Solve Sudoku (Without even thinking!)

This instructable provides step by step instructions to complete a Sudoku puzzle by simple process of elimination.

One of the things I that drove me crazy about Sudoku is how difficult it is to return to a puzzle if you get interupted. This system allows you to walk away from a puzzle at any time and return exactly where you left off.

Step 1 What you need to begin

You need:

1. A sudoku puzzle. I enlarged this one (which was the puzzle that appeared on Sudoku.com on July 8, 2006.) to make photography easier. I find this system works better on larger puzzles.

2. A pencil, Not a pen. A pencil.

3. An eraser.

Comments

[bobc4012]

While this approach is good for solving the simplest of Sudoku puzzles, it is only a start on the more difficult puzzles. Once you have "captured" some of the squares in the more difficult puzzles, you will find a number of squares where it looks like "guess work", when it actually requires "advanced techniques" to solve the puzzle There are techniques like "X-Wing", XY-Wing" an even "multi-color" solutions, to name a few. The site http://www.angusj.com/sudoku/ has an ".exe" file that can be downloaded (free) and is useful for solving hard Sudoku puzzles (it has a decent help guide along with googling some of the advanced solving techniques). When you get stumped, you can click on the "hint" button and it will indicate a square and the technique to derive the answer for that square. In certain instances, the technique isn't always straight forward and googling will help clarify.

One thing that is the case with most Sudoku puzzles is the symmetry. You notice the symmetry at the start and as the solution proceeds, you will notice a symmetry in the solution. In this case, this is where I agree with Eddie Chong on the "beauty" of the puzzle. What I detest most are those puzzles where you are reduced to guessing (rather than using logic and reasoning) which of 2 or 3 numbers to to choose in 2 or 3 squares and use "trial and error" to solve. This is typical in most Sudoku magazines that have "extreme" or "brutal" puzzles. To me, you should never have to resort to "trial and error" to solve the puzzle. As Sir Arthur Conan Doyle had Sherlock holmes say "Once you eliminate the impossible, whatever remains, no matter how improbable, must be the truth."

[10T]

It's great to see all the comments from so many dedicated puzzle solvers! I just started Sudoku, so using this technique along with tips from other websites really helped me to learn the methods quickly, for example, solving by pairs and triples. Thanks for creating this website.

[broper]

very nice
good job on you ible but one thing i want to menchon dosint this methoud take the fun out of sudoku? i tryed it on a hard puzzel and i didn't feel like i completed any thing when i was fineshed.

nice way to do it but it takes the fun away

[hrodriguez7]

you have a point


my dad is always sayin' that it has to be chalenging

[broper]

tx ( : i think the way you do it is awsom. and smart that you came up with it

[parallelchild]

I disagree... if you used this method on truly hard puzzles, even with "notes" (as I call them) they can be difficult. A puzzle is a problem meant to be solved, this happens to streamline the process. Goodluck :)

[broper]

yes thats a good point but i think it makes it to easy i mean i like doing hard puzzles and i enjoy the chalenge but if i do it the way you suggest i think it makes it two easy


ps

i think the methoud you made is a good one and i have nothing roung with it
it does its job it makes sudoku easy ( :

[mitchell12]

Try cross words, Seems like you'd have plenty of challenge with those :)

[WillyTheKid]

I try to get everybody I know to play Killer Sudoku (or Sum-doku). So much more of a challenge than even the hard level sudokus. but still there is a system.. always a system, even if it is complex. no need to guess!

[bjohnson1]

You should only use a pen and no eraser! That is how to do Sudoku.

[siamonsez]

I'm sorry, but this method feels like cheating to me, and at the very least defeats the purpose of doing the puzzle in the first place. You can resort to this method once the majority of the puzzle is done and you are totally stuck, but in my experience, you only get stuck because you either missed something obvious, made a mistake, of are trying a puzzle above your skill level that requires the use of techniques you are not familiar with.
Anyone that wants to learn to sudoku should avoid this technique because you will learn nothing by finishing puzzles this way and therefor will never be able to progress on to more challenging puzzles.

[vincent7520]

right !…
100% !…
I'm not against cheating, but it's got be worth it. In this example we didn't even try to begin to play sudoku : may as well play pencil and eraser on any piece of paper.
anyway… the example given is so basic that it's not even worth taking the pain to write all those numbers down : this sudoku can nearly be solved by itself !!!…

[bwrussell]

This is simply a visual representation of what your thought processes when finding the answers. The only reason it's easier is that you don't have to remember everything you've done. Of course more advanced puzzles will require more advanced techniques but a lot of those techniques are based on logical trial and error, which could be considered cheating just as much as this method.

This is a good way to start if you are learning sudoku, eventually you will learn the patterns and you wont need as many pencil marks. Besides if you are just starting and need this method to solve a puzzle there is little chance that you will know or be able to use the more advanced techniques.

[Eirinn]

I really cannot see what the difference is? The only difference i see is that this is systematic and the other way of doing it is less systematic.

This is nott a game of memory, it is a game of systematically solving a grid puzzle.

[siamonsez]

The difference is that if you rely on this method you will never get better. Once you start doing sudoku puzzles regularly, it's not just about completing them, but doing it quickly; anyone can finish , what makes it fun is doing it better.

[Eirinn]

The answer to this problem is not as straightforward as that :) It all depends in which way one views "getting better" and "fun".

[siamonsez]

Maybe I have a bit of ADD, but once something gets monotonous, it gets boring. So, for me, pushing myself to do better is what makes it fun; therefore, the two are interchangeable.

[Panta Rei]

Seriously, I've always thought that everyone in the world solves sudoku this way , just omitting the numbers already given in the related rows instead of writing them, and so I do. So...how the others do?!

[bjarvis2]

I'm creating a C++ sudoku solver for practice, and this algorithm looks great for a square 1 starting place! There are probably more efficient methods for automation, but this algorithm is a systematic approach to what a human does, albeit (or although, depending on who you're talking to) very explicitly stated and discrete.

[zheyman]

Here's another helpful technique.

Notice Row-3 has 2 squares whose only values can be 2 or 3. That means if one square is a 2, the other must be a 3. So no other square in that row can have a value of 2 or 3.

Removing all of the 2's and 3's from the other candidates in the row reveals the yellow square to be a 5. That forces the last square in the row to be a 4 and the remaining square in that row to be a 9.

Use this technique and more at http://www.SudokuByZol.com.  It has the best pencil mark support of any Sudoku website.

[aniketpatil]

Thank u so much for this technique.

[chinmoy samant]

he thanxxxxxxx it was just mind-blowing'awesome............................

[cat20]

hey thanx ,this is wat exact solution i was searching for

[eddie chong]

No doubt this procedure will give you the solution and the puzzle is "solved" as
quickly as possible. The uninitiated to this rather 'mechanical' method will be impressed, like I was a moment ago having followed your step by step approach.

I would not, however, resort to this 'protocol' The hallmark of Sudoku puzzles
lie in its intrigue and its beauty as a puzzle to be held in awe, respect and a
challenge to be taken. I often complete the day's puzzle and enjoy my successful
attempt with self satisfaction and look forward to the next day's fresh gauntlet, however long a time it took me. In Sudoku the journey is as enjoyable as its destination.

EddieC

[zheyman]

Don't mechanically enter them all first. Solve as much of the puzzle as you can. Only then resort to this method. Most puzzles of any difficulty require Pencil Marks (as they are called) for at least some of the squares.

If this method is too tedious for you, try out the many automatic Pencil Mark features at: http://www.SudokuByZol.com

[Ballooner]

@eddie chong While the idea that sudoku is good for mind, body, soul and spirit is nice. the interminable headache you get after trying to solve a more advanced puzzle without doing this is a slight problem for some of us, or at least myself due to a lack of an endless supply of patience. However this is a method that should be used as a last resort before finding the answer key.

and to the author thanks for the tips.

[nanma]

solve the columns with the most numbers first.
The fourth column going down.
has 6 numbers
1 3 4 9 2 5

so 6 7 and 8 are missing.
so the box between the 4 and 9 must
cant be a 6 or a 7 so it must be an 8.

[nanma]

use dots instead of numbers. and try to keep it to a minimum of 2 dots. per
blank box.

[atomic1269]

I guess this is the "beat it with a sick" method, so to speak. I find it easier just to use some real simple rules so you don't have to look at all the different possibilities. This site explains it pretty well - http://a-how.com/archives/247

[zheyman]

Pencil marks (as the potential candidates are called) are useful after the simple rules have been exhausted. For harder puzzles, they're almost impossible to do without.

For fast and efficient pencil mark support and free online sudoku puzzles, try http://www.SudokuByZol.com

[fasil]

Nice online sudoku at www.rci-jeux.com/jeuxwebdemo/sudoku/sudoku.html
rcy-jeux.com everyday sudoku

[Pentacle]

Well, it definitely works! It just takes a long time to write everything in and erase it.

[zheyman]

Too tedious ?? Just one click fills them all in at http://www.SudokuByZol.com

Comprehensive and extremely efficient pencil-mark support and user interface.

Play never ending supply of puzzles (5 Levels), or enter your own puzzle using the Solver link or button.

[camcuru]

This is the very first step to solving Sudoku puzzles, but it will only get you so far.
This method will NOT be sufficient to solve 197 of the 200 Sudoku puzzles found in the Extreme Sudoku book by Antoine Alary ( sudoku.alary.org ).
Get that book if you want a real challenge, it even has the pencilmarks pre-printed in half of the puzzles, as most of the advanced Sudoku solving techniques make use of them.

[zheyman]

You can enter the puzzles from that book into http://www.SudokuByZol.com and solve them online using the techniques here and others.

Efficient keyboard or mouse user interface that won't get in your way.

Just click the Solver button or link.

[ToddisI]

Pure amazingness!!!!

[KLUTZYengineer]

Is it plausible that this "system" not work properly.
If not - Go to websudoku, click "Evil" and attempt to solve 2,200,342,256, it should appear to be the first one it shows, if not search it. It would be very helpful if anyone is capable of solving this sudoku. Reply to me through my inbox if you have conquered the EVIL SUDOKU.

[puzzlemansc]

It is solvable You should download simplesoduko It will let you design your puzzle and teach you so many methods such as xy wing, coloring, etc. Answer is 849635721 271849653 563127984 714352869 382916547 695784312 157463298 436298175 928571436

[jss1701]

Nice puzzle.  My SudokuSolver program, which uses basically the same method proposed by this article, was unable to solve it, or even get very far, on its own.  I only had to solve a few numbers manually to get it there, though.

[axiesdad]

2,2etc. is solvable. My method is to go through putting a one in every square that isn't blocked by a one already in that row, column, or quadrant; then repeat with two, then three, etc. When you finish the nines, any square with only one number in it is solved. This should eliminate some additional numbers. When you have gone as far as you can with this, look for pairs of squares with two matching numbers. Example: if two squares in a column have 1 and 5 (only) in them then no other square in that column can have either a 1 or a 5. This can also be done for three squares that share the same three numbers. Finally, if a number appears multiple times in a quadrant, but only in one row or one column in that quadrant, then it cannot be in any other quadrant in that same row or column. I hope this helps and that I haven't only told you things you already knew. Thanks for the heads-up to this puzzle. It took me about a half hour to solve and was a lot of fun. Good puzzling to you.