# 99 questions/Solutions/3

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< 99 questions | Solutions(Difference between revisions)

m (Fix a typo: elementAt -> elementAt'') |
|||

Line 32: | Line 32: | ||

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> |

## Revision as of 10:24, 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''' xs n = head . drop (n - 1) $ xs

elementAt_w'

elementAt_w'pf = (last .) . take . (+ 1)