Thanks for the link TallMike - I still don't buy the idea that this is an invalid technique. It gives ambiguous results if the puzzle has more than one solution, but that same idea could be applied to other techniques. Thanks again.

Thinking about it, it is really more of a concern for puzzle constructors. To truly qualify as hard/expert/4star/5star the constructor should ensure that URs do not occur.

done yesterdays hard

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KT, you seem to be missing the point. There is no universally agreed standard for rating puzzle difficulty. More importantly, there is no rule that every puzzle must have a unique solution.
The modern requirement for a unique solution has nothing to do with the rules of Sudoku. It evolved from the needs of publishers to provide a solution to every puzzle they published.
It would be interesting to revert to allowing puzzles with more than one solution. The challenge would be to explain to the world why they could no longer rely on their favorite puzzle solving programs, and why IronSudoku no longer worked (unless it was redesigned to allow solutions in which some cells contain more than one possible number).

The point I was contesting was the assertion that URs are "an invalid" technique. So far I have heard or read nothing that supports that. If one goes on the assumption that there is only one solution to the puzzle - then it is a valid technique. If told in advance that there are multiple solutions, then the technique is invalid, but I can never recall a published puzzle claiming to have multiple solutions. If one is not told in advance, then I contend that URs are valid in the sense that it either solves the puzzle or reveals the ambiguity.

For the record, since IronSudoku and the 2 puzzles I do daily in the local fishwrap have no consequence other than personal enjoyment, this discussion is purely academic and along the lines of previous discussions concerning "guessing." I sometimes use guessing when no other technique that I know works. Other times when I can't crack a puzzle, I just leave it unsolved - primarily because of time constraints. In any case, I will continue to use URs when they present themselves in a puzzle.

A puzzle publisher has no obligation to announce that some puzzles may have multiple solutions. Players who try unsuccessfully to use the UR technique on such puzzles can hardly blame the publisher for the player's erroneous assumption that every puzzle has a single solution.

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I thought all sudoku puzzles could only have one resolution. I've never run into one that had multiple solutions.

Just for the record, I will happily use URs or guessing to solve an Expert puzzle, since I have not yet mastered any expert techniques. But I don't employ either method for Easy, Medium, or Hard, which should be solvable without their use.

I believe we may safely assume that all puzzles on this site have one unique solution, because otherwise the programming would get messy, as TallMike has pointed out.

dng

EZPZ fast clicker. 10.