Haskell Language Lists Accessing elements in lists
Example
Access the nth element of a list (zero-based):
list = [1 .. 10]
firstElement = list !! 0 -- 1
Note that !! is a partial function, so certain inputs produce errors:
list !! (-1) -- *** Exception: Prelude.!!: negative index
list !! 1000 -- *** Exception: Prelude.!!: index too large
There's also Data.List.genericIndex, an overloaded version of !!, which accepts any Integral value as the index.
import Data.List (genericIndex)
list `genericIndex` 4 -- 5
When implemented as singly-linked lists, these operations take O(n) time. If you frequently access elements by index, it's probably better to use Data.Vector (from the vector package) or other data structures.
