## Minimzing/Optimizing Pencilling In

Advanced methods and approaches for solving Sudoku puzzles

### Minimzing/Optimizing Pencilling In

Do you have any hints for minimiing/optimizing pencilling in. I love doing sudoku, but the pencilling in task is kind of tedious. Although I'm sure that with more practice,I'll improve, but at least for the time being I'm interested in techniques for optimizing the process. By looking at the puzzle, is it possible to identify areas where pencilling in is likely to have the best effect. And even if this is so, does this likely just defer later more extensive pencilling in, or does it likely reduce it? I understand that any intial solving for cells prior to pencilling reduces the overall pencilling. Just wondering if there are any other effective approaches for reducing it (other than using one of the solver programs, which do work great for automatically creating and updating the pencilling).
lennyh

Posts: 5
Joined: 26 August 2005

Often pays to choose between pencilling based on the box or the col / row. Often makes things clearer by agreeing your candidates based on the unit with maximum entries. ( probably mother / eggs / suck etc but as a newbie it helped me a lot)

stuartn

stuartn

Posts: 211
Joined: 18 June 2005

What's working for me, I'm likely to change as I improve (assuming I do), is reserving a line at the top of the cell for box-specific marks which I enter when I'm doing the first, fast go-through. I reserve the bottom of the cell for a line representing remaining candidates for that cell, I usually don't even start entering those until I'm well along, and I choose which cells to start with by a quick visual over-all so as to choose the cells with fewest candidates. Doing it that way (and only attempting as far as the published "Hards"), I rarely need to enter more than quads, maybe an occasional quint. Hope I made sense. This system is a main reason why I still sudoku (yes, verb) by hand, w/o computerized pencilmarks.
Doyle

Posts: 61
Joined: 11 July 2005

hi - I pencil right in the boxes but only in the boxes that hold only two possibilities. I find that if I do this carefully that there is no need to resort to any of the x-wing or swordfish devices (which I haven't mastered anyway). If I find myself marking 4 numbers in a box, then I haven't paid enough attention. Many boxes will be left empty. Enough boxes will contain 2 and only 2 possibilities, and from these you can solve enough that the boxes which held 3, 4, 5 possibilites start to narrow down to 2.

I'm sorry this is so vague. I haven't had to articulate it but I swear it works. I have only been doing The Times puzzles, but it works for all. I've only been stumped on one or two Sunday fiendish puzzles so far.
LittleBird

Posts: 3
Joined: 11 August 2005

LittleBird wrote:I've only been stumped on one or two Sunday fiendish puzzles so far.

That's because these require X-Wing, which you say you haven't mastered yet.

G
george-no1

Posts: 150
Joined: 20 May 2005

I may be rubbish at the hard stuff but I reckon I'm an ace at sifting thorugh the early stages. I'm even more abstemious that Little Bird in what I write in, limiting myself to pairs or triples in a line in a box. So I won't write a pair down if they aren't in the same row or column. That way you can trap those locked rows and columns that are so difficult to spot once you start littering the field with candidates (as you surely must at a certain point).

I found the little device described on this thread as a real boon when it comes to a speedy populating of lines in a box:

http://www.sudoku.org.uk/discus/messages/2/146.html?1124446262

and if you're lucky you may even find you've solved some new cells ahead of time.

And I'm with george-no1 on the x-wing stuff - it'll transform the amount of fun you can extract and the filtering facility in Angus's freeware helps hugely with the boring stuff while not diminishing the skill and satisfaction of discovery.
Max Beran

Posts: 57
Joined: 17 August 2005

### Reference to X-Wing and other advanced (for me) techniques

for Geaorge-no1

Our local newspaper only occationally requires advanced techniques for its Sudoku entries. This does not give me much practice.
Are there books or other learning tools with technique-specific practice puzzles, for each of the more advanced techniques?
Guest

Posts: 312
Joined: 25 November 2005

Pencelling revisited: After doing multiple sudoku using Angus Johnson's excellent Sudoku application, I tried going back to manual solving. with a random puzle from paulspages rated hard (22 pre-filled cells). Just to set the context, I've been having good results with "hard" puzzles generated using Angus's "Simple Solver". Perhaps I'm taking the wrong approach, but the required pencelling-in associated with my attempt at solving, sans PC support -- was just too tedious -- not fun at all. I was spending the majority of my time pencelling and erasing with little time spent doing the fun part -- actually solving. I assume that lots of folks out there are solving hard -- or worse (or better, depending on your point of view) puzzles w/o PC support? How do you minimize the laborious pencelling and erasing and keeping everything correct? .
lennyh

Posts: 5
Joined: 26 August 2005

Fred-the-novice, the following is excellent:

http://www.angusj.com/sudoku/hints.php
lennyh

Posts: 5
Joined: 26 August 2005

Guest wrote:Are there books or other learning tools with technique-specific practice puzzles, for each of the more advanced techniques?

When I want to solve a hard puzzle that requires X-Wing and above, I do the 'extreme' puzzles from angusj's Simple Sudoku program, and I find them quite hard enough (They require Swordfish too, I'm not sure about XY-Wing, maybe angusj can clarify this). This is freeware from www.angusj.com/sudoku . As for books, I haven't come across an anthology of really hard ones, but there are quite a few hard ones floating around on this site too.

G
george-no1

Posts: 150
Joined: 20 May 2005

george-no1 wrote:(They require Swordfish too, I'm not sure about XY-Wing, maybe angusj can clarify this)

Sorry, as a courtesy to Pappocom, I wont discuss features of Simple Sudoku in this forum.
angusj

Posts: 306
Joined: 12 June 2005

### Symmetrical vs. Non-symmetrical Sudoku

This is a bit off subject (and I'll repost as a new topic if that is preferable), but can someone clarify the distinction between a symmetrical and non-symmetrical (or asymmetrical) sudoku?
lennyh

Posts: 5
Joined: 26 August 2005

I'm not sure, but I think it goes like this:

Symmetrical = clues can be reflected in the line x=y

Non-symmetrical = clues are not in any sort of specific pattern.

Correct me if I'm wrong.

G
george-no1

Posts: 150
Joined: 20 May 2005

george-no1 wrote:I'm not sure, but I think it goes like this:
Symmetrical = clues can be reflected in the line x=y

Here's an excellent list of possible symmetries:
http://forum.enjoysudoku.com/viewtopic.php?p=7632#p7632

Anyhow, unless specified otherwise, symmetry in relation to Sudoku almost always refers to rotational symmetry.
angusj

Posts: 306
Joined: 12 June 2005

Thanks for the pointer to the clear symmetrical/non-symmetrical explanation. One related question -- is this strictly an esthetic consideration? That is, does it have any relationship to solvabilty/correctness? To put it another way, why would anyone care if the puzzle is symmetric or not?
lennyh

Posts: 5
Joined: 26 August 2005

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