问题
Probably an easy one, but I've looked through the docs and googled for examples and I'm still not sure of the answer.
If I have a list like this:
[1,2,3,4,5,6,7,8,9,0]
and I want to extract a slice, say from index 4 to index 8 i.e. I want:
[5,6,7,8,9]
What is the idiomatic way to do this in Haskell?
回答1:
First of all, that's not an array, it's a list. I'm not being (merely) pedantic, as arrays are much more problematic in Haskell than lists.
That said, one common way is to use take and drop together:
Prelude> drop 4 . take 9 $ [1,2,3,4,5,6,7,8,9,0]
[5,6,7,8,9]
Prelude> take (9-4) . drop 4 $ [1,2,3,4,5,6,7,8,9,0]
[5,6,7,8,9]
The latter is a bit more efficient.
回答2:
You may be interested in Data.Vector (slice).
ghci> import Data.Vector
ghci> let v = fromList [1..10]
ghci> v
fromList [1,2,3,4,5,6,7,8,9,10]
ghci> slice 4 5 v
fromList [5,6,7,8,9]
Note that slice in Data.Vector takes as inputs the beginning index and the length of the slice.
回答3:
> drop 4 (take 9 [1,2,3,4,5,6,7,8,9,0])
[5,6,7,8,9]
回答4:
Hmmm, not very practical, but maybe it can be improved?
(\(x,y) -> if 4 <= y && y <= 9 then [x] else []) =<< zip [1,2,3,4,5,6,7,8,9] [0..]
来源：https://stackoverflow.com/questions/8529814/get-a-sublist-in-haskell