Filtering a list based on a condition that depends on the value of multiple list elements-CodePudding

I have a list in F# that looks like this:

let x = [10; 9; 8; 7; 7; 6; 6; 5; 5; 4; 4; 4; 3; 3; 2; 2; 1; 0; 0; 0; -1; -2; -3]

I need to write a function that returns the elements of the list that come before and including two consecutive 0's. It should return:

[10; 9; 8; 7; 7; 6; 6; 5; 5; 4; 4; 4; 3; 3; 2; 2; 1; 0; 0]

This would be pretty easy to do in a for loop in a non-functional language, but what's the correct functional way to approach this?

CodePudding user response：

If you are learning fp you may prefer to do it directly (theres nothing wrong with doing it directly).

let x = [10; 9; 8; 7; 7; 6; 6; 5; 5; 4; 4; 4; 3; 3; 2; 2; 1; 0; 0; 0; -1; -2; -3]

let rec find : int list -> int list = function
    | 0 :: 0 :: _ -> [ 0; 0 ]
    | head :: tail -> head :: find tail
    | _ -> []

let foo = find x

and get

val foo: int list = [10; 9; 8; 7; 7; 6; 6; 5; 5; 4; 4; 4; 3; 3; 2; 2; 1; 0; 0]

CodePudding user response：

There's no single "correct" solution, but here's one that uses built-in functions instead of recursion:

let snip list =
    list
        |> List.pairwise
        |> List.takeWhile (fun pair ->
             pair <> (0, 0))
        |> List.map fst
        |> fun list' -> list' @ [0; 0]

let x = [10; 9; 8; 7; 7; 6; 6; 5; 5; 4; 4; 4; 3; 3; 2; 2; 1; 0; 0; 0; -1; -2; -3]
snip x
    |> printfn "%A"   // [10; 9; 8; 7; 7; 6; 6; 5; 5; 4; 4; 4; 3; 3; 2; 2; 1; 0; 0]

Or, if you like point-free style, you can do this:

let flip f b a = f a b

let snip =
    List.pairwise
        >> List.takeWhile ((<>) (0, 0))
        >> List.map fst
        >> flip List.append [0; 0]