Difference between revisions of "99 questions/Solutions/3"
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<haskell> |
<haskell> |
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− | elementAt'' xs n |
+ | elementAt'' xs n |
+ | | length xs < n = error "Index out of bounds" |
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− | + | | otherwise = fst . last $ zip xs [1..n] |
|
− | </haskell> |
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− | <haskell> |
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elementAt''' xs n = head $ foldr ($) xs |
elementAt''' xs n = head $ foldr ($) xs |
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$ replicate (n - 1) tail |
$ replicate (n - 1) tail |
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− | -- Negative |
+ | -- Negative indices not handled correctly: |
-- Main> elementAt''' "haskell" (-1) |
-- Main> elementAt''' "haskell" (-1) |
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-- 'h' |
-- 'h' |
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− | </haskell> |
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+ | elementAt'''' xs n |
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− | <haskell> |
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− | + | | length xs < n = error "Index out of bounds" |
|
+ | | otherwise = last $ take n xs |
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− | -- Main> map (elementAt_w' [1..4]) [1..10] |
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− | -- [1,2,3,4,4,4,4,4,4,4] |
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− | </haskell> |
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+ | elementAt''''' xs n |
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− | <haskell> |
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+ | | length xs < n = error "Index out of bounds" |
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− | + | | otherwise = head . reverse $ take n xs |
|
− | -- Main> map (elementAt_w'' [1..4]) [1..10] |
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+ | |||
− | -- [1,2,3,4,4,4,4,4,4,4] |
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+ | elementAt'''''' xs n |
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− | </haskell> |
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+ | | length xs < n = error "Index out of bounds" |
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+ | | otherwise = head $ drop (n - 1) xs |
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− | <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> |
||
Latest revision as of 02:32, 15 June 2016
(*) 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'' xs n
| length xs < n = error "Index out of bounds"
| otherwise = fst . last $ zip xs [1..n]
elementAt''' xs n = head $ foldr ($) xs
$ replicate (n - 1) tail
-- Negative indices not handled correctly:
-- Main> elementAt''' "haskell" (-1)
-- 'h'
elementAt'''' xs n
| length xs < n = error "Index out of bounds"
| otherwise = last $ take n xs
elementAt''''' xs n
| length xs < n = error "Index out of bounds"
| otherwise = head . reverse $ take n xs
elementAt'''''' xs n
| length xs < n = error "Index out of bounds"
| otherwise = head $ drop (n - 1) xs
or elementAt_w'
correctly in point-free style:
elementAt_w'pf = (last .) . take . (+ 1)
Pedantic note: the above definition of elementAt_w'pf
does not conform to the order of arguments specified by the question, but the following does:
elementAt_w'pf' = flip $ (last .) . take . (+ 1)