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wjrh

My solution to day 3 in Haskell. AoC is the first thing I have ever written in Haskell and it shows! I am an aerodynamics engineer who uses MATLAB and Python to basically just deal with matrix operations. It's been tricky to adjust to a functional way of thinking & where everything is not just a matrix of doubles!

main = do
    input <- fmap lines $ readFile "input_day03.txt"
    print $ countLetters (tail $ countTrees (every 1 input) [0,3..]) '#'
    print $ product $ countAllTrees input [1,1,1,1,2] [1,3,5,7,1]

countTrees :: [String] -> [Int] -> String
countTrees [] _ = []
countTrees (x:xs) (y:ys) = [x !! (y `mod` length x)] ++ countTrees xs ys

countLetters :: String -> Char -> Int
countLetters str c = length $ filter (== c) str

every :: Int -> [String] -> [String]
every n (x:xs) = [x] ++ every n (drop (n-1) xs)
every _ [] = []

countAllTrees :: [String] -> [Int] -> [Int] -> [Int]
countAllTrees input (x:xs) (y:ys) = 
        [countLetters (tail $ countTrees (every x input) [0,y..]) '#'] 
        ++ countAllTrees input xs ys
countAllTrees _ [] _ = []

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wence in reply to @wjrh
Looks good!

Try and train yourself out of the habit of adding a single entry to the start of a list by using ++ and instead cons it on the front with :.

E.g., write
```
every n (x:xs) = x : every n (drop (n-1) xs)
-- Rather than
every n (x:xs) [x] ++ every n (drop (n-1) xs)
```
(:) is O(1) because you're just adding a node to the head of a linked list, whereas ++ is O(n) in the length of the first list because Haskell has to walk to the end of the first list before appending the second one.

Although it doesn't really matter here because the first list is only length one, it's an antipattern one tries to avoid. e.g. compare the runtime performance of
```
rev :: [a] -> [a]
rev (x:xs) = rev xs ++ [x]
rev [] = []
-- with

rev' :: [a] -> [a]
rev' xs = revH xs []
  where
    revH (x:xs) ys = revH xs (x:ys)
    revH [] ys = ys
```
(Getting ready to teach intro Haskell again...)

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wjrh in reply to @wence

Thank you for this! Really useful. I'm planning to carry Haskell on past the advent of code, I feel like it is not a quick language to get competent in!

Out of interest, in what capacity do you teach into Haskell?

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