'Safe numbers'
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'Safe numbers' |
| Idéa
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Relativly easy wou
will grasp the four types of safe numbers that could exist in a
sudkou-problem. Only Place in the Box (OPB), means that in a box one and only one cell is possible for one of the missing digits. The
mathematician will probably argue like this: In the upper box 3,5,7
and 9 is missing. Starting with 3, could be in both upper and lower
row, in other wording not a safe digit. Let us look at 5. Hm.. it
could only be on one place in the box, making it safe. A more intuitive
approach to find this box could be. Which numbers are in the boxes to at the
right of this box. Anything in upper and lower rows there would make a
safe digit. Two 5:s, good let’s place another since there is only
one cell in the box. Only
Possible Number in this cell (OPN), this means that no other digit could be in this cell, all
the other digits are either in the row, column or box where this cell
is. These cells are rather tricky to find but you will probably need
find at least one or two for most sudokus.
Only
Place Across (OPA), means on of the missing digits in a row has only one place to go,
putting it elsewhere would produce a duplicate.
To find OPD:s and OPA:s is much more cumbersome than picking up OPR:s but you could train your eye and brain to find them. As you see these conditions could occur from a digit in any place of the sudoku and that is the main reason why they are harder to find. You will have to search for them a little bit longer.. Many easy
sudokus could be solved without finding OPD:s and OPA:s, often they
are only needed at the end when they are much easier to spot. With correct
hints activated in the Adviser the yellow cell will have a red
corner indicating futher assistans when hovering with the mouse,
telling 9 with reason OPD.
If you dislike our approach to pencil-marks and want to get deeper into sudoku theories there are many place on the Web to investigate. But to be complete as a sudoku-solver you need how and where to make good guesses when the safe digit stops to appear.
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In the above example the 5 is OPB
(Only Place in the Box) since the other fives block the other
empty cells of the box.
The pencilled 9 above is an example of an OPN (Only Possible Number). All the other eight digits either occurs in the row, column or box where the cell is.
This handwritten 9 is an example of an OPD (Only
Possible Down). The upper cell in the column is blocked by the 9 in
that box; next cell is blocked by the 9 to the left while the third
alternative is blocked by the 9 to the right. Only one place in this
column is possible, the selected.
There are many
opinions on how to solve a sudoku in the best way. Some people think
you should analyse candidates and reduce them to spot a safe digit
even in very locked situations. We prefer to do a small investigation
of candidates in a box or two and then try a good guess in locked situations. We like the idea of picking safe digit by hovering like a
helicopter instead of working with the error prone and boring method
of analysing and reducing candidates. The Adviser Nizze is built on
these assumptions making it possible to solve hard sudokus even if you
are a pure mortal. We believe this method combined with good
guessing leads to the solution much faster than the analytic
approach, might not be true for the most diabolic ones though, but who
cares… |
