Sudoku

Uh-Oh. Now I'm getting serious. Just did an "Evil" in under 15 minutes.

Damn you're good.

I did the evil in under 20 minutes, but I made one mistake early on - had to back track.

You guys must be pros.

I guess, after a while you know the moves.

You do learn the moves. It's harder on screen because I usually make little notes on the puzzle in the corners of the boxes.

Yeah, I find the screen a bit harder, too, I like being able to write little possible numbers in the boxes. I think I really got it a few weeks ago, when I started just seeing the patterns, e. g. there's 2 sevens in 2 rows, the third seven must go here, that sort of thing. A little tough to explain.

I started off doing the notes, as well.

The first one I did on line seemed harder at first, but when I "adjusted" I realized it was easier to see the "pattern" or take in the whole puzzle in order to pick out things like:

1. Which box had the most numbers to start with

2. Which number was most prevelant in the over all puzzle - makes it easy to figure where the last ones should go.

3. Which row or column is most complete - start with that if 1 & 2 don't jump out.

Very well done squinney. It took me over an hour to do an "evil" last night, but I was watching Arsenal v Villa Real and drinking a bottle of red wine...that's my excuse anyway.

Glad you guys are interested in Sudoku. I find it very relaxing...in between the bouts of mental torture.

I just discovered in the options that you can check something called "pencil marks" which allows you to enter more than one number in a box. That speeds things up considerably.

Cheater! Heh. I cheat by checking my progress.....

I confess to having done that when I was stuck. I try not to... but sometimes I just can't see.

I dont see how its possible to cheat! I often pencil in alternatives. Witht he harder ones its the only way forward imo.

the only "cheating" I've done is to print off a grid, take an hour filling it in, then go back to the online puzzle and fill it in in a couple of minutes...to go top of the class

but I only did this to confirm I'd got the right answer...honest.

I'm going to write some basic rules for sudoku...see if you agree with me...later.

If you know, for example that the 1 for a row goes in one of two boxes, you can pick one, enter a 1, and click "how am i doing" and know immediately if that is the right one. That's kind of cheating because it's guessing and asking the computer if you're right. I've done it when I was stuck, but I do try not to.

-for those who are new to Sudoku, and indeed for anyone else to comment on-

First some definitions. I call the puzzle a grid...(sounds more adult )

It has 9 vertical "columns"
and 9 horizonal "rows"
making 81 "squares" in total
with each 3*3 square making up a "box".
three boxes in a line (vertically or horizontally) make up one "segment" of the grid. The grid thus has six segments.

First quickly scan the grid to spot any frequently recurring numbers. There might be four or five 6's but only two 7s or no 8 at all. In that case it might be a good idea to try and fill in the remaining 6s rather than the 8s!


The first technique I call projection. Check out the position of all the 6s. Then for each one "project" the 6 along its column or row, into a box that has no 6. As the box must have a 6 in it, it cannot be in the three squares into which you have projected the 6 or there would be two 6s in the row or column. This works best where you can project two numbers into a box without that number, because it means you have eliminated six squares out of the nine in the box where the 6 cannot be. The 6 must be in one of three (maximum) positions. Often one square or more already has another number in it. Sometimes two squares are occupied...in that case bingo...the 6 must go in the single empty square. But if there is two or three positions it can go, look sideways i.e. through 90 degrees. Is there another 6 in an adjacent box that you can project into the box you are interested in? If so does it eliminate any of the squares where the 6 could go...? Is it possible to narrow it down to one square? Then that is where the 6 must live.

Use this technique all over the grid, working horizontally and vertically, projecting as many of the number you are interested in into the box that doesnt have that number. Sometimes its pretty hopeless, but if you spot two numbers in the same segment, its often straightforward to see where the third number in that segment must go.

Having got a number, immediately check for other similar numbers that can be forced out in that vertical or horizontal segment of the grid. Typically this means looking that the segment 90 degrees to the one you have been working on.

Start with the most commonly occurring number, in this case it was six. They look out for other common numbers.

Then methodically go through all the numbers 1 2 3 etc until you are sure you are not missing an obvious number location.

Sh1t this is taking longer than I thought...more later....if anyone is interested

Here are 2 other tips.

1) (a) When you have a pair of possible numbers in 2 squares in a box, row or column, then those numbers cannot go anywhere else. What I mean is, say you have a row of, I dunno, 4 5 3 7 _ _ _ _ _ . And let's say that you've eliminated every possible number from the last and second-to-last spaces except for 1 and 2. Therefore, for the three spaces immediately to the right of the 7, your only choices are 6, 8 or 9. 1 and 2 are eliminated by definition, because you have two numbers and two possible squares. They either go in as 1 2 or as 2 1 and there are no other alternatives.

(b) This is also true of three numbers for three squares, so let's say you have a row of 4 5 3 7 _ _ _ _ _ and you have 1, 2 and 6 in some combinations in the three squares all the way on the right. They can be 1, 2; 2,6; and 1,2,6 or can all be two alternatives or whatever, so long as they cannot be reduced down to two numbers for two squares (if that's the case, then just use rule 1a, above). Once you have these three numbers for three squares, that means that, by definition, they cannot go anywhere else. Therefore, in this example, the two squares immediately to the right of the 7 must be 8 and 9 in some combination. This is probably also true of four but after a while that gets very messy and complicated. I prefer to stick to these combinations of two or three numbers and not go any further as they tend to be all that's needed.

2) Another thing I've found helpful is to count the number of times that a number appears. Once the grid is completely filled in, you must have exactly nine instances of a number, no more, no less. Hence if you have eight instances of a number, just look for wherever the last instance should go. I don't use this as often as I use 1a and 1b but it can still help if you get really stuck.

Finally, one last thing, I don't pencil in more than three numbers in a square. I've found any more than that just get messy and are not very helpful. If I encounter a square that has more than three possible numbers that can go in it, I solve other parts of the grid until I get that number down to three or less.

I agree with those tips Jes. I was going to get on to the problem of alternative numbers next.

It took me ages to realise that if you have two alternatives in any two squares in a box row or column, then those two numbers cannot be in any other square in the B R or C.

Similiarly three or even four numbers.

Example (alternative numbers in a column)

345
478
45
34
3569

reduces to

345*
78
45*
34*
69

because the 3 4 and 5 MUST be distributed between positions (*) though you dont know exactly where.

So if you have two alternatives distributed between two squares, or three alternatives in three squares, or four in four, then you can eliminate the possiblity of those numbers being anywhere else other than the two three or four squares you have identified.

How are you getting on CJ?

Well Steve, I'm getting faster and faster, and thus I kind of lose interest already.

Over the first Sudoku, everyone went hungry to bed, we had no more fresh
clothes to wear, the indentation on the sofa got deeper and deeper, and I practically just lived until I had solved that darn Sudoku.

With each time, it got easier and easier and now I am searching for my
next fix...

try a hard one...as it were...that should keep you quiet for a while.

Me too.

I love sudoku.

I became hooked when the puzzle first appeared in our daily paper.

One-a-day was NOT satisfying. I was forced to search online to find MORE puzzles.

I found the daily USA Today Sudoku puzzle. But again, that's only one-a-day!

I continued to search and found Web Sudoku--Billions of Free Sudoku Puzzles at my fingertips! The website was immediately placed in my favorite places.

I'm NOT an addict. I can go a day or two without sudoku . . . . but they are like potato chips. Consuming one is never enough when I get started! I need more . . . more . . . more . . . .