Difference between revisions of "99 questions/Solutions/3"
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Raph amiard (talk | contribs) (Adding a new version for elementAt) |
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elementAt'' (_:xs) i = elementAt xs (i - 1) |
elementAt'' (_:xs) i = elementAt xs (i - 1) |
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elementAt'' _ _ = error "Index out of bounds" |
elementAt'' _ _ = error "Index out of bounds" |
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+ | </haskell> |
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+ | '''This does not work correctly on infinite lists, e.g.:''' |
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+ | <haskell> |
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+ | elementAt'' [1..] |
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</haskell> |
</haskell> |
Revision as of 07:42, 28 August 2011
(*) 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 infinite lists, e.g.:
elementAt'' [1..]