# 99 questions/Solutions/3

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< 99 questions | Solutions(Difference between revisions)

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elementAt'' _ _ = error "Index out of bounds" |
elementAt'' _ _ = error "Index out of bounds" |
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− | '''This does not work correctly on infinite lists, e.g.:''' |
+ | '''This does not work correctly on invalid indexes and infinite lists, e.g.:''' |

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− | elementAt'' [1..] |
+ | elementAt'' [1..] 0 |

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## Revision as of 00:05, 29 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 invalid indexes and infinite lists, e.g.:**

elementAt'' [1..] 0