completed Chapter 2 #538
completed Chapter 2 #538
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vrom911
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| @@ -336,8 +338,10 @@ from it! | |||
| ghci> :l src/Chapter2.hs | |||
| -} | |||
| subList :: Int -> Int -> [a] -> [a] | |||
| subList = error "subList: Not implemented!" | |||
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| subList x y z = if y<x || y>length z then [] else | |||
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It is more Haskell-way to separate all operators from the arguments with spaces 👾
| subList x y z = if y<x || y>length z then [] else | |
| subList x y z = if y < x || y > length z then [] else |
| contains _ []=[] | ||
| contains y (x:xs) = if elem y x then [x] ++ contains y xs else []++ contains y xs |
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You can use filter function here instead of the pattern matching, but this is nice solution already (you'll just need to replace ++ with : operator 🙂 )
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| -- Can you eta-reduce this one??? | ||
| pairMul xs ys = zipWith (*) xs ys | ||
| pairMul = zipWith (*) |
| rotate = error "rotate: Not implemented!" | ||
| rotate :: Int -> [a] -> [a] | ||
| rotate x s = | ||
| let y = cycle s |
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Unfortunately, cycle fails in runtime on empty lists ♻️ So you need to handle this case separately for this function to work properly.
| rewind = error "rewind: Not Implemented!" | ||
| rewind :: [a] -> [a] | ||
| rewind [] = [] | ||
| rewind(x:xs) = rewind xs ++ [x] |
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Your solution is correct! However, it is slow. In lists, it is quite slow to add anything at the end of the list. That is why it is always better to rewrite it with the : cons. Remember the explanation with trains? 🚂 🚋 🚋
That is why a more efficient solution is with the accumulator and the recursive function that will do the addition at the start of the list which is instant!
You can read a bit more about the go pattern in here: https://kowainik.github.io/posts/haskell-mini-patterns#recursive-go
Co-authored-by: Veronika Romashkina <vrom911@gmail.com>
Co-authored-by: Veronika Romashkina <vrom911@gmail.com>
Solutions for Chapter 2
cc @vrom911 @chshersh