Friday, 25 June 2010

Friday Puzzles #58

This week dear readers, I am appealing to you for a little feedback. It seems every puzzle designer worth their salt has their own pet sudoku variant they’ve created, nurtured and loved. Oh the long days I looked on in envy!

Anyhow, today that all changes as I’ve come up with something that is (a) original to the best of my knowledge and (b) isn’t totally lame. I call it Sudoku Islands. The idea is basically a hybrid of sudoku and nurikabe. You have an NxN grid, and the idea is to fill in each column, row and box with the digits 1-M, where M < N. Moreover these digits form a connected region of the grid, with no 2×2 blocks, which surround islands. The size of these islands is clued by the grey squares, and there is exactly one given clue per island in the solution. Unfortunately if you are used to nurikabe much, the roles of white and grey (black) squares have become reversed, but you’re playing by my rules now so I guess that’s hard cheese!

What you get is in my mind something that works out to be an improved version of extra space sudoku, where you not only have to fill in the grid, but also have to work out where the extra space is. That’s all beginning to sound a bit wordy, so here’s a really easy toy example for you to get the idea. Fill this grid with digits 1-4 in each row/column/box:
    #068 Sudoku Islands – rated very easy
And this one with digits 1-5. Enjoy!
    #069 Sudoku Islands – rated medium
All puzzles © Tom Collyer 2009-10


  1. Great puzzle! I was a bit skeptical for the first one, but by the end of the second you had me sold. I’d gladly solve more of these.

    I will say it’s a little weird seeing 2×2 regions of black squares, but I don’t think there’s much you can do to change that.

    One graphics request: can we have dark grey instead of black for the islands? My main criticism of extra space sudoku is that it’s often hard to see the thick lines that separate regions, and that’s especially true when the unused squares are black.

  2. Ah the 6×6 puzzle wasn’t really so much a puzzle as a simple explanation of the rules for those who don’t necessarily have perfect English. That and the solution I think ought to be enough to figure out what is going on.

    Dark grey shading sounds like a good idea!

  3. Also, whilst I think 9×9 and 1-5 has room for a bit of subtlety, I think the fun will really begin with say 12×12 and 1-7 or 16×16 and 1-9. I haven’t really had time to explore those idea properly though.

  4. I agree that the only place where the black/white reversal from Nurikabe makes me a little edgy is avoiding 2×2 white squares and allowing 2×2 black squares, but it’s not that big a deal. There are a lot of different little logical constraints in these – the black/white counts for each row, column, and region, the connectedness, and the basic “magic square” sudoku rules all combine in interesting ways. So for example, the chains of logic that go something like “the only other white square in this row has to be the 1, so these two here have to be black squares, but they have to connect like so…”. I think that’s what’s going to get tricky about the big ones is figuring out which particular logical constraint is supposed to come into play at any given moment.

    Fun variant. I actually do appreciate having a “just to make sure you understand the rules” example for the first one, and the second one was a very solid puzzle.

  5. The 6×6 is a perfect illustration of how to solve the puzzle and, by the looks of it, this could be an extremely entertaining puzzle (I got into Nurikabe quite a lot when I last done a mass order from Nikoli so will look forward to seeing more of these!)

    I’ll give you some better feedback once I’ve found the time to do the 9×9.

    On another point, and seeing as I don’t normally post, can I just thank you for all your work on your Friday puzzles. The best way to end the working week :)

  6. I’ll reserve judgement until I can try a half dozen more. (Hoping that convinces you to create more – quite enjoyable).

  7. Enjoyable construction. Good fun. Medium was right rating. Bring on the hard version of this entertaining mutant.




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