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helper (x:xs) count 1 = helper xs count count
 
helper (x:xs) count 1 = helper xs count count
 
helper (x:xs) count n = x : (helper xs count (n - 1))
 
helper (x:xs) count n = x : (helper xs count (n - 1))
  +
</haskell>
  +
  +
A similar iterative solution but using a closure:
  +
  +
<haskell>
  +
dropEvery :: [a] -> Int -> [a]
  +
dropEvery xs n = helper xs n
  +
where helper [] _ = []
  +
helper (x:xs) 1 = helper xs n
  +
helper (x:xs) k = x : helper xs (k-1)
 
</haskell>
 
</haskell>
   
Line 43: Line 53:
   
 
<haskell>
 
<haskell>
dropEvery n = map snd . filter ((n/=) . fst) . zip (cycle [1..n])
+
dropEvery = flip $ \n -> map snd . filter ((n/=) . fst) . zip (cycle [1..n])
 
</haskell>
 
</haskell>
   
Using take and drop:
+
Using zip and list comprehensions
 
 
<haskell>
 
<haskell>
dropEvery xs n
+
dropEvery :: [a] -> Int -> [a]
|n <= 0 = x
+
dropEvery xs n = [ i | (i,c) <- ( zip xs [1,2..]), (mod c n) /= 0]
|x == [] = []
+
</haskell>
|otherwise = take (n-1) xs ++ dropEvery (drop n xs) n
 
</haskell>
 
   
 
A more complicated approach which first divides the input list into sublists that do not contain the nth element, and then concatenates the sublists to a result list (if not apparent: the author's a novice):
 
A more complicated approach which first divides the input list into sublists that do not contain the nth element, and then concatenates the sublists to a result list (if not apparent: the author's a novice):
Line 64: Line 74:
 
safetail xs | null xs = []
 
safetail xs | null xs = []
 
| otherwise = tail xs
 
| otherwise = tail xs
  +
</haskell>
  +
  +
First thing that came to mind:
  +
<haskell>
  +
dropEvery xs n = map fst $ filter (\(x,i) -> i `mod` n /= 0) $ zip xs [1..]
  +
</haskell>
  +
The filter function can be simplified as seen above:
  +
<haskell>
  +
dropEvery xs n = map fst $ filter ((n/=) . snd) $ zip xs (cycle [1..n])
  +
</haskell>
  +
  +
And yet another approach using folds:
  +
<haskell>
  +
dropEvery :: Int -> [a] -> [a]
  +
dropEvery n xs = snd $ foldl (\acc e -> if fst acc > 1 then (fst acc - 1, snd acc ++ [e]) else (n, snd acc)) (n, []) xs
  +
</haskell>
  +
  +
Another very similar approach to the previous:
  +
<haskell>
  +
dropEvery :: [a] -> Int -> [a]
  +
dropEvery xs n = fst $ foldr (\x (xs, i) -> (if mod i n == 0 then xs else x:xs, i - 1)) ([], length xs) xs
  +
</haskell>
  +
  +
Another foldl solution:
  +
<haskell>
  +
dropEvery :: [a] -> Int -> [a]
  +
dropEvery lst n = snd $ foldl helper (1, []) lst
  +
where helper (i,acc) x = if n == i
  +
then (1,acc)
  +
else (i+1,acc++[x])
 
</haskell>
 
</haskell>

Revision as of 14:10, 9 September 2012

(**) Drop every N'th element from a list.

dropEvery :: [a] -> Int -> [a]
dropEvery [] _ = []
dropEvery (x:xs) n = dropEvery' (x:xs) n 1 where
    dropEvery' (x:xs) n i = (if (n `divides` i) then
        [] else
        [x])
        ++ (dropEvery' xs n (i+1))
    dropEvery' [] _ _ = []
    divides x y = y `mod` x == 0

An alternative iterative solution:

dropEvery :: [a] -> Int -> [a]
dropEvery list count = helper list count count
  where helper [] _ _ = []
        helper (x:xs) count 1 = helper xs count count
        helper (x:xs) count n = x : (helper xs count (n - 1))

A similar iterative solution but using a closure:

dropEvery :: [a] -> Int -> [a]
dropEvery xs n = helper xs n
    where helper [] _ = []
          helper (x:xs) 1 = helper xs n
          helper (x:xs) k = x : helper xs (k-1)

Yet another iterative solution which divides lists using Prelude:

dropEvery :: [a] -> Int -> [a]
dropEvery [] _ = []
dropEvery list count = (take (count-1) list) ++ dropEvery (drop count list) count

A similar approach using guards:

dropEvery :: [a] -> Int -> [a]
dropEvery xs n
  | length xs < n = xs
  | otherwise     = take (n-1) xs ++ dropEvery (drop n xs) n

Using zip:

dropEvery = flip $ \n -> map snd . filter ((n/=) . fst) . zip (cycle [1..n])

Using zip and list comprehensions

dropEvery :: [a] -> Int -> [a]
dropEvery xs n = [ i | (i,c) <- ( zip xs [1,2..]), (mod c n) /= 0]

A more complicated approach which first divides the input list into sublists that do not contain the nth element, and then concatenates the sublists to a result list (if not apparent: the author's a novice):

dropEvery :: [a] -> Int -> [a]
dropEvery [] _ = []
dropEvery xs n = concat (split n xs)
 where 
  split _ [] = []
  split n xs = fst splitted : split n ((safetail . snd) splitted)
   where 
    splitted = splitAt (n-1) xs
    safetail xs | null xs = []
    		| otherwise = tail xs

First thing that came to mind:

dropEvery xs n = map fst $ filter (\(x,i) -> i `mod` n /= 0) $ zip xs [1..]

The filter function can be simplified as seen above:

dropEvery xs n = map fst $ filter ((n/=) . snd) $ zip xs (cycle [1..n])

And yet another approach using folds:

dropEvery :: Int -> [a] -> [a]
dropEvery n xs = snd $ foldl (\acc e -> if fst acc > 1 then (fst acc - 1, snd acc ++ [e]) else (n, snd acc)) (n, []) xs

Another very similar approach to the previous:

dropEvery :: [a] -> Int -> [a]
dropEvery xs n = fst $ foldr (\x (xs, i) -> (if mod i n == 0 then xs else x:xs, i - 1)) ([], length xs) xs

Another foldl solution:

dropEvery :: [a] -> Int -> [a]
dropEvery lst n = snd $ foldl helper (1, []) lst
    where helper (i,acc) x = if n == i 
                             then (1,acc)
                             else (i+1,acc++[x])