:: [a] -> Int -> Maybe a -package:safe -package:rio -package:relude

A total variant of the list index function (!!).

[2,3,4] !? 1    == Just 3
[2,3,4] !? (-1) == Nothing
[]      !? 0    == Nothing

Lookup function (safe). O(min n index).

Safe version of (!!).

Array indexing with negative values allowed

O(1) Safe indexing.

A (!!) that fails using mzero

O(1) Indexing in a monad. The monad allows operations to be strict in the vector when necessary. Suppose vector copying is implemented like this:

copy mv v = ... write mv i (v ! i) ...

For lazy vectors, v ! i would not be evaluated which means that mv would unnecessarily retain a reference to v in each element written. With indexM, copying can be implemented like this instead:

copy mv v = ... do
x <- indexM v i
write mv i x

Here, no references to v are retained because indexing (but not the element) is evaluated eagerly.

O(1) Indexing in a monad, without bounds checks. See indexM for an explanation of why this is useful.

Deprecated: Use XMonad.Prelude.(!?) instead.

Safer version of !! -

O(n) Convert the first n elements of a list to a vector. It's expected that the supplied list will be exactly n elements long. As an optimization, this function allocates a buffer for n elements, which could be used for DoS-attacks by exhausting the memory if an attacker controls that parameter.

fromListN n xs = fromList (take n xs)

Examples

>>> import qualified Data.Vector as V

>>> V.fromListN 3 [1,2,3,4,5]
[1,2,3]

>>> V.fromListN 3 [1]
[1]

>>> fromReverseListN 3 [1,2,3] :: Data.Vector.Vector Int
[3,2,1]