# Difference between revisions of "99 questions/Solutions/18"

(**) 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, a concise solution using list comprehension:

(Incorrect - only works on sequential data, needs update.)

```--slice :: (Enum a) => [a] -> Int -> Int -> [a]
--slice [] _ _ = []
--slice xs m n = [(xs !! (m-1)) .. (xs !! (n-1))]
```

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' [] _ _ = []
slice (x:xs) n m = slice xs (n - 1) (m - 1)
slice []     _ _ = []
```

Or:

```slice :: [a] -> Int -> Int -> [a]
slice [] _ _  = []
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)
```

A solution using `zip`, `filter` then `map` seems straight-forward to me (NB: this won't work for infinite lists):

```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))
```

Zip, filter, unzip:

```slice :: [a] -> Int -> Int -> [a]
slice xs a b = fst \$ unzip \$ filter ((>=a) . snd) \$ zip xs [1..b]
```

Take and drop can be applied in the opposite order too:

```slice xs i k = drop (i-1) \$ take k xs
```

Using a fold:

```slice :: [a] -> Int -> Int -> [a]
slice (x:xs) begin end = snd \$ foldl helper (1, []) (x:xs)
where helper (i, acc) x = if (i >= begin) && (i <= end) then (i+1, acc ++ [x]) else (i+1, acc)
```