Difference between revisions of "99 questions/Solutions/3"
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< 99 questions  Solutions
Line 43:  Line 43:  
<haskell> 
<haskell> 

−  elementAt'' xs n 
+  elementAt'' xs n 
−  +   length xs < n = error "Index out of bounds" 

−  +   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 
+   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 

⚫  
+   length xs < n = error "Index out of bounds" 

−   Main> map (elementAt_w'' [1..4]) [1..10] 

⚫  
−   [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 !! (n1)
Except this doesn't quite work, because !! is zeroindexed, and elementat should be oneindexed. So:
elementAt :: [a] > Int > a
elementAt list i = list !! (i1)
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 pointfree 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)