Solving an easy puzzle - Using pencil marks
In this section we are going to see how pencil marks can help you to quickly complete an easy puzzle.
Pencil marks are usually small numbers that you pencil into the grid to list the possible candidates for each cell.
Candidates represent the numbers that can go into a cell without conflicting with the basic suodku rule that no number can appear twice in the same row, in the same column or in the same box.
The puzzle
If you have completed the first part of this tutorial you should now have a grid partially filled in that looks like the image on the left. To automatically calculate and display the pencil marks, press the "Make pencil marks" button above the grid. The candidates should display on each empty cell.
You can hide or display the candidates by clicking on the 'hide/show pencil marks button'.
If, after you display the pencil marks, any cell remains empty, it's an indication that a mistake was made somewhere during the previous phase.
The first thing we see is that cell E6 has only one candidate. This is because all other numbers are already present on the same row (3689), or on the same column (123468), or in the same box (13568). The only number that does not appear is 7. This would have been more difficult to see without using pencil marks.
We enter number 7 in E6.
On the left of the number we just entered, on cell E5, we see candidates 4 and 7. But we just entered a 7, so we know that cell E5 can only contain a 4. Note that the candidates don't update automatically. You can update all candidate on the grid by pressing the 'Update pencil marks' button. We don't need to do that now.
Enter a 4 in E5.
Again, cell E4, to left of E5, has candidates 2, 4 and 7. We just entered a 7 and 4 in the same box, so we know that cell E4 must contain a 2.
2 → E4 (type a '2' in E5).
Now we put a 9 in F4 because it's the last empty cell in the box and 9 is the only number that is still missing.
9 → F4
Cell H4 must contain a 7 because we just entered a 9 in the same column.
7 → H4
B4 takes a 1 because 7 and 9 already appear in the column.
1 → B4
4 → A4 (we just entered a 1 in B4).
4 → A3
1 → A2
We have entered lots of numbers, so it's time to update the pencil marks. To do that click on 'Update pencil marks'.
We have entered lots of numbers, so it's time to update the pencil marks. To do that click on 'Update pencil marks'.
The updated candidates show us lots more easy to fill cells:
5 → E9
4 → G3
5 → G2
1 → E1
4 → D1
5 → F1
Click again on 'Update pencil marks'.
2 → F3
9 → D2
9 → C3
2 → B2
4 → C2
4 → F9
Click 'Update pencil marks'.
3 → B2
2 → D9
5 → B7
1 → C8
2 → C7
5 → C5
9 → B6
7 → B5
9 → H5
5 → H6
Update pencil marks
You see now the puzzle is solved. Just for completion:
3 → D8
3 → I7
3 → I7
1 → D7
1 → D7
5 → I8
We are done!