To ease the resolving of Sudoku problems it is possible to mark candidates for fields. Just press the desired digit one up to nine in combination with the <Ctrl>-key. The program displays the candidate either using small digits or the Watch Hand Method according to the corresponding option. The pictures to the right shows a field with the candidates 1, 4, and 9 (the left one show candidates using digits, the right one using the "Watch Hand Method"). Certainly, these marks are printed out (however, if there is too little space to print digits, the candidates are printed out using the "Watch Hand Method"). To eliminate a candidate from a field, just press the candidate digit in combination with the <Ctrl>-key again. The validity of candidates is not checked by the program, i.e., it is without any difficulty possible to enter a candidate in a field for which the program knows that this is totally senseless...
In addition to mark cells with candidates it is also possible to mark cells with "nagative candidates", i.e., values that cannot -- according to the users opinion -- be in that cell. To enter such a negative candidate press the <Shift>- and the <Ctrl>-key together with the negative candidate digit. To remove a negative candidate from a cell just press the respective key combination again.
Normal candidates are shown (or printed out) in green, negative ones in red.
Since the marking of candidates uses the same marks as the options "Show Possible Values" or "Print Possible Values" these options are switched off when entering a candidate.
To remove all candidates select the menu entry "Problem|Clear Candidates".
The Sudoku application allows both solving problems as far as generating new ones. The application supports both normal classic 9*9 Sudokus and X-Sudokus (also with an edge length of nine). The use of the Sudoku program is completely self-describing: Select simply a field with the keyboard or the mouse and enter the desired number. This also could be done using the mouse: Turning the mouse-wheel upwards increases the value, downwards decreases the value in the cell. And if you have made a mistake: Just undo the last input and try again. When solving the problem interactively it is possible to mark "candidates" for cells. The language of the user interface of the application can either be English or German. It is possible to save problems and reload them later. Problems can be printed out with or without their solutions. It is also possible to generate "Booklets" with a definable number of problems (an example can be downloaded). These booklets can either contain newly generated problems or existing problems generated in advance. All newly generated booklet-problems are by default automatically stored in a definable directory (see "Other Options"). When solving a problem interactively you can either check the validity of the problem on demand (by pressing <F7>) or set the "Auto Check" option. In the latter case, the application automatically gives you a hint, if the entered number leads to an unresolvable problem. If you need help, the application can give you hints by pressing <Ctrl-F1>. This feature marks those cells, in which only one number can be entered and the problem remains valid. The problem can always be automatically solved by pressing <F4>. If the corresponding option is checked, this can be tracked. Another option lets you define whether or not all possible solutions for a given allocation should be found. (Certainly, an allocation with more than one solution is not a valid Sudoku problem, however, these allocations are not rejected from being solved. BTW: The empty Sudoku problem has roundabout 6.7*1021 solutions...) Pressing <F8> shows an info box about the current problem, including a "Severity Level" ("trivial", "easy", "intermediate", and "hard"). Another nice feature of the Sudoku application is the possibility to set the option that an indicator of the possible values of each cell can be showed and/or be printed out either using digits or "watch hands". For detailed information about the "Sudoku" game, please refer to this article.
The Sudoku application computes the severity level of a problem on the basis of the number of occupied fields and the distribution of the pre allocations on the playing field. Furthermore the tactics necessary to solve the problem are taken into account. The computed values are standardized on the range from 0 to 100 and assigned then to the four different severity levels ("trivial" < 10, "easy" < 27, "intermediate" < 60, and "hard" for all the rest). The info box, which can be called with the help of the function key <F8>, displays both the severity level and the internal numeric value. Problems with the severity levels "trivial" and "easy" can be completely solved without backtracking, i.e. at any time there is at least one cell which has a definite assignment. However, it might happen that a problem which is very easy to solve for somebody is rated as "hard" or the other way around.
A X-Sudoku is a variant of the normal classic Sudoku. For X-Sudokus additionally to the rules of the classic Sudoku the two diagonals may only contain the numbers one to nine only once. The application marks the diagonals for X-Sudokus with a light contrast (depending on the contrast setting). For printouts the diagonals are also highlighted. New X-Sudokus are created either by selecting the menu item "File|New|X-Sudoku" (empty X-Sudoku) or by selecting "Problem|Generate|X-Sudoku". For booklets it is also possible to define whether or not it should contain X-Sudokus.
The "Watch Hand" method draws a little dash for each candidate value, which can be entered in a cell or for a manually entered candidate. The picture to the right shows which dash corresponds with which number (this looks a little bit like a watch, thus the name of this method). The idea for this is from the German Sudoku-Wikipedia article.