Revision: 50900

at September 8, 2011 00:23 by rtperson

**Initial Code**

import Data.List list1 = [1,2,3,4,5,6] -- Problem 1: find the last element of a list lastList :: [a] -> a lastList [] = error "list cannot be empty" lastList (x:[]) = x lastList (x:xs) = lastList xs -- Problem 2: Find the last but one element of a list. lastButOne :: [a] -> a lastButOne [] = error "list cannot be empty" lastButOne (x:[]) = error "list must be larger than one" lastButOne [x,_] = x lastButOne (_:xs) = lastButOne xs -- Problem 3: Find the K'th element of a list. The first element in the list is number 1. elementAt :: [a] -> Int -> a elementAt [] n = error "list cannot be empty" elementAt [email protected](x:xs) n | n > (length s) = error "n cannot be larger than list" | otherwise = if (length s) == n then x else elementAt xs n -- Problem 4: find the number of elements of a list countList :: [a] -> Int countList [] = 0 countList (x:xs) = 1 + countList xs -- Problem 5: reverse a list (I assume, not using "reverse") reverse' :: [a] -> [a] reverse' [] = [] reverse' (x:xs) = reverse' xs ++ (x : []) -- Problem 6: find out whether a list is a palindrome isPalindrome :: (Eq a) => [a] -> Bool isPalindrome [] = True isPalindrome xs = let xs2 = reverse xs in (xs2 == xs) -- Problem 7: flatten a nested list structure -- NOTE: This doesn't strictly solve the problem. May want to revisit later. flatten :: [[a]] -> [a] flatten = foldr (++) [] {- Problem 8: Eliminate consecutive duplicates of list elements. If a list contains repeated elements they should be replaced with a single copy of the element. The order of the elements should not be changed. > compress ["a","a","a","a","b","c","c","a","a","d","e","e","e","e"] ["a","b","c","a","d","e"] -} -- excellent opportunity to play around with mapAccumL compress :: (Eq x) => [x] -> [x] compress xs = fst $ mapAccumL (\ys s -> if (length ys > 0) && (last ys == s) then (ys, s) else (ys++s:[], s)) [] xs -- the simplest solution from the Wiki compress' :: Eq a => [a] -> [a] compress' = map head . group {- Problem 9: Pack consecutive duplicates of list elements into sublists. If a list contains repeated elements they should be placed in separate sublists. *Main> pack ['a', 'a', 'a', 'a', 'b', 'c', 'c', 'a', 'a', 'd', 'e', 'e', 'e', 'e'] ["aaaa","b","cc","aa","d","eeee"] -} pack :: (Eq a) => [a] -> [[a]] pack = group

**Initial URL**

http://www.haskell.org/haskellwiki/99_questions/1_to_10

**Initial Description**

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

**Initial Title**

Haskell 99 Problems, numbers 1 through 9

**Initial Tags**

**Initial Language**

Haskell