## Friday 13 August 2010

### Friday Puzzles #65

Welcome once more to another edition of Friday Puzzles! To those who are just (de)tuning in, I’d like to introduce you to my own pet sudoku variant, Sudoku Islands. The idea is the result of blending Sudoku and Nurikabe together. I’ve realised that since introducing them earlier on in the summer I’ve not really given a proper description of the rules. So here goes:
Place the digits 1-N in the grid such that each digit appears exactly once in each row, column and marked S×T box. The cells containing digits form a connected region of the grid, not containing any 2×2 block of cells. The remaining cells are shaded islands, whose sizes are given. Each island is clued in the grid.
Previously, N has equalled 5, and S and T have both been 3. Here’s a little recap in case you were wondering how that looks practically.
#079 Sudoku Islands – rated medium
This example ramps up N to 7, and S and T have become 4 and 3. 12×12 classic Sudoku puzzles are something of a drag; I’ve intentionally made this one fairly difficult, although I’m not quite sure whether this size again makes things a little bit fiddly. Let me know what you think!
#080 Sudoku Islands – rated hard
All puzzles © Tom Collyer 2009-10

1. Wonderful. Really glad to see more of these!

I didn’t find the second one too difficult. Also, interesting to see so a river with so many loops; I don’t think that happens often in regular Nurikabe of this size.

2. Tom,

This puzzle design seems to scale up successfuly. I thoroughly enjoyed solving both puzzles.

David.

3. Very interesting concept – I didn’t find the large one too difficult either. The complexity/ difficulty could be increased further by making the solver having to go back and forth between the Nurikabe part and the Sudoku part:

All six Sudoku Islands puzzles are solvable by completing the Nurikabe first without taking into account the actual Sudoku numbers (by using only the clues that certain cells are part of the river, and that each row, column, and area contains the same number of river cells), and the Sudoku part afterwards, i.e. they are each actually two consecutive puzzles.

I’d love to see a ‘fully integrated’ hybrid!

4. That’s certainly an interesting point you raise about trying to blend the two puzzles. The main challenge in constructing these that I’ve found so far is the fact the Sudoku givens at the start of the puzzle must be sufficient to solve the finished Nurikabe puzzle. Now, given that a non-trivial puzzle has to start off with some Nurikabe logic, the only opportunity for hybrid logic is in the middle of the solve.

The other issue is the set of valid Nurikabe patterns as a whole. It takes a little bit of work to even come up with something valid to clue, and that’s not generally the way I create me normal Nurikabe puzzles. It suggests to me that there’d be a bit of work involved coming up with a grid which doesn’t solve uniquely as a Nurikabe by itself, especially when you have the Sudoku givens acting as constraints to the Nurikabe puzzle too.

Having said all that, I will try and give it a go!

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