It is becoming clear fom the poll that there are many people who find that just getting started with sudoku is their biggest problem and so I thought I’d start a case study – showing how to play sudoku from the start. What I have decided to do, is to actually solve an easy sudoku puzzle for the next few days, step-by-step. I hope that this will show you what happens for me, when I am trying to play sudoku. Please feel free to explore the sudoku technique pages as we look at this puzzle. They will help you to understand the process I am following.
I have created a starting grid which looks like this:
Initial steps in learning how to play sudoku
The most basic thing to do when you start learning how to play sudoku is to see if there are any rows, columns or boxes that have just one cell left to fill. That will always be an easy win when you are getting started. It is unusual to see this but it does happen, particularly for easy puzzles.
When I see a grid of any kind, easy or hard, my next reaction is to see if there are a lot more of one given number than any other. This grid does not seem to have many like that, except for the number 8 but that always seems to be in a row of three boxes where they have all the 8’s filled.
The next thing I normally do, is to see if there are any givens in two boxes out of three (across or down). Checking through the numbers 1 to 9 in order, here I eventually get to 9 and see that there are two 9’s already filled in the top boxes. Checking to see what those 9’s affect looks like this:
I have highlighted the effects from one 9 in red, the other in green. You should be able to see that they cross over in the top-center box and leave only one cell where the 9 can then go in that box.
Once I have found a value, I always go back round and check that this has not made any obvious changes to things I have already looked at. So, for example, you may find a house with only one cell unfilled after any change you have made.
After this, I usually look to see if the givens that exist have other interactions of the kind described in the box-line post and the box-box post. In the following diagram, I have highlighted the areas of influence of the 7’s in place already.
Look closely at the top-left box. You can see two possible locations for the 7 in the cells 1,2 and 2,2 and no other places in this box. These two possibilities mean that there can be no other 7 in the second column and so if we look down to 8,2, this cannot be a 7.
This is one way we can use this kind of “pointing” to eliminate other possibilities. In this case it means that there must be a 7 in the only cell left in the lower-left box, in cell 8,1.
Your homework for today is to check the rest of the grid to see if any other interactions exist. Look back at the sudoku techniques series if you want some clues to these easy steps and head to “learning to speak like a sudoku solver” if some of the terms I am using are unfamiliar.
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