F# filtering list

Question

How would I filter out a list by the index ? List: [400; 5; 401; 6; 403; 7] Filtered List: [5; 6; 7;]

I want to filter out the odd index numbers. So I could compare the values and then print out the largest value.

Answer 1

A simple and straight forward approach would be to recurse through the list and pick every second element:

let getOddIndexed list =
    let rec aux acc xs =
        match xs with
        | _::x::tail -> aux (x::acc) tail
        | _ -> acc |> List.rev
    aux [] list

Answer 2

In the specific case you asked about (keeping the odd index numbers and dropping the even ones), John Reynolds' answer will work. But in the general case where your index-based filter is more complicated, you'd want to use Seq.indexed, which turns any list of items into a list of (index, item) pairs. E.g.:

["apple"; "banana"; "cherry"] |> Seq.indexed |> List.ofSeq
// Produces [(0, "apple"); (1, "banana"); (2, "cherry")]

With this approach, you would then use Seq.filter to do the filtering you want, then turn the sequence back into a list at the end:

let keepOdd (idx, item) =
    // A more complicated filter might use the item parameter too
    idx % 2 <> 0

let input = [400; 5; 401; 6; 403; 7]
input |> Seq.indexed |> Seq.filter keepOdd |> List.ofSeq