Day 2: Red-Nosed Reports
Megathread guidelines
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FAQ
- What is this?: Here is a post with a large amount of details: https://programming.dev/post/22323136
- Where do I participate?: https://adventofcode.com/
- Is there a leaderboard for the community?: We have a programming.dev leaderboard with the info on how to join in this post: https://programming.dev/post/6631465
Haskell
This was quite fun! I got a bit distracted trying to rewrite
safein point-free style, but I think this version is the most readable. There’s probably a more monadic way of writinglessOneas 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) inputLove to see your haskell solutions!
I am so far very amazed with the compactness of your solutions, your
lessOneis 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?<$>is justfmapas an infix operator.>>> fmap (+1) [1,2,3] [2,3,4] >>> (+1) <\$> [1,2,3] [2,3,4]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
negateandgradual.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.