Lesson 1
What is Sudoku? Well it is a number puzzle. I will not bore you with any history because you would not be here is you did not know about it already.
So What is this page for. Well when I was looking on the web I found information about sudoku online and some limited information about how to solve the puzzle. Most left you wondering what they meant. So this is my attempt to help people understand the logic be hind how to solve sudoku.
My First peace of advice is to go and get a sudoku program. right now the best one that I have found is simple sudoku. It is free and easy to use. Why use a program. Because filling in all the candidates by hand suck and is very time consuming. It makes sudoku a chore and not something fun to do. Most of the time if you can not solve the puzzle with paper and pencil it is because you make a non logical mistake. Witch means that all the work you have done is for nothing. The program solves this issue and if you get stuck with the logic the program can help you with that too. After a wile you will use the programs help less and less.
Okay where to start. This is an easy puzzle.
So you understand me when I explain things let me define some words for you. A square is the smallest unit on the sudoku board there are 81 of them. A row is the has 9 squares and go left to right or right to left. A column also has 9 squares in it and goes up and down and down and up. Direction does not matter. A box has 9 squares in 3 by 3 there are 9 boxes on the board. So as we should know that the idea is that there can only be 1-9 in a row, column and box. Each one has every number and the numbers never repeat. This is the basic logic behind the whole puzzle. A candidate is any number a square can be. When we lock a number we make it a big number and that is the number the square is in the final solution.
So now back to our puzzle. As we can see there are 3 squares than can only be 8. So we will lock them as 8's. Then We get this:
Because we did this we can now see that there are 4 more squares that only have one candidate in them. two 9's a 1 and a 4. So we are going to lock the squares with these numbers.
Now we have 5 squares that have single candidates. We do the same thing here and keep doing it until we no longer have any single candidates left.
This is the easiest of the logic in sudoku so we will call it logic rule # 1 and we will state it this way.
Rule #1: If there is only one candidate left in a square then the square must be that candidate.
Ok after we take care of all of the single candidates the come up we get to this stage.
So what do we do form here? Well the next step I do is I isolate the 1's and see if there is any thing easy to deal with.
In this case there is not. Lets look at 2's.
Once again nothing easy so we move to 3's.
Nothing here ether so we move on to 4's.
Here we have something. there is only one possible place for a 4 left. The row, column, and box all need a 4 so this has to be it there is no where else it could be.
Here we get the second rule
Rule #2: If there is only one square in a row, column, or box that can be a number then it must be that number.
So we lock it and move to 5's.
Nothing easy with the 5's so lets move on.
Here we have Rule #2 again so we lock it and move on.
Her again we have Rule #2. We only need a number to be the only possible in the row or column or box not all 3 so here we have the only possible number in a column and a box so we know it is a 7 and we can lock it. When we do that we eliminate the other 7 in its row. Now we could have used Rule #2 with the 7 in the middle row. It is the only 7 in that row so it is clearly a 7. But now it is the only 7 in the row, column, and box.
We have nothing clear cut here to do so lets move on to 9's.
Rule #2 we have here too. Lock it and move on. Back to look at the 1's.
Still nothing easy and simple so we move on to 2's.
Nothing so we move to 3's.
Nothing... We do not have any 4 candidates left so we move to 5's.
Still nothing... No candidates for 6's, and 7's so we move to 8's
Nothing simple here there are things we could do but lets not worry about them yet and move on for now if we need to we can come back no need to make the puzzle more difficult than needed.
We are going to look at pairs now.
Rule #3: If you have the same pair of candidates in two square in the same row, column, or box then you can exclude those candidate form the other squares in the row, column, or box the paired candidates are in.
Let's look at what we have above. In the center row we have two squares that have 1 and 3 in them so we know that one of these square has to be 1 and the other has to be 3. Right now it dose not matter witch one is witch what does matter is that we know that 1 and 3 can not be any where else in this row so we can exclude any 1's and 3's form this row with out a problem. That will leave us with this.
Ok this is the start of the puzzle ending chain reaction. We go back to Rule #1. We keep applying Rule #1 until the puzzle is solved.
This one was easy we did not have to do much thinking at all.
This is lesson 1. When you are ready you can move to lesson 2.