The New Sudoku Players' Forum

[Lily]

Hi!!!

I don't know how to solve Sodoku Puzzles, and I now I have to do it, otherwise I am going to be in trouble.

Could anyone help me?



I would be very grateful.

[Cec]

Hi Lily and welcome to the Forum,

If you click on the "how to solve" window box in the ladder menu on the left side of the screen you can read up on the basic idea to fill in the vacant cells.

To help you along the way, the sudoku grid comprises 81 cells having nine rows and nine columns. You'll notice the grid is also 'divided' into nine 3X3 "boxes" numbered in this sequence:

123
456
789

Each row, column and "box" must only contain each digit 1 to 9. Notice there is already a 1 in the first and second row. The missing 1 in the third row must go in "box" 2 because "boxes" 1 and 3 already contain a 1. The 1 must go in row3 column 4 (r3c4) as column 5 already has a 1.

Using this same logic, because the existing 3's in rows 2 and 3 occur in boxes 1 and 2, there is only one place for a 3 in box 3, ie in cell r1c9.
Again, 3's already exist in boxes 7 and 9 (rows 8 and 9) which leaves only one place in r7 for a 3 to go.

Also check the numbers in each row and column to find whether there is only one "missing" number called a "naked single" to fill a cell.

You could also read up on more solving techniques by clicking on HERE

Post again to the Forum if you need more help.

Cec

[QBasicMac]

Cec covered all of what you need to know. Here is another view in case you are still stuck: You now know the basic rule that every row, column and box must finally have one each of the digits 1-9. Well, one thing to look for is a cell which can only contain one of those values.

For example, look at r1c2 (row 1, column 2). What values can it have?

1 No. There is already a 1 in the box (r2c1)
2 No. There is already a 2 in the box (r1c3)
3 No. There is already a 3 in the box (r2c3)
4 No. There is already a 4 in the row (r1c4)
5 No. There is already a 5 in the row (r1c5)
6 No. There is already a 6 in the box (r3c3)
7 Yes. There is no 7 in the row, column or box.
8 No. There is already a 8 in the column (r2c4)
9 No. There is already a 9 in the row (r1c7)

So we see that r1c2=7 because it can't be anything else and it has to be something.

Fill in that cell.

Now look at r1c1.
In its box is 1236 plus the 7 we just solved.
In its row is 124579.
In its column is 12356.
Only missing digit: 8

Fill in that cell.

Now look at r2c3.
In its box is 12367 plus the 8 we just solved.
In its row is 139.
In its column is 1269.
Missing digits: 45

Rats! Two digits! Well, we can't do anything here yet. We want cells with only one digit left. Just go on to other cells and ignore this one for now. Come back later.

Many puzzles require additional techniques, but the techniques described by Cec and I will solve the puzzle you posted. Come back when you get stuck on a more difficult puzzle.

Don't get stuck on this one. It will solve if you don't make a mistake.

Mac

[QBasicMac]

Hi, Lily. You posted a good puzzle to illustrate two of the three techniques for solving simple puzzles:

1) Select a digit and try to place it
Cec showed how to solve r3c4 and r1c9 using this technique

2) Select a cell and hope it has only one legal digit.
I showed how to solve r1c2 and r1c1 using this technique.

Now let's look at the third:
3) Select a row, column or box and try to fill in a missing digit.

This is easiest when the row, column or box has only one missing digit. In fact, it is automatic. The more missing digits there are, the harder this technique is.

Assume we somehow got this far:

Code: Select all

872  45-  91-
13-  9--  ---
-96  --3  28-

58-  -1-  3--
--1  --7  -98
6--  82-  --1

2-9  -6-  45-
35-  -9-  --2
---  2-5  -3-


We easily see that 4 and 5 are missing from box 1.

So we look at cell r2c3. Rats! Both candidates will fit. No progress can be made.

But we look at cell r3c1. Great! We see that 5 will not fit due to the 5 in row 4. So we know that r3c1 must be 4.

Now we have only one cell left to fill and only one candidate. No need to check the rows or columns. Just stick 5 in r2c3.

So you need to master three techniques:
1) Select a digit and try to place it
2) Select a cell and hope it has only one legal digit.
3) Select a row, column or box and try to fill in a missing digit.

Don't force yourself to use only one all the time. Swap around freely. You will eventually get a hunch in any situation of which is best to use for the next cell to be solved.

If this is a single homework assignment and you hope never to see SuDoku again the rest of your life, fine - you have the information you need to do it.

If you get really interested, you will soon run into more difficult puzzles that cannot be solved by the three techniques. We can help then.

Mac

P.S. To watch a puzzle solve itself using the three techniques, go to
http://www.sudoku.funurl.com/
and download "A SuDoku Tutorial"
written by me using (what else) QBasic.