This page explains Sudoku Solver's GUI controls and interface and how to use it in solving Sudoku puzzles and exploring solution strategies.
The solver window has three main sections:
The Sudoku board is in the upper left. The starting clues are displayed in black; all other cell values are shown in blue. The starting clues are locked and cannot be modified.
The current cell is outlined in red. A value can be entered into the current cell from the keyboard using keys 1-9, plus a-f for larger puzzles. Invalid choices are rejected with a beep. The current cell is advanced automatically after a value is entered into a cell.
The current cell can be moved using the mouse or the arrow, HOME, PAGE UP and PAGE DOWN keys. SPACE or 0 move the current cell forward, wrapping through successive rows. BACKSPACE removes the value just entered, moving the current cell back one; DELETE removes the value in the current cell. Locked or empty cells will not be changed.
The Puzzle Info tab below the board provides detailed information about the current puzzle. More detailed information will be shown if the Detail box is checked.
A new 9x9 puzzle is provided daily, ranging in difficulty from Easy on Monday to Insane on Sunday. A selection of puzzles of all difficulty levels is available under the File menu.
The Undo button in the button panel to the right of the board will undo the last action. Unlimited undo is supported, all the way back to the starting state.
The Redo button in the button panel to the right of the board will redo the next action in the move sequence, up to the last move made. You can move freely back and forward through your move sequence with Undo and Redo. Making a new move or checkpoint clears the memory of Redo actions.
The Go To choice box allows you to display any move result or checkpoint in the move sequence, undoing or redoing multiple steps in a single operation. The choice box shows the move result or checkpoint currently being displayed.
The Checkpoint button allows you to name the current board position for easier access later. If you want to explore the choices at a particular cell, create a checkpoint first so you can easily return to the choice point if a value doesn't pan out.
The Move History tab contains the complete list of steps taken to reach the current position. If a step contains multiple actions they are listed individually, indented under the step. The move history includes any checkpoints you have created.
File>Select Puzzle offers several months of daily puzzles providing a range of difficulties and layouts. The filters on the left allow one to restrict the puzzles listed by group, size, difficulty, and/or layout.
File>New Puzzle creates a new puzzle. You can specify the board size, configuration, puzzle name, group, and comments. The solver supports board sizes from 6 to 16, in regular and jigsaw configurations. See board configurations for a full description.
The optional starting state is entered as a text string of the values of all the cells, one character per cell in row order, with 0's indicating empty cells. Characters other than digits and letters are ignored, so spaces, newlines, and the like can be inserted to improve readability.
To enter a puzzle directly on the board, leave the Starting state blank. Then type the clues into the board, using spaces or zeros to skip cells. If you mistype, use BACKSPACE to back up to the last cell entered and clear it. Once you are done entering the clues, use Edit>Set Starting Position to tell the Solver this is the starting position and lock those values in place. (If needed, Edit>Edit Starting Position will unlock them again).
File>Duplicate makes a copy of the current puzzle under a new name.
File>Export writes the current board state as a string to the Suggestion tab. (The string for the starting state can be found in the Puzzle Info tab.)
File>Load Library loads puzzles from a local sudoku library on your computer. A file chooser will be presented to allow you to select a file. (Web Start only)
File>Save Library save puzzles back to a local library file. A file chooser will be presented to allow you to select a file; the default file will be the last library file loaded. (Web Start only)
File>Exit closes the solver (Web Start only).
Sudoku Solver can support the solving process by tracking possibilities and providing views of the board that focus on the most relevant details.
The solver keeps track of what values are still possible for each open cell. Tracking possibilities accurately is essential for moving beyond the basic strategies, for example identifying naked sets and various kinds of chains. To include this information on the board, check the Notes box in the button panel.
In the example below, the notes make it clear that cell G6 has only one possible value, 5. Single values in a group, such as that the only place for value 2 in the center box is cell E6, can also be found this way.
Notes display simplifies board updates - clicking a note removes that possibility from the cell, SHIFT-click on a note enters that value, and SHIFT-click in the blank sections restores the corresponding possibility (if it is consistent). If you mis-click, just Undo. Clicking on the blank parts of a cell makes no changes - it just moves the cursor to that cell.
Clicking on a column heading highlights or un-highlights the corresponding value in all open cells. This helps identify singletons in rows, columns, and groups.
In the example below, 9's have been highlighted. It is easy to see that the only cell in box 2 (and column 6) that can contain a 9 is A6, so A6 must contain 9.
Shift-Clicking on a column heading highlights that value without removing highlighting of other values; otherwise highlighting of all other values is cleared first. Clicking in the upper-left corner of the board border highlights or un-highlights all values at once.
Checking Links in the button panel graphically displays the dependencies between the currently highlighted values. (When no values are highlighted, Links shows all the dependencies.) A green link between two choices indicates that they are mutually exclusive (same row, column, box, or cell). We call this a weak link. A red link between two choices indicates a strong link, meaning that they are mutually selective - eliminating either choice forces the other. Strong links are the foundation of all chaining strategies, which are the fundamental techniques for solving advanced Sudoku.
In this example, the strong link between 6 at D8 and 6 at D9 (a Pointing Pair) tells us that the 6@D3 can be removed: the strong link means that if D8 is not a 6, D9 must be, and vice-versa - either way, 6@D3 is excluded.
Highlighting plus link display is an effective aid for finding chains over a single value. More specialized views are provided under View>Select View to help in finding more complicated chains such as XY and bilocation chains. See the Solving pages for more details.
Highlight, link display, and View selections are preserved across board steps and undos. To remove them, use the Clear button.
The Hint button provides a general hint as to what strategy to apply next. It is also useful as a quick check that nothing simpler has been overlooked.
The Detailed Hint button provides a more specific hint as to what to look for next. Use it to focus your solving efforts.
If you get stuck, Show Me will give you the next step. The suggestion will be described in a pop-up and illustrated graphically on the board. Click OK to acknowledge or Do It to perform the suggestion immediately.
To generate hints, the solver applies its library of strategies in order of difficulty until one of them finds a suggestion. In many cases, multiple suggestions will be generated. All suggestions generated will be listed in the Suggestions tab sorted by difficulty, and the first one will be illustrated on the board. A value highlighted in red is eliminated by the strategy; a green value is forced. Other color highlights indicate values and cells that participate in the strategy, such as the cells of an X-cycle chain, or the intersecting groups for Group Intersection strategy. See the strategy pages for additional detail.
The Next and Previous buttons can be used to select which of the current suggestions is illustrated on the board. The Detail check box controls the level of detail provided on the board and in the Suggestions tab.
Pressing Clear when a hint or illustration is displayed will remove the hint and restore the highlighting, link display, and view that was in effect before the hint was displayed.
The individual buttons under Strategies - Single Value, X-Cycles, etc - allow you to apply specific strategies to the board to see what they produce. See the strategy pages for details on the individual strategies. As with the Show Me button, the results will be illustrated on the board and explained in the Suggestions pane, and the Next and Previous buttons can be used to cycle through multiple suggestions.
Do It will apply the current suggestion, and Do All will apply all the current suggestions. For example, you can get the Solver to fill in forced choices by selecting strategy All Singles plus Do All. Do It and Do All are enabled only when there are suggestions available.
SHIFT+Strategy does the first suggestion from that strategy. This is useful for quickly executing the simpler strategies, allowing one to move to the harder parts of a puzzle.
Solve>Puzzle Status checks whether the board is correctly solved, complete but incorrect, unsolvable or solveable.
Solve>Start Over returns the puzzle to the starting position.
Solve>Do All Singles will find and fill in all the Singles; Solve>Do All Sets and Solve>Do All Intersections do the same for Naked/Hidden Sets and Group Intersections. These commands get you to the harder parts of a puzzle quickly.
Solve>Solve Puzzle will solve the puzzle from the current position, if possible.
Solve>Rate Puzzle provides a difficulty rating for the current puzzle, from the starting state. Puzzles are rated on a scale from 100 to 700 based on the strategies required to solve them. The rating results include the difficulty level (Easy, Medium, etc), the difficulty score on a scale of 100-700, the number of steps required, and the most difficult step and its rating.
The difficulty scale is:
Help>About provides general program information.
Help>What's New provides a list of recent changes.
Help>Settings>Naming controls the naming presented for cells. The options are "A1" style (the default) or "R1C1" style.
Help>Settings>Colors lets you adjust the coloring of all GUI components.
Help>Information>User Guide brings up the User Guide Web page for the Sudoku Solver (applet only).
Help>Information>Solving Sudoku brings up the Strategy Web page for the Sudoku Solver (applet only).
Excellent explanations and illustrations of Sudoku solving strategies can be found at Andrew Stuart's Web site SudokuWiki.org.