I suspect most of you have seen these “logical” puzzles
in many newspapers (not to mention little books).
They consist of putting numbers into squares so each
big square of nine squares has every number, one
through nine. Likewise, so does every row and every column.
The easy ones are too easy, and the hard ones are substantially
impossible. But the moderate ones are fairly challenging
and satisfying. Sudoku is a big time-waster, and I won’t recommend
anybody to take it up. But if you have some time to burn,
well, sudoku will take up a lot of it. Each puzzle may take 20 to
60 minutes—if you don’t make any mtakes.
How come my wife can do a puzzle, but when I try it and get
stalled, she can’t show me where to find the next legal move? A
move that she surely just made? Maybe she can’t recall where
to look for the next move. Last night, I was working on a fivestar
puzzle, and I found it easy. She started on the same puzzle,
and she got stuck! She could not find the
next legal move.
It took me about nine minutes to find
the legal moves that would let her continue.
It was, of course, in the ~ last place
I looked. She had overlooked a couple of
small moves. Then she had no problem
finishing it. But it sure was challenging
for me to find that move!
TIPS AND TRICKS
I hate to guess. Many very difficult or
five-star puzzles force you to guess,
because after a while, there are no unique legal moves you can
make. (Or, to be precise, too many legal moves, and none that is
uniquely permitted.) I have looked for advanced sudoku techniques,
but most of them aren’t helpful.
Recently I’ve decided to cheat a little and put in one number
from the published solution. I can usually finish most of them
if I make a reasonable guess of which number to choose.
Some books recommend using “Ariadne’s Thread,” which is
just a form of guessing. You make your guess and see if the puzzle
plays out. If it doesn’t work, you “back up” to where you got
stuck and started guessing. I do this by putting a small number
in the lower right-hand corner of a square. If that doesn’t work,
I put my second guess in the lower left-hand corner. If that
doesn’t work, I give up.
My wife does them with pencil and eraser. I use a ball-point
pen and just cross out numbers that have become forbidden.
She likes to work on a grid about 0.75 in. square. I find that
very hard, and I use a grid that’s 1.2 in. wide by 0.9 in. tall, as
big as I can fit on a 8.5- by 11-in. sheet.
She likes to start by putting in all the easy numbers when
there are several numbers given. I like to start with the ones
and progress right up to the nines. That way, I know that a tiny
number between a four and a six must be a five. But that does
not explain our differences. We have checked several times, and
even though we use different techniques, we come to the same
intermediate states. Usually.
GETTING A DO-OVER
Sometimes I find I have made a mistake. I wish I could take
back a lot of my moves, sequentially, back to a certain point
where there are no errors and then start forward again. But
without an infinite amount of record-keeping, you can’t back
up a sudoku game.
Can I get a computer program to let me “back up”? To enable
me to go back and see where I made a mistake? And then
list all the “possible” numbers and erase
them when they become forbidden?
Most computers aren’t set up for this.
I don’t want a computer that can play
the game for me, but I think it would
be fun to document the flow. Easier to
back up! And I am certainly not going
to start writing software for a project as
monstrous as this!
Sometimes a two-star puzzle can be
quite hard or tricky. Sometimes puzzles
that are alleged to be four or five stars
can be easy. These “ratings” are quite
arbitrary and inconsistent. Beware of some “Latin Squares”
that look like a sudoku but may have two or more solutions. I
once did a “monster” sudoku that was four by four by (four by
four) squares, but I don’t need to do that very often. Maybe if
I’m stuck on a desert isle.
Anyhow, it is a nasty time-killer. When I have a short plane
trip, I bring several sheets of graph paper so I can do the puzzles
from the airline’s in-flight magazine. When I have a long
flight, it just ruins my spare time!
And I promise to not bore you about my adventures solving
cryptoquips. But these, too, make pretty good mental exercises
to keep your mind sharp.
See associated figure