# Sudoku impasse **URL:** \ **Category:** The Game Room\ **Created:** [April 27, 2013, 3:47pm UTC](https://boards.straightdope.com/t/sudoku-impasse/656844 "2013-04-27T15:47:16Z")\ **Posts on this page:** 9\ **Page:** 1
**Author:** ![BwanaBob](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/bwanabob/32/2985_2.png) [@BwanaBob](https://boards.straightdope.com/u/BwanaBob)\ **Post date:** [April 27, 2013, 3:47pm UTC](https://boards.straightdope.com/t/sudoku-impasse/656844/1 "2013-04-27T15:47:16Z")
Often I get to a point where I have a puzzle completely filled in except for a bunch of boxes with pairs of numbers that chain around the puzzle. I’ve photo copied them and tried picking one of the two numbers in a box and let the chain collapse according to that choice, and sure enough, one of the two tries completes the puzzle. The other winds up with a contradiction. To me that’s a guess. I’ve been dying to find an analytical way of choosing the correct one. Does that situation have a name? Can anyone steer me to a site that might explain the way to analyze this situation? Or can anyone explain it themselves? Thanks. ---
**Author:** ![Robot\_Arm](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/robot_arm/32/18280_2.png) [@Robot\_Arm](https://boards.straightdope.com/u/Robot_Arm)\ **Post date:** [April 27, 2013, 5:00pm UTC](https://boards.straightdope.com/t/sudoku-impasse/656844/2 "2013-04-27T17:00:41Z")
There’s an interesting page [here](http://www.sudokuwiki.org/sudoku.htm). Along the right side are a list of solving strategies, starting with “Hidden Singles” and continuing in increasing complexity. Clicking on the name will give a detailed description. Be warned, it’s going to be a lot more complicated than you think. ---
**Author:** ![BwanaBob](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/bwanabob/32/2985_2.png) [@BwanaBob](https://boards.straightdope.com/u/BwanaBob)\ **Post date:** [April 27, 2013, 5:17pm UTC](https://boards.straightdope.com/t/sudoku-impasse/656844/3 "2013-04-27T17:17:23Z")
This looks promising. Thanks. ---
**Author:** ![BigT](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/bigt/32/12044_2.png) [@BigT](https://boards.straightdope.com/u/BigT)\ **Post date:** [April 28, 2013, 3:26am UTC](https://boards.straightdope.com/t/sudoku-impasse/656844/4 "2013-04-28T03:26:36Z")
Do you have an example Sudoku so we can use that solver to analyze it? I’ve always considered this a feature of the more complex Sudoku puzzles, BTW. I thought they wanted you to have to guess a bit in order to make it harder. ---
**Author:** ![Gatopescado](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/gatopescado/32/5711_2.png) [@Gatopescado](https://boards.straightdope.com/u/Gatopescado)\ **Post date:** [April 28, 2013, 9:30pm UTC](https://boards.straightdope.com/t/sudoku-impasse/656844/5 "2013-04-28T21:30:46Z")
There is _NO GUESSING_ in Sudoku! ---
**Author:** ![Tom\_Tildrum](https://avatars.discourse-cdn.com/v4/letter/t/e95f7d/32.png) [@Tom\_Tildrum](https://boards.straightdope.com/u/Tom_Tildrum)\ **Post date:** [April 29, 2013, 12:07am UTC](https://boards.straightdope.com/t/sudoku-impasse/656844/6 "2013-04-29T00:07:41Z")
It’s proof [reductio ad absurdum](http://en.wikipedia.org/wiki/Reductio_ad_absurdum), and I don’t think that amounts to guessing. ---
**Author:** ![Snarky\_Kong](https://avatars.discourse-cdn.com/v4/letter/s/a183cd/32.png) [@Snarky\_Kong](https://boards.straightdope.com/u/Snarky_Kong)\ **Post date:** [April 29, 2013, 12:22am UTC](https://boards.straightdope.com/t/sudoku-impasse/656844/7 "2013-04-29T00:22:10Z")
That’s not guessing. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [April 29, 2013, 4:37am UTC](https://boards.straightdope.com/t/sudoku-impasse/656844/8 "2013-04-29T04:37:20Z")
The situation exactly as you describe, with every box containing a pair of possible numbers, is impossible in a well-formed Sudoku. If every box has a pair of possible numbers, it will always be the case that there are at least two solutions. I’m guessing that you overlooked one box that had three possibilities. ---
**Author:** ![Disheavel](https://avatars.discourse-cdn.com/v4/letter/d/e274bd/32.png) [@Disheavel](https://boards.straightdope.com/u/Disheavel)\ **Post date:** [April 29, 2013, 6:49pm UTC](https://boards.straightdope.com/t/sudoku-impasse/656844/9 "2013-04-29T18:49:22Z")
Might I recommend [sudokuslam](http://www.sudokuslam.com/), it has a pretty good cheat function that will show you where and what logical step must be taken at any point. You can also enter in your own puzzle to solve alongside it. It helped my skills quite a bit to see some of the strategies in a graphical real-time example.