How can I select a random value from a list using F#

Question

I\'m new to F# and I\'m trying to figure out how to return a random string value from a list/array of strings.

I have a list like this:

Answer 1

I wrote a blog post on exactly this topic a while ago: http://latkin.org/blog/2013/11/16/selecting-a-random-element-from-a-linked-list-3-approaches-in-f/

3 approaches are given there, with discussion of performance and tradeoffs of each.

To summarize:

// pro: simple, fast in practice
// con: 2-pass (once to get length, once to select nth element)
let method1 lst (rng : Random)  =
    List.nth lst (rng.Next(List.length lst))

// pro: ~1 pass, list length is not bound by int32
// con: more complex, slower in practice
let method2 lst (rng : Random) =
    let rec step remaining picks top =
        match (remaining, picks) with
        | ([], []) -> failwith "Don't pass empty list"
        //  if only 1 element is picked, this is the result
        | ([], [p]) -> p
        // if multiple elements are picked, select randomly from them
        | ([], ps) -> step ps [] -1
        | (h :: t, ps) ->
            match rng.Next() with
            // if RNG makes new top number, picks list is reset
            | n when n > top -> step t [h] n
            // if RNG ties top number, add current element to picks list
            | n when n = top -> step t (h::ps) top
            // otherwise ignore and move to next element
            | _ -> step t ps top
    step lst [] -1

// pro: exactly 1 pass
// con: more complex, slowest in practice due to tuple allocations
let method3 lst (rng : Random) =
    snd <| List.fold (fun (i, pick) elem ->
               if rng.Next(i) = 0 then (i + 1, elem)
               else (i + 1, pick)
           ) (0, List.head lst) lst

Edit: I should clarify that above shows a few ways to get a random element from a list, assuming you must use a list. If it fits with the rest of your program's design, it is definitely more efficient to take a random element from an array.