# Sudokus don't always have unique solutions **URL:** \ **Category:** The Game Room\ **Created:** [November 19, 2024, 8:18am UTC](https://boards.straightdope.com/t/sudokus-dont-always-have-unique-solutions/1010447 "2024-11-19T08:18:24Z")\ **Posts on this page:** 13\ **Page:** 2
**Author:** ![RitterSport](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/rittersport/32/6326_2.png) [@RitterSport](https://boards.straightdope.com/u/RitterSport)\ **Post date:** [November 20, 2024, 12:52am UTC](https://boards.straightdope.com/t/sudokus-dont-always-have-unique-solutions/1010447/21 "2024-11-20T00:52:18Z")
Simon is so nice and fun to watch, but as I’ve gotten better at spotting stuff, he can get frustrating. Pencil Mark is way better at scanning. ---
**Author:** ![Richard\_Pearse](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/richard_pearse/32/13144_2.png) [@Richard\_Pearse](https://boards.straightdope.com/u/Richard_Pearse)\ **Post date:** [November 20, 2024, 12:53am UTC](https://boards.straightdope.com/t/sudokus-dont-always-have-unique-solutions/1010447/22 "2024-11-20T00:53:31Z")
I plugged your puzzle into sudoku.coach (you can import from a picture) and it has more than one solution. [![Imgur](https://live.staticflickr.com/65535/54152395629_0ca646d551_b.jpg "Untitled | Jet Blaster | Flickr") ](https://www.flickr.com/photos/200199460@N02/54152395629/) ---
**Author:** ![Richard\_Pearse](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/richard_pearse/32/13144_2.png) [@Richard\_Pearse](https://boards.straightdope.com/u/Richard_Pearse)\ **Post date:** [November 20, 2024, 12:55am UTC](https://boards.straightdope.com/t/sudokus-dont-always-have-unique-solutions/1010447/23 "2024-11-20T00:55:33Z")
Yes he will sometimes miss a naked single while working out a swordfish or something. ---
**Author:** ![Atamasama](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/atamasama/32/12961_2.png) [@Atamasama](https://boards.straightdope.com/u/Atamasama)\ **Post date:** [November 20, 2024, 1:01am UTC](https://boards.straightdope.com/t/sudokus-dont-always-have-unique-solutions/1010447/24 "2024-11-20T01:01:00Z")
I do recall with the old Sudoku app I was using previously (which no longer works properly in a newer iOS), it was basically “teaching” me as I went along. And it was clearly leaning on bifurcation, because one of the things it would often mention for a hint was along the lines of, “Since having these numbers here would mean there is more than one solution, that can’t be correct, therefore they have to go here instead.” ---
**Author:** ![Mijin](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mijin/32/9369_2.png) [@Mijin](https://boards.straightdope.com/u/Mijin)\ **Post date:** [November 20, 2024, 2:05am UTC](https://boards.straightdope.com/t/sudokus-dont-always-have-unique-solutions/1010447/25 "2024-11-20T02:05:44Z")
> [@Richard\_Pearse](#): > > I plugged your puzzle into sudoku.coach (you can import from a picture) and it has more than one solution Thank you! I’d looked at this puzzle for a long time but still had the suspicion that I’d share it and within a minute someone would post “You do know you have three 8s on one row, right?” ---
**Author:** ![mattgg](https://avatars.discourse-cdn.com/v4/letter/m/ccd318/32.png) [@mattgg](https://boards.straightdope.com/u/mattgg)\ **Post date:** [November 20, 2024, 2:49pm UTC](https://boards.straightdope.com/t/sudokus-dont-always-have-unique-solutions/1010447/26 "2024-11-20T14:49:00Z")
> [@Mijin](#): > > Having said that I was reluctant to post up the image of the point I got stuck but screw it, let’s hear where I went wrong Which numbers are the ones you filled in? The red ones or the dark ones? ---
**Author:** ![mattgg](https://avatars.discourse-cdn.com/v4/letter/m/ccd318/32.png) [@mattgg](https://boards.straightdope.com/u/mattgg)\ **Post date:** [November 20, 2024, 2:51pm UTC](https://boards.straightdope.com/t/sudokus-dont-always-have-unique-solutions/1010447/27 "2024-11-20T14:51:24Z")
> [@Richard\_Pearse](#): > > I plugged your puzzle into sudoku.coach (you can import from a picture) and it has more than one solution. You’re assuming that the answers filled in up to this point are correct. They could be incorrect without violating the constraints. ---
**Author:** ![RitterSport](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/rittersport/32/6326_2.png) [@RitterSport](https://boards.straightdope.com/u/RitterSport)\ **Post date:** [November 20, 2024, 2:56pm UTC](https://boards.straightdope.com/t/sudokus-dont-always-have-unique-solutions/1010447/28 "2024-11-20T14:56:35Z")
I just entered only the black ones and that Coach site still says Number of Solutions \>1. Someone else can try entering all the red ones. ---
**Author:** ![Mijin](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mijin/32/9369_2.png) [@Mijin](https://boards.straightdope.com/u/Mijin)\ **Post date:** [November 20, 2024, 6:38pm UTC](https://boards.straightdope.com/t/sudokus-dont-always-have-unique-solutions/1010447/29 "2024-11-20T18:38:20Z")
I think the dark ones are the ones I filled in ---
**Author:** ![Richard\_Pearse](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/richard_pearse/32/13144_2.png) [@Richard\_Pearse](https://boards.straightdope.com/u/Richard_Pearse)\ **Post date:** [November 20, 2024, 6:40pm UTC](https://boards.straightdope.com/t/sudokus-dont-always-have-unique-solutions/1010447/30 "2024-11-20T18:40:43Z")
If the numbers entered don’t violate constraints and they lead to more than one solution then there was always more than one solution. Edit: To put it another way, the definition of an incorrect answer is that it leads to a contradiction. Incorrect answers can’t lead to a valid solution. ---
**Author:** ![pjd](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/pjd/32/3009_2.png) [@pjd](https://boards.straightdope.com/u/pjd)\ **Post date:** [November 20, 2024, 6:46pm UTC](https://boards.straightdope.com/t/sudokus-dont-always-have-unique-solutions/1010447/31 "2024-11-20T18:46:09Z")
> [@Jas09](#): > > How is “taking a guess and seeing if it works out” different than the other types of logical solving steps? Isn’t it just more layers of deduction before you hit the contradiction (or don’t) to determine what the first number should be? It seems to be a form of _Reductio ad absurdum_. ---
**Author:** ![Mijin](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mijin/32/9369_2.png) [@Mijin](https://boards.straightdope.com/u/Mijin)\ **Post date:** [November 20, 2024, 6:54pm UTC](https://boards.straightdope.com/t/sudokus-dont-always-have-unique-solutions/1010447/32 "2024-11-20T18:54:49Z")
> [@mattgg](#): > > You’re assuming that the answers filled in up to this point are correct. They could be incorrect without violating the constraints True but I think unlikely; there were no guesses, Id only filled in what I knew from deduction. Of course it’s possible that I made a visual error though. ---
**Author:** ![mattgg](https://avatars.discourse-cdn.com/v4/letter/m/ccd318/32.png) [@mattgg](https://boards.straightdope.com/u/mattgg)\ **Post date:** [November 20, 2024, 7:27pm UTC](https://boards.straightdope.com/t/sudokus-dont-always-have-unique-solutions/1010447/33 "2024-11-20T19:27:24Z")
> [@Richard\_Pearse](#): > > If the numbers entered don’t violate constraints and they lead to more than one solution then there was always more than one solution. > > Edit: To put it another way, the definition of an incorrect answer is that it leads to a contradiction. Incorrect answers can’t lead to a valid solution. Thank you. I didn’t think that through. [Previous page](https://boards.straightdope.com/t/sudokus-dont-always-have-unique-solutions/1010447.md?page=1)