99 questions/Solutions/3
From HaskellWiki
(Difference between revisions)
m (Fix a typo: elementAt -> elementAt'') |
(Tweak code layout; add an example showing erroneous behavior.) |
||
| Line 13: | Line 13: | ||
<haskell> | <haskell> | ||
elementAt :: [a] -> Int -> a | elementAt :: [a] -> Int -> a | ||
| - | elementAt list i = list !! (i-1) | + | elementAt list i = list !! (i-1) |
</haskell> | </haskell> | ||
| Line 20: | Line 20: | ||
<haskell> | <haskell> | ||
elementAt' :: [a] -> Int -> a | elementAt' :: [a] -> Int -> a | ||
| - | elementAt' (x:_) 1 = x | + | elementAt' (x:_) 1 = x |
| - | elementAt' [] _ = error "Index out of bounds" | + | elementAt' [] _ = error "Index out of bounds" |
elementAt' (_:xs) k | elementAt' (_:xs) k | ||
| - | + | | k < 1 = error "Index out of bounds" | |
| - | + | | otherwise = elementAt' xs (k - 1) | |
</haskell> | </haskell> | ||
| Line 31: | Line 31: | ||
<haskell> | <haskell> | ||
elementAt'' :: [a] -> Int -> a | elementAt'' :: [a] -> Int -> a | ||
| - | elementAt'' (x:_) 1 = x | + | elementAt'' (x:_) 1 = x |
elementAt'' (_:xs) i = elementAt'' xs (i - 1) | elementAt'' (_:xs) i = elementAt'' xs (i - 1) | ||
| - | elementAt'' _ _ = error "Index out of bounds" | + | elementAt'' _ _ = error "Index out of bounds" |
</haskell> | </haskell> | ||
'''This does not work correctly on invalid indexes and infinite lists, e.g.:''' | '''This does not work correctly on invalid indexes and infinite lists, e.g.:''' | ||
| Line 55: | Line 55: | ||
<haskell> | <haskell> | ||
| - | + | elementAt_w''' xs n = head . drop (n - 1) $ xs -- wrong | |
| + | -- Main> map (elementAt_w''' [1..4]) [0..10] | ||
| + | -- [1,1,2,3,4,*** Exception: Prelude.head: empty list | ||
</haskell> | </haskell> | ||
Revision as of 11:08, 12 September 2012
(*) Find the K'th element of a list. The first element in the list is number 1.
This is (almost) the infix operator !! in Prelude, which is defined as:
(!!) :: [a] -> Int -> a (x:_) !! 0 = x (_:xs) !! n = xs !! (n-1)
Except this doesn't quite work, because !! is zero-indexed, and element-at should be one-indexed. So:
elementAt :: [a] -> Int -> a elementAt list i = list !! (i-1)
Or without using the infix operator:
elementAt' :: [a] -> Int -> a elementAt' (x:_) 1 = x elementAt' [] _ = error "Index out of bounds" elementAt' (_:xs) k | k < 1 = error "Index out of bounds" | otherwise = elementAt' xs (k - 1)
Alternative version:
elementAt'' :: [a] -> Int -> a elementAt'' (x:_) 1 = x elementAt'' (_:xs) i = elementAt'' xs (i - 1) elementAt'' _ _ = error "Index out of bounds"
This does not work correctly on invalid indexes and infinite lists, e.g.:
elementAt'' [1..] 0
A few more solutions using prelude functions:
elementAt_w' xs n = last . take n $ xs -- wrong -- Main> map (elementAt_w' [1..4]) [1..10] -- [1,2,3,4,4,4,4,4,4,4]
elementAt_w'' xs n = head . reverse . take n $ xs -- wrong -- Main> map (elementAt_w'' [1..4]) [1..10] -- [1,2,3,4,4,4,4,4,4,4]
elementAt_w''' xs n = head . drop (n - 1) $ xs -- wrong -- Main> map (elementAt_w''' [1..4]) [0..10] -- [1,1,2,3,4,*** Exception: Prelude.head: empty list
elementAt_w'
elementAt_w'pf = (last .) . take . (+ 1)
