dailysudoku.com

nataraj

Wouldn't it be nice ...

... to see solving techniques in action ?
... "real" time ?

... to watch while the solving process is going on ?

This test might well develop into a full tutorial. For now, I am experimenting with animation support in PhotoDraw / ImageReady.

Showcase: dailysudoku.com puzzle of Sept. 13 (very hard).

keith · Posted: Wed Dec 26, 2007 2:43 am Post subject:

Nataraj,

Merry Christmas!

This is very interesting! I looked at your message a couple of times before I figured out to go and click on the thiumbnails. Tha animations are great!

What you are showing is how I start a puzzle. First, I sweep the floors - sweeping is carrying two candidates into the third house. Next, I sweep the towers. Except, if I find a candidate in the towers, I sweep that floor again.

Then, I do it again. Sweep the floors. Except, now I am also looking for pairs of candidates. If you find a pair, pencil it in. Then, sweep the towers for pairs.

Next, go row by row. For any row that has four or more solved cells, consider the candidates for the unsolved cells. Look at how those candidates occur in the box and column, seeking candidates that can only be in one cell of the row. Do the same for the columns. Only pencil in candidates if they are pairs.

Now, go box by box. Look at each box that has four or more solved cells, for the candidates in each unsolved cell. Pencil in the candidates for each cell that has only two candidates.

Then, row by row, again. Pencil in candidates for cells that have only two. And, for the columns.

Now, look at the grid. You should have pencil marks in most of the two-candidate (bivalue) cells. Are there pairs you have not yet recognized?

I find it helpful to write the numbers

123
456
789

outside the puzzle, and to black out each candidate as it is solved in all 9 houses.

Look at each candidate. If it is solved in eight houses, you MUST be able to complete the ninth. If it is solved in most houses, look to see how you can complete the solution in the others.

With the above, you can solve almost any "easy" to "hard" puzzle, that requires only singles.

If you have a hard puzzle requiring more than basic techniques:

Scan the pairs to find possible triples, UR's and XY-wings. Look for intersections.

Then, go back to sweeping. Except, now fill in candidates in those cells that have three. Examine the grid for possible triples or quads, and for XY- and XYZ-wings. Also look for possible remote pairs and W-Wings.

Still not done? Well, my friend, it is time to fill in all the candidates and start slogging. (Man, I hate looking for X-wings!) But, coloring is fun!

Keith

nataraj · Posted: Fri Jan 11, 2008 6:43 pm Post subject:

Thanks for the encouragement, keith - glad you liked the animations! Unfortunately (luckily?), right now I have a lot of work so very little spare time... The animations were fun to do, too. Sooo, for the moment, consider this the pilot but don't hold your breath for the next episode (although it might well be worthwhile, what with naked pairs appearing and hidden singles in the closet - uh oh Wink

)

I like your phrase "sweeping the floors". It is very graphic and actually much better than "squeeze". Only problem: "sweep" seems to be already used (in the "play online" section of daily sudoku) in the sense of "Sweep - show all possible numbers for each cell, overwriting your current pencil marks." which is definitely what I DO NOT do at this stage. Maybe we can settle for "mopping up" ? nah, no good.