Code-Doku puzzles blend the logical challenge of normal sudoku puzzles with a surprise message that is revealed as you work toward a solution. Instead of using the digits one through nine, Code-Doku puzzles use letters. Other letter-based puzzles such as Go Doku (or Godoku), Sudoku Word or Wordoku have a hidden word, often down the diagonal. Code-Doku puzzles have shaded squares that contain the hidden message. When you have determined the solution to the puzzle, the letters in the shaded squares will spell out the hidden message.

This puzzle was suggested by Arnie Miller in San Mateo, California. This is also the first symmetric code-doku that I've posted.

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A Special Treat

Place each of the letters of BE SPINACH exactly once in each row, column and 3×3 box. When you have completed the puzzle, the shaded squares can be read left to right, then top to bottom to form a phrase describing a special treat.

Anagram: BE SPINACH

Difficulty: Easy

Here is a print-friendly version of this puzzle and a solution.

Other difficulties:
All of these can be solved using the sudoku placement rules alone, and they all have the same hidden message. Choose a puzzle difficulty that fits your interest, or solve the easy puzzle then try a harder one after you know the message.

• Medium — Requires no advanced techniques
• Hard — Cannot be solved with elimination alone
• Tricky — Requires something beyond row/box and column/box interactions
• Harder — Cannot be solved with elimination alone
• Fiendish — Requires something beyond row/box and column/box interactions
• Extreme — Requires advanced techniques beyond forced chains

The puzzles below are the first jigsaw code-dokus. If you've been following the site, you might have noticed that I made a few non-standard puzzles when I did the holiday greeting puzzles. The 5×5 puzzle below, is a good introduction to jigsaw puzzles. The 11×11 is harder than the 9×9 puzzles. Here are a couple of things to know about jigsaws if you're going to go beyond the easy and moderate puzzles:

• A single cell can neighbor cells in a different box because they are in the same row, or a cell can neighbor cells in a different box because they are in the same column. In a regular sudoku, only one can happen at a time. A cell can cause eliminations in a single row in another box, or in a single column in another box. In a jigsaw, a cell can cause eliminations in another box in a single row and in a single column. Keep this in mind when you're looking for block interactions. A cell cannot take a value that would eliminate all locations for that value in another box. Two-block interactions can happen with row/column neigbors in another box, or with row/box neighbors in another column, or with column/box neighbors in another row.
• Know the Law of Leftovers. In these 5×5 jigsaws, it is crucial for the tricky and fiendish puzzles. I'll say no more.

You might want to consult the FAQ. There are additional puzzles in the Code-Doku archive, including several “super code-doku” puzzles with overlapping grids.

If you'd like to be notified when new puzzles are posted, just send me an e-mail at the address below. Good Luck!

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Code-Doku Joke #20

How does a pig get to the hospital?

Place each of the letters of CAMEL IN HUB exactly once in each row, column and box. In addition to the letters, each row, column and box will also contain one empty square. When you have completed the puzzle, the shaded squares will form the punch line of the joke read left to right, then top to bottom.

Difficulty: Easy (Caution: This is an easy 11×11 jigsaw. It is harder than a moderate 9×9.)

Here is a print-friendly version of this puzzle.

Other difficulties:
All of these can be solved using the sudoku placement rules alone, and they all have the same hidden message. Choose a puzzle difficulty that fits your interest, or solve the easy puzzle then try a harder one after you know the message.

• Moderate — Requires no advanced techniques
• Subtle — Cannot be solved with elimination alone, but otherwise easy
• Tough — Cannot be solved with elimination alone
• Tricky — Requires something beyond row/box and column/box interactions
• Tougher — Requires something beyond row/box and column/box interactions
• Strenuous — Does not require techniques beyond the Law of Leftovers, but requires great patience.
• Extreme — Requires advanced techniques beyond forced chains.

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Code-Doku Joke #19

What clothes does a house wear?

Place each of the letters of READS exactly once in each row, column and box. When you have completed the puzzle, the shaded squares will form the punch line of the joke read left to right, then top to bottom.

Difficulty: Easy

Here is a print-friendly version of this puzzle.

Other difficulties:
All of these can be solved using the sudoku placement rules alone, and they all have the same hidden message. Choose a puzzle difficulty that fits your interest, or solve the easy puzzle then try a harder one after you know the message.

• Moderate — Requires no advanced techniques
• Subtle — Cannot be solved with elimination alone, but otherwise easy
• Tough — Cannot be solved with elimination alone
• Tougher — Cannot be solved with elimination alone
• Tricky — Requires something beyond row/box and column/box interactions
• Fiendish — Requires more than row/box and column/box interactions

Other Sudoku Puzzles and Links