Teach Yourself Sudoku
Learn the secrets that let you easily solve Sudoku puzzles faster!
Lesson #3. Test the numbers 1 through 9 in each cell against all the numbers in the cellís associated RCB.† Write down any POSSIBLE numbers that are not already present in the associated RCB.
On Your Mark, Get Set, Ö.Whereís the Starting Line?
OK, youíre ready to play Sudoku.† Where do you start?† Well, anywhere you like.† Sudoku puzzles donít have an official† Begin Here location. Your ability to solve the puzzle will be the same if you start in the first cell, the last cell, or any of the other 79 cells in between.
However, if youíre like most smart players, youíll look for an area of the puzzle that appears to be more likely to yield some quick RESOLUTIONS (i.e. the placement of a single number in a cell).† How do you find such an area?† Look for a Block that has lots of numbers already in it AND is connected to intersecting Rows and Columns that are also heavily populated.† Iíve found that the intersecting Rows and Columns are just as important as the Block and that a Block that may not be quite as full as some others can still turn out to be the best place to start because of its Row and Column connections.† So be sure to consider both factors when selecting a starting place.
With a little practice, youíll be able to quickly scan a Sudoku puzzle and make a pretty good guess as to where to begin. There is a slight advantage to being able to pick a good place to start as any Cells that you can quickly be solved will simply make it that much easier to solve the remaining cells but that advantage only lasts until youíve written in all of the POSSIBLE numbers and then re-scanned the grid to identify any cells that can be solved now that everything has been filled in.
OK, Count From One to Nine 81 Times in a Row
Sounds exciting and glamorous, doesnít it?† At this point, you might be asking yourself ďCount from One to Nine 81 times? Why is Sudoku so popular?Ē† Well, despite the simplistic and boring mechanics of starting to play the game, itís the brain stimulating fun of using your logic and pattern recognition skills that makes it worth your while to play Sudoku. Plus, after a few puzzles, all of this mechanical stuff becomes second nature..
So, hereís an example of how to get started on a Sudoku puzzle.† Look at the highlighted areas in the grid below.† It represents a Cell located at Row 6, Column 7 (itís empty and shown in dark gray) and all of the associated cells in its RCB group (indicated in light gray.) To help focus your attention, Iím only showing selected parts of this Sudoku puzzle which would normally have lots of additional numbers in the other grid areas.
Hereís our goal: Find out† what are the POSSIBLE numbers that we can write in the darkened square at location Row 6, Column 7.† To know this, we need to take into account all of the numbers in the highlighted areas: all of Row 6, all of Column 7, and all of Block Right Middle.† Why?† Because the Cell at Row 6, Column 7 is a part of each of those groups and as such, the numbers in any them are automatically eliminated as being possible for the Cell at Row 6, Column 7.† Why? Because each Row, Column, or Block can only have one occurrence of any number form 1 to 9.† So we want to find out what numbers are already ďspoken forĒ so that we find whatís missing.
To start, letís make a list of the numbers in Row 6.† They are (working left to right) : 7,4,5, and 9.
And in Column 7 (Working top to bottom) : 8,3,7,1, and 5.
In Block Right Middle (working left to right, top to bottom): 7, 3, 8, and 9.
Now put them all of the numbers together in one list: 7,4,5,9,8,3,7,1,5,7,3,8,and 9.
Now letís eliminate any duplicates (taking the first occurrence): 7, 4, 5, 9, 8, 3, and 1.
Now put them in ascending order: 1, 3, 4, 5, 7, 8, and 9.
(Note: the number 7 actually appeared 3 times, once in the Row, Column, and Block.† Thatís perfectly acceptable in Sudoku and it doesnít matter if you have multiples of any number as long as it appears at least once.)
So what numbers are missing from our list for this cell?† The answer is 2 and 6. These are the POSSIBLES for our cell so letís write them down.
Voila!† Youíve figured out the POSSIBLES for a cell and weíre done with it for the time being.† Youíd now repeat that process for all of the other empty cells in the puzzle.
Note; Iíve gone to great lengths in this example to show you the step-by-step the details of the process.† In real life, most folks do the ďCount from 1 to 9Ē process in their heads and just write down the final results on paper.† Youíll be doing that soon (if youíre not doing that already) but if you need a little more practice, try using a piece of scrap paper and use it to write out each of the steps shown here (writing down the numbers for a cellís RCB, taking out the duplicates, ordering them low to high, etc.)† Do that a few times on paper and youíll quickly catch on.
Before you rush off to try this on your favorite Sudoku puzzle, be sure to take a quick look at the next lesson..† It has some great tips on writing POSSIBLES that will save you lots of time and trouble and make it easier for you to solve Sudoku puzzles faster.
NEXT UP: Lesson #4.† The Write Way
Copyright 2006 Gary Ward All Rights Reserved