Difference between revisions of "99 questions/Solutions/3"

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Line 43: Line 43:
   
 
<haskell>
 
<haskell>
elementAt'' xs n | length xs < n = error "Index out of bounds"
+
elementAt'' xs n
| otherwise = fst . last $ zip xs [1..n]
+
| length xs < n = error "Index out of bounds"
</haskell>
+
| otherwise = fst . last $ zip xs [1..n]
   
<haskell>
 
 
elementAt''' xs n = head $ foldr ($) xs
 
elementAt''' xs n = head $ foldr ($) xs
 
$ replicate (n - 1) tail
 
$ replicate (n - 1) tail
-- Negative indeces not handled correctly:
+
-- Negative indices not handled correctly:
 
-- Main> elementAt''' "haskell" (-1)
 
-- Main> elementAt''' "haskell" (-1)
 
-- 'h'
 
-- 'h'
</haskell>
 
   
<haskell>
 
  +
elementAt'''' xs n
elementAt_w' xs n = last . take n $ xs -- wrong
 
  +
| length xs < n = error "Index out of bounds"
-- Main> map (elementAt_w' [1..4]) [1..10]
 
  +
| otherwise = last $ take n xs
-- [1,2,3,4,4,4,4,4,4,4]
 
</haskell>
 
   
<haskell>
 
  +
elementAt''''' xs n
elementAt_w'' xs n = head . reverse . take n $ xs -- wrong
 
  +
| length xs < n = error "Index out of bounds"
-- Main> map (elementAt_w'' [1..4]) [1..10]
 
 
| otherwise = head . reverse $ take n xs
-- [1,2,3,4,4,4,4,4,4,4]
 
  +
</haskell>
 
  +
elementAt'''''' xs n
  +
| length xs < n = error "Index out of bounds"
  +
| otherwise = head $ drop (n - 1) xs
   
<haskell>
 
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
 
 
</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)