this post was submitted on 02 Dec 2024
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Advent Of Code

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Advent of Code is an annual Advent calendar of small programming puzzles for a variety of skill sets and skill levels that can be solved in any programming language you like.

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๐ŸฆŒ - 2024 DAY 2 SOLUTIONS -๐ŸฆŒ (programming.dev)
submitted 3 weeks ago by [email protected] to c/[email protected]
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Day 2: Red-Nosed Reports

Megathread guidelines

  • Keep top level comments as only solutions, if you want to say something other than a solution put it in a new post. (replies to comments can be whatever)
  • You can send code in code blocks by using three backticks, the code, and then three backticks or use something such as https://blocks.programming.dev/ if you prefer sending it through a URL

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[โ€“] [email protected] 9 points 3 weeks ago (1 children)

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
[โ€“] [email protected] 4 points 3 weeks ago (3 children)

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?

[โ€“] [email protected] 3 points 3 weeks 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.

[โ€“] [email protected] 2 points 3 weeks ago* (last edited 3 weeks ago)

<$> is just fmap as an infix operator.

>>> fmap (+1) [1,2,3]
[2,3,4]
>>> (+1) <\$> [1,2,3]
[2,3,4]
[โ€“] [email protected] 1 points 3 weeks 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!