SuDoKu Solver Instructions

Short of space?

Clicking on the 'New window' link opens up a new window without any unnecessary clutter, i.e. there's no menubar, no toolbars and nothing on the page except the puzzle itself. If you're running at a resolution of 800 by 600 you might find this a useful alternative to playing in the main window.


Click anywhere on the Welcome page to start. This will display the familiar 9 by 9 SuDoKu grid in Setup mode. The focus, indicated by the cell having a light blue background, will initially be in the top, left corner.

Setting up the puzzle

The focus cell can be changed by using the left / up / right / down arrow keys, or by clicking on the required cell with the mouse pointer. Typing a number (i.e. 1 to 9) will enter that number into the focus cell and move the focus to the next cell. If the cell already contains a number it will be replaced by the new number.

There is some basic validation performed to help you avoid mistyping a number. It is impossible to enter the same number twice in any row, column or region.

However, as I have discovered, it is still possible to enter a puzzle incorrectly. This will not become apparent until some point in, possibly well into, the solving process. Typically the program will detect that there are now no possible values for an unsolved cell and display the error message 'Invalid state detected'.

You can remove the contents of the focus cell using any of these keys: Delete, Return, Backspace, Enter, Zero, Space.

Typing an 'I' will invert the puzzle along the top/left bottom/right diagonal. A great aid to testing but otherwise probably not of great interest!

To reset, type an 'R' - you will be asked to confirm! Typing 'Y' causes all input to be removed.

When you have completed setting up, type an 'S' to enter solver mode. At this point the basic rules of SuDoKu are applied and for all unsolved cells the possible solutions for these cells are shown.

If you're playing with the downloaded version, and if enabled, typing a 'Q' will load a puzzle from a file. See the ReadMe.txt file for more details.

Solver mode

This is the easy bit! A mouse click or pressing either the Enter or Return key takes you one step down the path to an eventual solution (most of the time). See the Explanation page for a detailed discussion on the solving process.

To reset, type an 'R' - you will be asked to confirm! Typing 'Y' causes all input to be removed and returns you to the setup mode.

Typing an 'S' takes you back into setup mode at exactly the same point as you had got to when you entered solver mode. All solved cells are emptied.

Possible errors

Error messages are displayed in red and (mostly) don't need further explanation. However, for a complete run down of what can go wrong, click here.

Download SuDoKu Solver

If you would like to play offline: download the solver (82kb). Instructions in ReadMe.txt in the zip file. (May only work in a Windows environment.)

Version history

Version 1 - 20/11/2005
Basic setup and presentation plus the first four algorithms, last in row/column/region, only possibility, third row/column solve and by elimination (row/column/region).

Version 2 - 07/05/2006
Presentation improved to better show how the solution, particularly for the later algorithms, is arrived at.

Added exclusive pair algorithm.

Version 2.1 - 28/05/2006
Program enhanced to better detect invalid starting positions.

Version 2.2 - 04/06/2006
Removed the 'feature' whereby it can be necessary to reapply some of the advanced solver algorithms following the solution of a cell.

Version 3.0 - 28/08/2006
Added region/row, region/column elimination and exclusive triple algorithms.

Version 3.1 - 10/09/2006
Couple of minor enhancements to the setup mode:-
1) Set focus cell with mouse click.
2) Invert the puzzle.

Version 3.2 - 17/09/2006
Created downloadable version with the facility to load a puzzle from a file.

Version 3.3 - 08/10/2006
Enhanced the region/row, region/column eliminations to deal with their converse.

Future enhancements

I'm thinking about an undo function so that you can go back a few moves.

I've more sophisticated algorithms to program that will improve the solving ability.

I'm also toying with adding the ability to select the puzzle to be loaded (downloadable version only).

Any other changes will depend on feedback received. All feedback to the webmaster and is most welcome.

Copyright © Bernard Hawkes 2005, 2006.