99 questions/Solutions/18

(Difference between revisions)

(**) 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`

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```