Getting a hint on basic logics
Ask for a hint
You can ask for a hint by typing i. Go back to the last page and take this first problem as an example (you can go back to the initial position by initial).
Now type i and this hint is displayed.
kaidoku-1.0.1> i
Look at R1C2. What number is available?
This is a format of hint to be given when naked single logic can be used. You can just look at R1C2 and think what number is available. Whenever this type of hint is given, only one number is available and the number to be filled in the cell is determined. In this case, you will find that only 6 is available. Naked single is one of the basic logics in sudoku, and the program first evaluates if naked single logic can be applied. Therefore this type of hint is most likely to be shown.
Hidden single
Hidden single is another basic logic in sudoku. When naked single is not available, kaidoku checks if this logic can be applied. Therefore when you ask for a hint, this hint is likely to be given next to the naked single.
Actually in some situations hidden single can be found more easily than naked single for humans. Take Level 3 No. 1 for example. You can change to level 3 by l 3 and this problem is shown (this problem is shown at the top page).
Now if you ask for a hint, this hint is given.
kaidoku-1.0.1> i
Look at R2C2. What number is available?
and if you check R2C2, you will find only 6 is available. You can find more naked single pattern in R2C5 as 3, R2C6 as 4. However, it is easier to find a hidden single pattern for me, in the following way.
Look at 2 in R7C8 and R8C6, and also look at box 7, and think where 2 can be filled in box 7. Here box numbers are defined as follows.
| Box 1 | Box 2 | Box 3 |
| Box 4 | Box 5 | Box 6 |
| Box 7 | Box 8 | Box 9 |
2 in R7C8 eliminates the possibility of 2 in row 7, and 2 in R8C6 eliminates the possibility of 2 in row 8. Therefore the possibility of 2 only remains in R9C3 as in the following table.
| x | x | x |
| 1 | 8 | x |
| 4 | 7 |
Therefore R9C3 = 2 is determined and you can fill it by 932. This is the hidden single logic. Suppose that in a certain row, line or box, therere is only one place that a certain number can be filled. The cell is a hidden single. In this case, R9C3 is a hidden single of 2 in box 7. If you get used to this logic, you can easily find this pattern by scanning 2 in R7C8 and R8C6 horizontally. Hidden single in a box is often easier to find than naked single.
Actually, after filling 2 in R9C3, there remains only 1 cell in box 1 that 2 can be filled, R1C2. Hidden single in 2 in box 1. Now type 122. Then you can keep on finding hidden single of 2 in boxes. Type 652, 592, 372. Now all of the 2s are determined. Next you can determine all 4s successively by applying hidden single in boxes. Confirm it by typing 334, 264, 744. Then you can determine all 1s successively by applying hidden single in boxes. Confirm it by typing 131, 361, 671, 521, 791. Like this way, you can solve this problem by using only the hidden single logic. Often it is possible to successively apply hidden single logic for a same number.
When you type i to ask for a hint in the situation that naked single is not available and hidden single is available, hidden single is shown as a hint. For example, go back to the initial position of Level 3 No. 1 by typing initial, and then type 226, 253, 264, 118, 277, 317, reaching this position
Here you get a hint like this.
kaidoku-1.0.1> i
Hidden single in box 1 can be found.
Now you can examine box 1 and you will find that 4 is only available in R3C3.
Almost all of the problems up to level 3 can be solved with the basic logics of naked single and hidden single. Most of the problems in level 4 can also be solved with the basic logics, while in some cases some more advanced logics might be needed.
If you are not familiar with solving sudoku, I would recommend solving the easy problems with kaidoku, in the level you feel comfortable, and get used to the basic logics. Get hints when you get stuck and you will get used to the logics.
When advanced logic is required
When you try to solve problems in higher levels, you will meet the positions when advanced logics other than naked single and hidden single is required. In such cases, following message appears. It will be discussed in a later page of this document.
kaidoku-1.0.1> i
Think candidates of the cells.
See image by "html".
For more hints, type ii.
When you made a mistake
Suppose that you made a mistake without realizing it, and you get stuck. In this situation, i command will return this message.
kaidoku-1.0.1> i
There is no solution for this position. You can take back one move with b.
You can take back one move with b and think again. You can get a hint again in this position and you may get a similar message. You can keep this until you get a proper hint. Now this is the position where you made a mistake.