This is by far the most frequently asked question. I have left it as a deliberate puzzle. I encourage you to consider what you do when a numeric sudoku has only eight numbers, then go work the second introductory puzzle. Make any reasonable choice to solve that puzzle, and the hidden message will answer your question. (How cool is that?)
If you are really stuck and just want the answer, then here it is: If a puzzle only gives eight letters (or characters, if the puzzle contains an asterisk), then the ninth space in each row, column or 3×3 box is just that—it's a space. The spaces in the shaded squares form the spaces between the words of the hidden message.
The hardest part of creating these puzzles is coming up with a good message. It can't contain too many different letters, and certain letter patterns can make it difficult or impossible to form a puzzle around any particular message. That's where the asterisk comes in. Typically, the asterisk stands for a few different letters, each of which only appears once. Here's an example: Suppose I wanted to create a puzzle with the message "Asterisks are unknown characters". This message contains the letters A, C, E, H, I, K, N, O, R, S, T, U, and W. That's four or five letters too many (depending on whether I want to give eight or nine characters). If I want to give nine characters, then I could use asterisks to represent the letters H, I, O, U, and W. This gives the message 'ASTER*SKSARE*NKN**NC*ARACTERS', which has only nine different characters (including the asterisk).
The asterisk may also stand for punctuation, numbers or special characters. For example, an asterisk may be used to represent the apostrophe in the word don't (DON*T). It may be used represent both the comma and the ampersand in Peter, Paul & Mary, and also to reduce the number of overall characters (PETER* PA*L * MAR*). It could be used to represent the number 9 in the Beatles song title Revolution 9 (because the numeral 9 in a Beatles song title would be a dead giveaway), and to reduce the number of overall characters (RE*OL*TION *).
I have placed some restrictions on the use of the asterisk. The asterisk will never represent one of the letters that is given in the same grid. It may, however, represent a letter given in another grid of the same Super Code-Doku. An asterisk may represent an apostrophe, a hyphen, a numeral or a special character such as & or $. But an asterisk will never represent a space, and an asterisk will never represent punctuation such as a comma or an exclamation point unless the puzzle instructions say that it does.
You can try the set of introductory puzzles. These gradually introduce you to the features of Code-Doku puzzles. If you have trouble with those puzzles, or if you'd rather shorten the learning curve, then you should consult the FAQ entry about puzzles with only eight letters and then consider the following suggested approach:
Code-Doku puzzles first appeared on the MathRec web site, although the name Code-Doku was coined by Top Notch.
You can work your way through the Code-Doku archive. You can also find a few code-doku puzzles at Top Notch. After that, you have to convince me to make more. Send me e-mail. If you're really interested, then you can help me make the puzzles. The usual 9×9 Code-Doku puzzles are easy for me to generate (using a lot of software that already exists), but I have to have a message to encode. Send me suggestions for messages. Better yet, send me a suggestion and have a friend send me one. Then you can solve each other's puzzles.
The Super Code-Doku puzzles take some real work. The software that I've developed can't generate those in one or two steps. If you want to make a Super Code-Doku, then send me an e-mail. I'll tell you more.
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