Revision as of 21:54, 4 January 2012
(**) 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 k | i>0 = take (k-i+1) $ drop (i-1) 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)
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)
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
slice xs (i+1) k = snd (split (fst (split xs k)) i)
slice xs i j = map snd $ filter (\(x,_) -> x >= i && x <= j) $ zip [1..] xs
A solution using list comprehension:
slice xs i k = [x | (x,j) <- zip xs [1..k], i <= j]
Another simple solution using take and drop:
slice :: [a] -> Int -> Int -> [a] slice l i k | i > k =  | otherwise = (take (k-i+1) (drop (i-1) l))