a solution and it certainly would not fulfil definition 2 of solving.

Quite often people see some solving techniques as guessing i.e. Forcing chains. I view them as elimination, just more complex than the normal. Even when solving simple Sudokus you are essentially "guessing" whereby you go through the numbers 1-9 for a square and conclude that only one of them can fit.

Other solving techniques rely on the assumption that the puzzle only has one solution, which again would not fulfil definition 2 of solving.

Longwinded answer to what appears a simple question :-)

Short answer - Yes, all Soduko should be able to be solved without guessing, it may just take you a long time for some.

Yes I was wrong that you have to guess. I have found Andrew Stewart's site SUDOKUWIKI.ORG.
I am amazed at the number of techniques available and will probably never master them all but so far I have used X WING and UNIQUE RECTANGLES to good effect.
In relation to Jimmy's comment I have a friend who is satisfied to find the solution by guessing but I find that not worth doing.
There is only one solution to a legitimate SUDOKU and that is why UNIQUE RECTANGLES is usefull.

Hi Graysmith,

Yes, Unique Rectangles are useful; if you like those, you certainly will find Steve's recent blog on uniqueness tecniques very useful.