Solving a Sudoku PuzzleEach sudoku-cell has a number of possible candidates, i.e., digits that are allowed according the sudoku rules. Our SuDoku Assistant can display those candidates and keep them updated during the game. The candidates will sometimes form patterns that are good to know about. We will consider them below.Determine the candidateIt is sometimes possible to determine which digit a certain cell must contain. Let us look at two examples.Lonely candidateThe first example is trivial: If a cell only contains one candidate that must be the cell's digit!Lonely cell
The next example is somewhat more sophisticated.
Assume that only one cell in a 3x3-block allows the digit 7.
Even if this cell also has other candidates
we can conclude that it must contain the block's 7,
since no other cell has 7 as a candidate.
The same thing can be said about lonely cells in rows and columns.
Eliminate candidatesIt is often possible to eliminate some candidates in a cell. Below, we look at some examples:IntersectionsTwo rows cannot cross each other, and neither can two columns or two blocks. A row can cross a column but the intersection is only one cell. It is more interesting to focus on the intersection between a block and a row or a column. Let us focus on rows, and on the digit 7.
Double twins
Assume that a certain cell only has two candidates, for example
3 and 7.
If there is another cell one the same
row/column/block that also only allows 3 and 7,
then one of the cells must contain the digit 3 and the other cell 7.
We may therefore eliminate the candidates 3 and 7
from all other cells in
the block, row or column.
Rectangles
Assume that a certain row only allows
the digit 7 in two cells (in other words in two columns).
Assume that there is another row that also
only allows the digit 7 in those two columns.
Look at the figure! It has four green corner cells.
Two of them must contain the digit 7,
and they must occupy both columns.
We can therefore eliminate 7 as a candidate
in all the red areas.
More informationThe wikipedia's SuDoku information.Our own SuDoku Assistant |
 svensk sudoku
|
Assume that all 7:s one a certain row are constrained to
the same 3x3-block!
The row should contain exactly one 7, and that digit
must belong to the intersection between the row and the block.
This means that the remaining six cells of the block cannot contain a 7
(since the block only has one 7).
The figure illustrates this. No cell in the gray area has 7 as a candidate.
It means that the 7 of the row must belong to the green area.
We conclude that the red area cannot contain a 7.
The opposite pattern is also important.
Assume that only one row within a block has 7 as a candidate.
The one and only 7 on that row must belong to the intersection between the row and the block, and hence we can eliminate
the row's 7:s outside the block.
