151 lines
2.6 KiB
Org Mode
151 lines
2.6 KiB
Org Mode
#+TITLE: 99 Haskell Problems
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#+Author: Joseph Ferano
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https://wiki.haskell.org/H-99:_Ninety-Nine_Haskell_Problems
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*** #1 Find the last element of a list.
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#+begin_src haskell
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λ> myLast [1,2,3,4]
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4
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λ> myLast ['x','y','z']
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'z'
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#+end_src
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**** Solution
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#+begin_src haskell
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myLast [] = error "Cannot accept an empty list"
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myLast [x] = x
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myLast (_:xs) = myLast xs
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myLast [1..5]
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#+end_src
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*** #2 Find the last but one element of a list.
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#+begin_src haskell
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λ> myButLast [1,2,3,4]
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3
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λ> myButLast ['a'..'z']
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'y'
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#+end_src
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**** Solution
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#+begin_src haskell
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myButLast x
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| length x < 2 = error "Cannot accept an empty list"
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| otherwise = case x of
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[x,y] -> x
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(x:xs) -> myButLast xs
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#+end_src
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*** #3 Find the K'th element of a list.
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The first element in the list is number 1.
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#+begin_src haskell
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λ> elementAt [1,2,3] 2
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2
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λ> elementAt "haskell" 5
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'e'
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#+end_src
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**** Solution
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#+begin_src haskell
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elementAt (x:xs) i = if i == 1 then x else elementAt xs (i - 1)
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#+end_src
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*** #4 Find the number of elements of a list.
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#+begin_src haskell
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λ> myLength [123, 456, 789]
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3
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λ> myLength "Hello, world!"
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13
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#+end_src
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**** Solution
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#+begin_src haskell
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myLength [] = 0
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myLength xs = foldl (\acc _ -> acc + 1) 0 xs
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-- or
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myLength' [] = 0
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myLength' [x] = 1
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myLength' (_:xs) = 1 + myLength xs
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#+end_src
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*** #5 Reverse a list.
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#+begin_src haskell
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λ> myReverse "A man, a plan, a canal, panama!"
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"!amanap ,lanac a ,nalp a ,nam A"
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λ> myReverse [1,2,3,4]
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[4,3,2,1]
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#+end_src
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**** Solution
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#+begin_src haskell
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myReverse = go []
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where go acc [] = acc
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go acc (x:xs) = go (x:acc) xs
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#+end_src
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*** #6 Find out whether a list is a palindrome
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#+begin_src haskell
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λ> isPalindrome [1,2,3]
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False
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λ> isPalindrome "madamimadam"
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True
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λ> isPalindrome [1,2,4,8,16,8,4,2,1]
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True
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#+end_src
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**** Solution
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#+begin_src haskell
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isPalindrome xs = xs == reverse xs
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#+end_src
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*** #7 Flatten a nested list structure.
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Transform a list, possibly holding lists as elements into a `flat' list by replacing each list with its elements (recursively).
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Example:
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#+begin_src haskell
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,* (my-flatten '(a (b (c d) e)))
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(A B C D E)
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#+end_src
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Example in Haskell:
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We have to define a new data type, because lists in Haskell are homogeneous.
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#+begin_src haskell
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data NestedList a = Elem a | List [NestedList a]
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#+end_src
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#+begin_src haskell
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λ> flatten (Elem 5)
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[5]
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λ> flatten (List [Elem 1, List [Elem 2, List [Elem 3, Elem 4], Elem 5]])
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[1,2,3,4,5]
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λ> flatten (List [])
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[]
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#+end_src
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**** Solution
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#+begin_src haskell
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flatten = reverse . go []
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where go acc (List []) = acc
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go acc (Elem x) = x:acc
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go acc (List (x:xs)) = go (go acc x) (List xs)
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#+end_src
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