81 lines
1.3 KiB
Org Mode
81 lines
1.3 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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#+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 (_:xs) = 1 + myLength xs
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#+end_src
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