Why? And if that’s not valid, what is? Ultimately, all sudoku techniques can be thought of as just special cases of guess-and-check.
One could probably design a Sudoku that requires some extremely large number of operations (10^20, say) to solve by guess-and-check, but where a uniqueness assumption bypasses all that. Sudokus are generally supposed to be solvable by human-level deduction, which might include shallow guess-and-check but nothing that requires more than dozens of operations.
I think you should be able to see the logic ahead of time. Guessing a number and carrying on to see what happens without having any idea of what the result will be is both uninteresting and distinctly different from seeing a pattern of candidates and knowing that the pattern allows you to rule out candidates from certain cells without having to follow a process of trial and error.
My 7 year old daughter could brute force any sudoku given enough time and motivation.
In KenKen (a Sudoku variant) I have used the fact that solutions must be unique to eliminate possibilities.
But “carrying on to see what happens” is exactly what “seeing a pattern of candidates” is.
Heck, even trivial techniques like “when 8 of the numbers in a row are filled in, fill in the last one” are based on guess-and-check: “Is this a 1? Check… No, that means two 1s in the row. Is this a 2? Check… No, that means two 2s in the row”, etc.
One can certainly say that it should be possible to do all of the work in one’s head… but then we have to ask whose head. It’s possible to do guess-and-check in one’s head, if one has a good memory and it’s not too many steps. On the other hand, for many “logical” techniques, most solvers find they can’t do all of the work for the technique in their head.
But that’s not what you do is it? You look at it and see a 1 is missing, you don’t literally go through each number and mentally fill it in.
Look at a naked pair of a 3 and 5. If I see a 35 naked pair, I know I can remove those two candidates from all other cells in the region without having to literally put a 3 in one of the cells, see that a 5 is forced to be in the other cell and then see that there is no way for 3 and 5 to go in the regions that see the pair, even though that is the underlying logic. This is totally different from not knowing about naked pairs, guessing a 3 in one of the other cells then eventually finding there’s nowhere to put 3 in the naked pair cells.
In one case you’re using a generalised technique that has been proven to work, in the other you are blindly trying stuff until it doesn’t work and then trying something else.
Same with an X-wing. I know the X-wing pattern eliminates all candidates in the same row/column as the X-wing, I’m not thinking about what happens if I put a particular number in a cell, I don’t even need to know the number, I could just colour the applicable cells blue.
I want to use techniques that can be generalised, I don’t want to be in a position where I’m literally putting a number in a cell without knowing what will happen.
This paragraph suggests you know very well the difference between guessing and using “logical” techniques.
I like that style, but but the English cultural references usually stump me.
Have they ever done puzzles by Cox and Rathvon? Those are my favorite constructors; they recently reitred and put their whole archive online. I’m working my way through it, although I’ve probably done most of them before.
No idea on that front. I’ve only ever occasionally done puns and anagrams in Games magazine, and in the NY Times, when they used to post their extra puzzle on the website.
Thanks, all. Wife claims the Sudokus she does (in the Chicago Trib) have had such multiple answers in the past. She has discussed it with her friend - a more involved puzzler, who agrees.
I personally am not interested in the puzzles (or my wife’s interpretation of them) enough to take this further. Sorry I missed the previous thread.
No, it suggests that I know that others think there’s a difference, and sort some techniques into the category they call “logical” and others into the category they call “non-logical”, even though they all work the same way.
Well, if you can share just one of the ambiguous puzzles here, I bet we can figure it out for sure.
Thanks. I’ll see if I can easily pull up a link to the Sudoku in yesterday’s Chicago Trib, but I doubt it is publicly accessible.
I appreciate that you guys enjoy these so much, and I intend no disrespect, but I’ve surpassed my very limited interest in Sudoku, so I won’t be putting in much effort to share them. Nor am I interested in proving my wife right or wrong. I’m happy she enjoys doing them. And Wordle - another puzzle I do not do.
No disrespect to your wife but if it’s happening to her regularly, especially with a large paper like the Chicago Tribune, I’m pretty certain that the error is on her end, the “selective blindness” I mentioned earlier.
I don’t know if this is the puzzle but it’s the one for 24 March on the Tribune’s website. It is fully solvable with only 1 unique solution.
That is entirely cool. As you might imagine, since Sudoku is not something that is at all important to me, I’m not terribly interesting in proving her (and her friend) right or wrong. 
Maybe later I’ll see if the puzzle in question is still readily accessible in our recycling bin… But other than checking to see if it is the same puzzle you linked, I’m not going to “check her math.” After 39 years of marriage, we have no difficulty finding plenty of other things to fight about! 
Other considerations notwithstanding, perhaps a puzzle designer might exploit the existence of multiple solutions to make things more challenging.
Pulled it out of the recycling - it was a different puzzle. In fact, the Business section has a “bonus puzzle page” that had a different Sudoku than the one you found on line.
Curious - they seem to be skimping on all aspects of news reporting, yet they appear to be Sudoku RICH!
Do you think there’s such a thing as fun Sudokus and less fun Sudokus?
A fun sudoku is one that’s easy enough that I’m able to finish it, but difficult enough that I’m not able to finish it too quickly or easily. Of course, by this definition, different sudokus will be fun to different people.
But easy and hard isn’t a 1-dimensional spectrum. Pure guess-and-check is easy–it just takes a while. And you can solve any Sudoku that way. At least for me, the satisfaction comes from seeing a pattern that logically constrains the solution. And also discovering the general form of these patterns. A Sudoku that didn’t have any of these patterns would not be fun for me.