Teach Yourself Sudoku

Learn the secrets that let you easily solve Sudoku puzzles faster!

Lesson #4. Some tips on writing POSSIBLE numbers in cells


In theory, you can write down the POSSIBLES in any way you want.  You can write them down in random order, write them so tiny they fit on one line, write them vertically, horizontally, in groups of 3 separated by semi-colons, etc.  As long as you can keep track of them, do it in any way that pleases you.


However, I’d like to offer a few suggestions for your consideration that can save you lots of time and that will effortlessly increase your accuracy and ultimately help you solve more Sudoku puzzles in less time. I make that claim after playing tons of Sudoku puzzles and anything that reduces your chances of making an error is welcomed by me and perhaps you’ll feel the same way.


1. ERASE THAT! Since the game play of Sudoku is based on reducing a list of  numbers until only one is left in each Cell, be sure that you have enough separation between your written numbers to be able to get at each one individually with your eraser. The alternative technique is to put a slash or dash through the numbers that have been eliminated. To me, that’s like leaving chess pieces that have been captured on the board but marking them with a red dot. It creates a very cluttered puzzle grid. Of course, you can always try the slash technique for yourself and see if you prefer that to erasing.  Either way is perfectly valid.  If you don’t leave enough space between numbers, you’ll find yourself erasing adjacent numbers and you’ll have to stop and recalculate / rewrite the missing numbers.  Even worse, you might not notice that you erased another number and if it was the right answer for that cell, you’ve now made solving the puzzle impossible to do.  (Of course, you won’t know that until you’ve spent lots of time pulling your hair out trying to figure out why you can’t solve this one.) And while it is possible to erase all of the numbers and then immediately rewrite the remaining ones, most experienced players tend to avoid doing that as it is both time consuming and error prone (every time you have to look at a list of numbers, erase them, and rewrite them, you introduce the possibility of writing down the wrong numbers.) So make it easy on yourself.    Leave some room between the numbers so you can easily erase them and save yourself the trouble of rewrites.

2. ORDER IN THE CELL: If you put your POSSIBLES in ascending numerical order, it makes checking the contents of a cell much faster and less prone to error.  Here’s an example using two number strings: 35789 and 73985.  Now imagine that these are written into a tiny cell on a Sudoku grid surrounded by 79  others cells containing lists of numbers and you need to know if there is a 5 in either Cell.  In the first instance, you start checking “3…5…Stop!.  In the second, you’d have to go all the way to the end “7…3…9…8…5…Stop!” before you found your number.  Now you might say that this sounds like I’m getting really picky, after all, how much time does it take to check all the way to the end of a string of 5 numbers? And the truth is, not much if you’re doing it only once.  But when playing Sudoku, you’ll be doing that hundreds of times each puzzle.  That’s when the time savings of putting things in numerical order add up.  Plus, using ordered numbers will increase your accuracy and being accurate is a MUST when playing Sudoku as a single wrong number makes the entire puzzle wrong and that can cost you enormous amounts of time to correct which can lead to frustration which is “not fun.”  Better to put the odds in your favor and get your numbers in order.

3. UPPERS / LOWERS:   If a cell has more than 4 POSSIBLES, I prefer to divide them into 2 segments: 1 through 5 and 6 through 9.  I then write the entries for the 1 through 5 segment in the upper half of the cell and 6 through 9 in the bottom half.  This cuts my number checking time in half as I only have to check either the upper or lower string depending on the number I need to know about.  When there are just 2 or 3 numbers, I usually write them all on the same line even if they are a mix of low and high numbers (e.g. “2 7”, “3 9”, “2 4 8”, etc.)  I usually write these slightly larger than when I write using the Uppers / Lowers technique just because that makes them easier to read and erase.


Here are some sample ways that show some of these ideas in action (not a real Sudoku puzzle.)





NEXT UP: Lesson #5.  ONLY ONCE


Copyright 2006 Gary Ward All Rights Reserved