Difference between revisions of "99 questions/Solutions/18"
< 99 questions | Solutions
Jump to navigation
Jump to search
m (Updated my solution to a more paranoid one) |
(added a cleaner version of the final solution) |
||
Line 48: | Line 48: | ||
chunk = fst $ splitAt (k - i') chop -- Remove the part after k |
chunk = fst $ splitAt (k - i') chop -- Remove the part after k |
||
i' = i - 1 |
i' = i - 1 |
||
+ | </haskell> |
||
+ | A little cleaner, using the previous problem's split (a.k.a. <hask>splitAt</hask>): |
||
+ | <haskell> |
||
+ | slice xs (i+1) k = snd (split (fst (split xs k)) i) |
||
</haskell> |
</haskell> |
Revision as of 14:47, 17 September 2010
(**) Extract a slice from a list.
Given two indices, i and k, the slice is the list containing the elements between the i'th and k'th element of the original list (both limits included). Start counting the elements with 1.
slice xs (i+1) k = take (k-i) $ drop i xs
The same solution as above, but the more paranoid (maybe too paranoid?) version of it (uses guards and Maybe):
slice :: [a] -> Int -> Int -> Maybe [a]
slice [] _ _ = Just []
slice xs k n | k == n = Just []
| k > n || k > length xs || n > length xs || k < 0 || n < 0 = Nothing
| k == 0 = Just (take n xs)
| otherwise = Just (drop (k-1) $ take n xs)
Or, an iterative solution:
slice :: [a]->Int->Int->[a]
slice lst 1 m = slice' lst m []
where
slice' :: [a]->Int->[a]->[a]
slice' _ 0 acc = reverse acc
slice' (x:xs) n acc = slice' xs (n - 1) (x:acc)
slice (x:xs) n m = slice xs (n - 1) (m - 1)
Or:
slice :: [a] -> Int -> Int -> [a]
slice (x:xs) i k
| i > 1 = slice xs (i - 1) (k - 1)
| k < 1 = []
| otherwise = x:slice xs (i - 1) (k - 1)
Another way using splitAt
, though not nearly as elegant as the take
and drop
version:
slice :: [a] -> Int -> Int -> [a]
slice xs i k = chunk
where chop = snd $ splitAt i' xs -- Get the piece starting at i
chunk = fst $ splitAt (k - i') chop -- Remove the part after k
i' = i - 1
A little cleaner, using the previous problem's split (a.k.a. splitAt
):
slice xs (i+1) k = snd (split (fst (split xs k)) i)