# How do you verify, that there is no error (made by the creator of the sudoku) in the sudoku you are planning to solve?

Asked by

flo (

12974)
December 2nd, 2010

One time, I found the error immediately, which was nice. Is there a way of making sure that it is error free before starting?

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## 11 Answers

No good way. As far as I know, its NP-Complete (aka Hard).

There are sudoku solvers out there. You can probably feed your particular puzzle into one of them and see if it can find a (unique) solution. Some of them probably will check without giving you the answer.

P.S. If you are into programming, a sudoku solver isn’t too hard to write.

@flo Same thing.

There are two ways a sudoku problem can go wrong.

1. There may not be any solution. E.g. if the guy who messes up puts two “3’s” in the same row.

It can be more subtle though. E.g. you start solving the problem and you say “ahh, this cell needs to have the number 7.” Then you keep going and realize that a 7 in that cell conflicts with something else. As long as you didn’t make any mistake, this is a messed up puzzle (or as you put it, a number placed in the wrong box).

2. There might be more than one solution. E.g. a totally blank puzzle. There are a huge number of solutions. But it doesn’t have to be this simple. For example, if the problem you are given has neither a 3 or 5 in it, as long as you don’t run into problem 1, there are at least two solutions. (If you find one, you can find another by swapping all the 3’s for 5’s and vice-versa).

If this happens, there is a solution but it isn’t “unique.”

You may or may not consider this to be a “bad” puzzle…

A sudoku solver is a program where you enter the problem (i.e. type in the numbers you are given) and hit “go.” It then runs through every possible combination until it finds a solution. Some of them stop once they find a solution. Some of them keep going and find all the solutions. If you can find one that does this and just tells you how many solutions there are (but doesn’t display them) then you have a solution to your issue. I’m sure there are some other there… its something anyone with a few programming classes under their belt can probably pull off.

@roundsquare thanks. I am not that dedicated to Sudoku though. You helped the ones who are though I am sure.

I suspect it is very rare. My husband has done hundreds, if not thousands of them and never found an error.

Thank you for reminding me of my love for this game. It’s been years since I played!

Are you all doing it on the computer? I do the ones in the newspapers

I’ve only been doing sudoku puzzles for a few months, but boy…I am SO hooked.

Does anyone know of any good sudoku message boards? I don’t know anyone in real life that’s a fan.

I have never found a problem with a printed Sudoku.

If you are looking for a similar puzzle, the Sujiken is amusing.

Thanks @filmfann but I might look into it.

By the way, some people pass on trying Sudoku it because they think it is an arithmetic thing.

@flo it’s true. Neither Sujiken or Sudoku are math puzzles. I do a Sudoku printed in my local paper that uses letters, not numbers.

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