Difference between revisions of "99 questions/Solutions/3"
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m (Fix a typo: elementAt -> elementAt'') |
(Tweak code layout; add an example showing erroneous behavior.) |
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<haskell> |
<haskell> |
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elementAt :: [a] -> Int -> a |
elementAt :: [a] -> Int -> a |
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− | elementAt list i = list !! (i-1) |
+ | elementAt list i = list !! (i-1) |
</haskell> |
</haskell> |
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Line 20: | Line 20: | ||
<haskell> |
<haskell> |
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elementAt' :: [a] -> Int -> a |
elementAt' :: [a] -> Int -> a |
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− | 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 |
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− | + | | k < 1 = error "Index out of bounds" |
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− | + | | otherwise = elementAt' xs (k - 1) |
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</haskell> |
</haskell> |
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<haskell> |
<haskell> |
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elementAt'' :: [a] -> Int -> a |
elementAt'' :: [a] -> Int -> a |
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− | elementAt'' (x:_) 1 = x |
+ | elementAt'' (x:_) 1 = x |
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" |
</haskell> |
</haskell> |
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'''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.:''' |
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<haskell> |
<haskell> |
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− | + | elementAt_w''' xs n = head . drop (n - 1) $ xs -- wrong |
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+ | -- Main> map (elementAt_w''' [1..4]) [0..10] |
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+ | -- [1,1,2,3,4,*** Exception: Prelude.head: empty list |
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</haskell> |
</haskell> |
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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
or elementAt_w'
correctly in point-free style:
elementAt_w'pf = (last .) . take . (+ 1)