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## CHAT LOG for Wednesday, November 29, 2006

12:42 am
WHB

Done
4:58 am
bzikofski

hard one
6:35 am
NJL

a very hard medium
7:15 am
peternl

hi all
7:30 am
peternl

7:36 am
eyeman

challenging for me, but I got it after a couple of guesses.
8:18 am
drwho

Difficulty score 30. Requires cross hatching, missing digit scans and region intersection analysis (scanraid calls it pointing pairs).
8:43 am
saratara

are there really strategies and terms for a puzzle?
8:45 am
drwho

Yes. An elementary introduction can be found at Wikipedia. A more advanced treatment is at http://www.scanraid.com/sudoku.htm.
9:21 am
Minski

Done!
9:25 am
drwho

Saratara question is a good one, because unfortunately there is no universal agreement on terms for strategies and other concepts related to Sudoku.
9:26 am
drwho

I define region much like the Wikipedia article on Sudoku; the 3x3 squares that must contain one of each of the 9 digits.
9:26 am
drwho

However, I think it is useful to also use the term region a little more loosely to also refer to the rows and columns.
9:27 am
drwho

Hence, region interesection is looking at the place where a 3x3 region and a row or column intersect.
9:28 am
drwho

If a particular digit can only appear in 2 or 3 empty cells in a row and those empty cells all lie in the same 3x3 region, then that digit can be eliminated from all other cells in the 3x3 region.
9:30 am
drwho

It works for columns and 3x3 regions, and you can reverse the logic to eliminate the possibility of a digit appearing in the row or column outside of an intersecting 3x3 region.
9:33 am
drwho

I use cross hatching as defined in the Wikipedia article on Sudoku. Wikipedia has pictures and once you see it I think you will agree it is the most descriptive term to describe a techique everone had to figure out before they could solve their first puzzle.
9:35 am
drwho

A missing digit scan is when you consider an empty square by tabulating all the digits used in the row, column and region containing the cell. If 8 of the 9 digits are present, then the missing digit MUST go in your empty cell.
9:38 am
drwho

I don't remember where I saw it, but I agree with the idea that the word "square" should not be used in relation to Sudoku, it's too ambiguous.
9:43 am
spellacked

I usually use the word "block" to refer to the 3x3 areas.
9:46 am
drwho

I like it. Maybe I will too. We could reserve regions to refer to rows, column or blocks collectively.
9:46 am
spellacked

a note on the missing digit scan: sometimes you only have 5 or 6 of the 9 filled in (say everything but 4,6,7 and 9), but two of them must be, say, 6 and 7, because they're both the only missing digits in an intersecting row (, column, or block) and of the two remaining, one *can't* be a 4, so the other *has to* be a 9.
9:50 am
drwho

I think you are referring to the Naked Pairs technique (see http://www.scanraid.com/sudoku.htm).
9:51 am
drwho

Or more accurately, you seem to be describing an application of Nake Pairs analysis followed by a cross hatching scan.
11:02 am
beertje

Yay I'm pro.
11:02 am
zemoxian

congrats!
11:02 am
hurshy

congrats
11:03 am
beertje

Thanks :) Hmm, no Icelandic flag?
11:04 am
beertje

That was the number one reason for registering :P
11:06 am
poreid

Why don't you email Dr God - maybe he can add it for you
11:11 am
beertje

good idea.. right after I finish this puzzle, hehe
11:16 am
UnikeTheHunter

Rats, I made some mistake and I don't have any checkpoint.
11:20 am
UnikeTheHunter

Rats, I didn't recover from that mistake after all. I thought I was doing okay.
11:22 am
UnikeTheHunter

Nuts, I think I'll quit.
1:40 pm
rycaut

done without greens or checkpoints - nice puzzle though, harder I think than some of the recent "hards"
2:11 pm
aki2403

I can't believe it, I forgot to do the sudoku yesterday, and it was a hard too. *cries* :(
4:20 pm
skizzybiz

don't worry aki, this one is more fun anyway :)
4:21 pm
skizzybiz

(he says, 2 hours later)
6:00 pm
Stephanie

Hi!
6:03 pm
beertje

hey
6:05 pm
brittlefish.com

in
6:17 pm
Stephanie

done!
6:34 pm
skizzybiz

hey look, i keep forgetting to finish this
6:34 pm
skizzybiz

stupid work, getting in the way of sudoku
6:37 pm
chebuctonian

hi all
6:37 pm
skizzybiz

how goes?
6:37 pm
chebuctonian

pretty decent
6:37 pm
chebuctonian

you?
6:38 pm
skizzybiz

frazzled
6:38 pm
skizzybiz

but good :)
6:48 pm
chebuctonian

there we go... off to archives now
6:49 pm
skizzybiz

yeah, i've got all of thanksgiving vacation to go back and finish
6:49 pm
skizzybiz

that will have to wait until i'm less busy
6:50 pm
beertje

I just upgraded to pro... I've got 8 still to finish in march, and then most of the puzzles since then
6:50 pm
chebuctonian

I nearly gave up on March 28th's
6:53 pm
beertje

still haven't tried that one.. got something to look forward to then :P
6:53 pm
chebuctonian

some solvers can't break it
6:54 pm
chebuctonian

I'll be resorting to some brute force
7:06 pm
brittlefish.com

crap
7:08 pm
brittlefish.com

finally
8:16 pm
Playwoman

Finally. That was a little tough for a "medium"
8:20 pm
helenkeller

go
8:36 pm
helenkeller

done
9:06 pm
greengeeny

go
10:32 pm
thedave

go
10:53 pm
thedave

pause
10:54 pm
thedave

unpause
11:08 pm
thedave

done!
11:08 pm
thedave

That was tough
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