99 questions/Solutions/3
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Revision as of 23:38, 2 February 2011 by Raph amiard (talk | contribs) (Adding a new version for elementAt)
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(*) 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"