Question

I have the following demand: getting the array of tuples from the first array according to the elements’ appearance in the second array:

let totals = [| ("old1", "new1"); ("old2", "new2");  ("old3", "new3"); ("old4", "new4")  |]
let changes = [| "new1"; "new4" |]

I want to have this:

let updates = [| ("old1", "new1"); ("old4", "new4") |]

If the both arrays totals and changes have the same length, then I think it is easy:

let updates = Array.zip  changes totals
            |> Array.choose(fun (a, B) -> if a = fst(B) then Some (B) else None)

Unfortunately, totals and changes have different number of elements; therefore, I can not find an easy way to get the elements I need.

Answer 1

The solution posted by pad is correct and will work fine for small number of elements in changes. However, it iterates over the array changes for every element in total, so it may be inefficient for large arrays.

As an alternative, you can turn changes into an F# set type which allows more efficient membership test:

// Create set containing 'changes'
let changesSet = Set.ofArray changes

// Filter totals where the second element is in 'changesSet'
totals |> Array.filter (fun (_, v) -> changesSet.Contains(v))

// Same thing using function composition
totals |> Array.filter (snd >> changesSet.Contains)