How to choose a random element from an array in Scala?

Question

For example, there is a Scala array val A = Array("please", "help", "me"). How to choose a random element from this array?

Answer 1

import java.util.Random
// ...
val rand = new Random(System.currentTimeMillis())
val random_index = rand.nextInt(A.length)
val result = A(random_index)

Answer 2

import scala.util.Random

val A = Array("please", "help", "me")
Random.shuffle(A.toList).head

Answer 3

import scala.util.Random

val A = List(1, 2, 3, 4, 5, 6)
A(Random.nextInt(A.size))

Answer 4

We can also add some safety with the Option monad (using the lift function, and a condition)

Actually, if you use this function on Arrays (that could be empty), your result will always be an Option.

Referencial Transparency FTW \o/

def getRandElemO[T](arr: Array[T]): Option[T] =
  if (arr.isEmpty) None
  else arr lift util.Random.nextInt(arr.length)

Answer 5

A better answer that does not involve reshuffling the array at all would be this:

import scala.util.Random

object sample {
  //gets random element from array
  def arr[T](items:Array[T]):T = {
    items(Random.nextInt(items.length))
  }
}

This also works generically

Answer 6

If you want a more idiomatic solution, consider using the typeclass pattern (implicit classes in scala).

implicit class ListOps[A](list: List[A]) {
  def getRandomElement: Option[A] = list match {
    case Nil => None
    case _ => list.lift(scala.util.Random.nextInt(list.size))
  }
  def randomChoice(n: Int): Option[List[A]] =
    (1 to n).toList.foldLeft(Option(List[A]()))((acc, e) => getRandomElement.flatMap(r => acc.map(a => a :+ r)))
}

Now if the implicit class is in scope, you can:

val randomElement: Option[String] = List("this", "is", "a", "list").getRandomElement

If you are sure that the option contains some value, you can use the get method.

randomElement.get // This will return a String (or a NotSuchElementExeption)

Nonetheless, pattern matching or getOrElse are recommended:

randomElement match {
  case None => ??? // This is what you do when a None is encounter (e.g. for empty lists)
  case Some(result) => ??? // The variable result contains a string. 

Note that the randomChoice method assumes substitution of elements.