Posted By

rtperson on 09/08/11

Viewed 3 times

Haskell 99 Problems, numbers 1 through 9

I had originally started these problems from #10 (Run-length encoding). I went back and did 1-8 for completeness.

Copy this code and paste it in your HTML
1. import Data.List
2.
3. list1 = [1,2,3,4,5,6]
4.
5. -- Problem 1: find the last element of a list
6. lastList :: [a] -> a
7. lastList [] = error "list cannot be empty"
8. lastList (x:[]) = x
9. lastList (x:xs) = lastList xs
10.
11. -- Problem 2: Find the last but one element of a list.
12. lastButOne :: [a] -> a
13. lastButOne [] = error "list cannot be empty"
14. lastButOne (x:[]) = error "list must be larger than one"
15. lastButOne [x,_] = x
16. lastButOne (_:xs) = lastButOne xs
17.
18.
19. -- Problem 3: Find the K'th element of a list. The first element in the list is number 1.
20. elementAt :: [a] -> Int -> a
21. elementAt [] n = error "list cannot be empty"
22. elementAt s@(x:xs) n
23. | n > (length s) = error "n cannot be larger than list"
24. | otherwise = if (length s) == n then x else elementAt xs n
25.
26. -- Problem 4: find the number of elements of a list
27. countList :: [a] -> Int
28. countList [] = 0
29. countList (x:xs) = 1 + countList xs
30.
31. -- Problem 5: reverse a list (I assume, not using "reverse")
32. reverse' :: [a] -> [a]
33. reverse' [] = []
34. reverse' (x:xs) = reverse' xs ++ (x : [])
35.
36. -- Problem 6: find out whether a list is a palindrome
37. isPalindrome :: (Eq a) => [a] -> Bool
38. isPalindrome [] = True
39. isPalindrome xs = let xs2 = reverse xs
40. in (xs2 == xs)
41.
42. -- Problem 7: flatten a nested list structure
43. -- NOTE: This doesn't strictly solve the problem. May want to revisit later.
44. flatten :: [[a]] -> [a]
45. flatten = foldr (++) []
46.
47. {-
48.   Problem 8: Eliminate consecutive duplicates of list elements.
49.   If a list contains repeated elements they should be replaced with
50.   a single copy of the element. The order of the elements should not be changed.
51.
52.   > compress ["a","a","a","a","b","c","c","a","a","d","e","e","e","e"]
53.   ["a","b","c","a","d","e"]
54.
55. -}
56.
57. -- excellent opportunity to play around with mapAccumL
58. compress :: (Eq x) => [x] -> [x]
59. compress xs = fst \$ mapAccumL (\ys s -> if (length ys > 0) && (last ys == s) then (ys, s) else (ys++s:[], s)) [] xs
60.
61. -- the simplest solution from the Wiki
62. compress' :: Eq a => [a] -> [a]
63. compress' = map head . group
64.
65. {-
66.   Problem 9: Pack consecutive duplicates of list elements into sublists.
67.   If a list contains repeated elements they should be placed in separate sublists.
68.
69.   *Main> pack ['a', 'a', 'a', 'a', 'b', 'c', 'c', 'a', 'a', 'd', 'e', 'e', 'e', 'e']
70.   ["aaaa","b","cc","aa","d","eeee"]
71. -}
72.
73. pack :: (Eq a) => [a] -> [[a]]
74. pack = group