Given a list, I would like to produce a second list of elements selected from the first one.
For example:
let l1 = [1..4]
let n = [0; 2]
l1.[n]
should return 1
and 3
, the first and third element of l1
. Unfortunately, it returns an error:
error FS0001: This expression was expected to have type
int but here has type int list
Now, I wonder, does exist a way to pass an argument n
representing a list or, even better, an expression?
F# doesn't offer syntactical support for that sort of indexing. You can ask for a continuous slice of an array like below, but it doesn't cover your exact scenario.
let a = [|1 .. 4|]
let b = a.[0..2]
// returns [|1; 2; 3|]
You can easily write a function for that though:
let slice<'a> (indices: int list) (arr: 'a array) =
[| for idx in indices do yield arr.[idx] |]
slice [0; 2] a
// returns [| 1; 3 |]
I'm leaving proper handling of out-of-bounds access as an exercise ;)
With indexed properties , you should be able to define this for your types: just define a member Item with get(indexes)
which accepts a list/array. But standard lists and arrays already have Item
defined, and you can't have two at once...
I solved it this way:
List.map (fun index -> l1.[index]) n
It's inefficient for large lists, but works for me.
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