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HoDoKu (Bernhard Hobiger)

by Jocelyn Doire

Sudoku is a game that plays on a 9-by-9 grid that requires putting the digits from 1 to 9 in every row, in every column and in every sub 3-by-3 grid. The rules are pretty simple but trying to solve some of those little puzzles can get terribly hard, making the game a good exercise for your mind, and a good way to pass some time with an interesting game.

The easy levels can be solved all in your mind, but for the harder levels it becomes necessary to write down all the "candidates" that a cell can have in order to solve the puzzle, and anyone who has tried to use a pen and paper knows that it gets pretty tedious to keep track of all of them. Lots of programs exist to play Sudoku, but one that I find particularly good is called HoDoKu, a free and GPL program written in Java that is available on SourceForge at

HoDoKu lets you play the game like you do on paper, select the level you wish and click on the “create a puzzle” button (with the 2/6 in a grid) to create a sudoku grid that is guaranteed to have a unique solvable game. If desired, you have the option to have all the candidates shown in light grey in every cell, which are updated immediately whenever you add a new valid digit (in blue).



If you get stuck you can ask the program to give you a "vague hint" (Puzzle > Vague Hints…) to help you move forward without revealing everything, or click on the "Next Hint" button to show you the next possible step, with a full explanation and with all the relevant candidates highlighted.

You can click on any of the digit filters, at the top, to highlight all the cells with that digit in green, helping you in your search for candidates that can be eliminated.

Where the program really shines is that HoDoKu can be your teacher. For example, you can view the complete path to a solution for any given puzzle, and by double clicking on any of the steps, HoDoKu fills the grid with all the previous steps, highlights all the relevant candidates, all colour coded, and even adds arrows for very complicated techniques.



If that is not enough for you, you can even ask HoDoKu to calculate every single possible step to solve a grid from any configuration and to sort them all in 5 different ways, letting you study in depth the various ways a game can be solved. If you are interested in a particular technique you can select it using the configuration menu and then search for its every occurrence.

If desired, you can work on specific techniques by selecting Mode > Practising… and then select all those that you want, and when you create your next game it will include those techniques (note that it can take a while to create a game depending on the selections).



Further help is also available on the HoDoKu web site with a complete manual that explains all the features of the program in detail. The site also has an extensive list of all the techniques that HoDoKu is able to use, and believe me, that list is a lot more extensive than I could ever imagine was possible.

Other sites exist also on the web, but one I find particularly interesting is an equivalent to Wikipedia but dedicated only to the game:

If you are looking for a guide on how to help solve Sudoku games you can check from the author of another program called "Simple Sudoku".

If you want to study a particular game, you can enter it manually or use the clipboard, select the analyser mode to view all the steps that are needed to solve it, move them around, and play different "what if" scenarios. You can ask the program to execute the all simple moves so that you can concentrate on the harder ones, for example.

The only complaint I have about HoDoKu, and it's a minor one, is that it's a very serious program, maybe a bit too much. For example, when you complete a game there are no fireworks, no sounds, no message, just the grid with all the digits and no indication that you’ve won.

There is a lot more in that program than I can cover in this article, but let’s just say that if you do take the time to explore everything that HoDoKu has to offer then you are going to become a much better Sudoku player.


Bottom Line:

HoDoKu (Freeware)
Bernhard Hobiger

Originally published: June, 2011

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