Day 2: Red-Nosed Reports

Megathread guidelines

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FAQ

  • lwhjp@lemmy.sdf.org
    9·
    21 days ago

    Haskell

    This was quite fun! I got a bit distracted trying to rewrite safe in point-free style, but I think this version is the most readable. There’s probably a more monadic way of writing lessOne as well, but I can’t immediately see it.

    safe xs = any gradual [diffs, negate <$> diffs]
  where
    diffs = zipWith (-) (drop 1 xs) xs
    gradual = all (`elem` [1 .. 3])

lessOne [] = []
lessOne (x : xs) = xs : map (x :) (lessOne xs)

main = do
  input :: [[Int]] <- map (map read . words) . lines <$> readFile "input02"
  print . length $ filter safe input
  print . length $ filter (any safe . lessOne) input
    • VegOwOtenks@lemmy.worldEnglish
      4·
      21 days ago

      Love to see your haskell solutions!

      I am so far very amazed with the compactness of your solutions, your lessOne is very much mind-Bending. I have never used or seen <$> before, is it a monadic $?

      Also I can’t seem to find your logic for this safety condition: The levels are either all increasing or all decreasing, did you figure that it wasn’t necessary?

      • mschwennesen@discuss.tchncs.de
        3·
        21 days ago

        For the last point, it isn’t needed since the differences between elements should be all positive or all negative for the report to be safe. This is tested with the combination of negate and gradual.

        I am also enjoying these Haskell solutions. I’m still learning the language, so it’s been cool to compare my solution with these and grow my understanding of Haskell.

      • kintrix@linux.communityEnglish
        2·
        21 days ago

        <$> is just fmap as an infix operator.

        >>> fmap (+1) [1,2,3]
[2,3,4]
>>> (+1) <\$> [1,2,3]
[2,3,4]
      • lwhjp@lemmy.sdf.org
        1·
        21 days ago

        Thanks! The other two posters already answered your questions, I think :)

        Haskell makes it really easy to build complex operations out of simple functional building blocks, skipping a lot of boilerplate needed in some other languages. I find the compactness easier to read, but I realize that not everyone would agree.

        BTW, I’m a relative Haskell newbie. I’m sure more experienced folks could come up with even more interesting solutions!