Pick a Random Value from an Enum

Pick a random value from an enum?

The only thing I would suggest is caching the result of values() because each call copies an array. Also, don't create a Random every time. Keep one. Other than that what you're doing is fine. So:

public enum Letter {
  A,
  B,
  C,
  //...

  private static final List<Letter> VALUES =
    Collections.unmodifiableList(Arrays.asList(values()));
  private static final int SIZE = VALUES.size();
  private static final Random RANDOM = new Random();

  public static Letter randomLetter()  {
    return VALUES.get(RANDOM.nextInt(SIZE));
  }
}

How can i pick Random Value from an Enum?

To generate the random number, use ThreadLocalRandom.current().nextInt(0, Animal.values().length) and retrieve the value from the enum using Animal.values()[randomNumber], although it appears you're provided with RandomTools.randomValue. I've written my own for completeness.

Declare Animal[] values = Animal.values() once, outside your loop and perform the operations on that to avoid values() being called more than once.

It should look something like this:

import java.util.concurrent.ThreadLocalRandom;

public class SOExample {
    private enum Animal {
        ELEPHANT, LION, TIGER, WASP, SNAKE, MONKEY, EMU
    }

    // It's not clear from the question if you're provided with this or if you have to write it
    private static class RandomTools {
        public static int randomValue(int start, int end) {
            return ThreadLocalRandom.current().nextInt(start, end);
        }
    }

    public static void main(String[] args) {
        Animal[] zoo = generateRandomZoo(100);
        // Printing to STDOUT to check results
        for (int i = 0; i < zoo.length; i++) {
            System.out.println(zoo[i]);
        }
    }

    private static Animal[] generateRandomZoo(int numberOfAnimals) {
        Animal[] animals = new Animal[numberOfAnimals];
        Animal[] values = Animal.values();
        for (int i = 0; i < animals.length; i++) {
            int random = RandomTools.randomValue(0, values.length);
            animals[i] = values[random];
        }
        return animals;
    }
}

How to randomly select an enum value?

// get an array of all the cards
private Card[]cards=Cards.values();
// this generates random numbers
private Random random = new Random();
// choose a card at random
final Card random(){
     return cards[random.nextInt(cards.length)];
}

How to get random value from assigned enum in c++?

There is no way to enumerate the values of an enum.

You can use a table:

std::vector<int> colors = {red, black, pink, rainbow};

and then pick a random element from it.

Picking a random element left as an exercise.

How to generate random value from enum and put it in array?

An enum should be named in the singular, and should start with an uppercase letter.

enum Item { KARTE, MONSTERBALL, MONSTERDEX, FAHRRAD, VM03, HYPERHEILER, AMRENABEERE, TOPGENESUNG, ANGEL, TOPSCHUTZ }

And I suggest you work at devising a more descriptive name than “Item”.

Get an array of all the enum objects by calling values.

Item[] allItems = Item.values() ;

Generate a random number. Use that number as the index into your array of all items.

int randomIndex = ThreadLocalRandom.current().nextInt( 0 , allItems.length ) ;  // ( inclusive , exclusive )
Item rando = allItems[ randomIndex ] ;

Or perhaps you meant to randomly sort (shuffle) all the elements of the enum. If so, make a List backed by the array. Call Collections.shuffle. Changes made to the list also affect the backing array. So the array is shuffled.

Item[] allItems = Item.values() ;
List< Item > list = Arrays.asList( allItems );
Collections.shuffle( list );

More briefly:

Item[] allItems = Item.values() ;
Collections.shuffle( Arrays.asList( allItems ) );

See this code run live at Ideone.com.

[MONSTERBALL, TOPSCHUTZ, KARTE, FAHRRAD, VM03, MONSTERDEX, ANGEL, AMRENABEERE, TOPGENESUNG, HYPERHEILER]

How do I choose a random value from an enum?

Your own enum

Like most abstractions in Rust, random value generation is powered by traits. Implementing a trait is the same for any particular type, the only difference is exactly what the methods and types of the trait are.

Rand 0.5, 0.6, 0.7, and 0.8

Implement Distribution using your enum as the type parameter. You also need to choose a specific type of distribution; Standard is a good default choice. Then use any of the methods to generate a value, such as rand::random:

use rand::{
    distributions::{Distribution, Standard},
    Rng,
}; // 0.8.0

#[derive(Debug)]
enum Spinner {
    One,
    Two,
    Three,
}

impl Distribution<Spinner> for Standard {
    fn sample<R: Rng + ?Sized>(&self, rng: &mut R) -> Spinner {
        // match rng.gen_range(0, 3) { // rand 0.5, 0.6, 0.7
        match rng.gen_range(0..=2) { // rand 0.8
            0 => Spinner::One,
            1 => Spinner::Two,
            _ => Spinner::Three,
        }
    }
}

fn main() {
    let spinner: Spinner = rand::random();
    println!("{:?}", spinner);
}

Rand 0.4

Implement Rand for your enum, then use any of the methods to generate a value, such as Rng::gen:

extern crate rand; // 0.4.2

use rand::{Rand, Rng};

#[derive(Debug)]
enum Spinner {
    One,
    Two,
    Three,
}

impl Rand for Spinner {
    fn rand<R: Rng>(rng: &mut R) -> Self {
        match rng.gen_range(0, 3) {
            0 => Spinner::One,
            1 => Spinner::Two,
            _ => Spinner::Three,
        }
    }
}

fn main() {
    let mut rng = rand::thread_rng();
    let spinner: Spinner = rng.gen();
    println!("{:?}", spinner);
}

Derive

The rand_derive crate can remove the need for some of this boilerplate, but does not exist for Rand 0.5.

extern crate rand;
#[macro_use]
extern crate rand_derive;

use rand::Rng;

#[derive(Debug, Rand)]
enum Spinner {
    One,
    Two,
    Three,
}
    
fn main() {
    let mut rng = rand::thread_rng();
    let spinner: Spinner = rng.gen();
    println!("{:?}", spinner);
}

Someone else's enum

Since you don't control the enum, you have to copy something into your code in order to reference it. You could create an array of the enum and choose from that:

use rand::seq::SliceRandom; // 0.8.0

mod another_crate {
    #[derive(Debug)]
    pub enum Spinner {
        One,
        Two,
        Three,
    }
}

fn main() {
    let mut rng = rand::thread_rng();
    let spinners = [
        another_crate::Spinner::One,
        another_crate::Spinner::Two,
        another_crate::Spinner::Three,
    ];
    let spinner = spinners.choose(&mut rng).unwrap();
    println!("{:?}", spinner);
}

You could replicate the entire enum locally, implement Rand for that, and then have a method that converts back into the other crates representation.

use rand::{
    distributions::{Distribution, Standard},
    Rng,
}; // 0.8.0

mod another_crate {
    #[derive(Debug)]
    pub enum Spinner {
        One,
        Two,
        Three,
    }
}

enum Spinner {
    One,
    Two,
    Three,
}

impl From<Spinner> for another_crate::Spinner {
    fn from(other: Spinner) -> another_crate::Spinner {
        match other {
            Spinner::One => another_crate::Spinner::One,
            Spinner::Two => another_crate::Spinner::Two,
            Spinner::Three => another_crate::Spinner::Three,
        }
    }
}

impl Distribution<Spinner> for Standard {
    fn sample<R: Rng + ?Sized>(&self, rng: &mut R) -> Spinner {
        match rng.gen_range(0..=2) {
            0 => Spinner::One,
            1 => Spinner::Two,
            _ => Spinner::Three,
        }
    }
}

fn main() {
    let spinner = another_crate::Spinner::from(rand::random::<Spinner>());
    println!("{:?}", spinner);
}

You could count the number of spinners and do a match:

use rand::Rng; // 0.8.0

mod another_crate {
    #[derive(Debug)]
    pub enum Spinner {
        One,
        Two,
        Three,
    }
}

fn rando<R: Rng>(mut rng: R) -> another_crate::Spinner {
    match rng.gen_range(0..=2) {
        0 => another_crate::Spinner::One,
        1 => another_crate::Spinner::Two,
        _ => another_crate::Spinner::Three,
    }
}

fn main() {
    let mut rng = rand::thread_rng();
    let spinner = rando(&mut rng);
    println!("{:?}", spinner);
}

You can implement a newtype and implement the random generation for that:

use rand::{distributions::Standard, prelude::*}; // 0.8.0

mod another_crate {
    #[derive(Debug)]
    pub enum Spinner {
        One,
        Two,
        Three,
    }
}

struct RandoSpinner(another_crate::Spinner);

impl Distribution<RandoSpinner> for Standard {
    fn sample<R: Rng + ?Sized>(&self, rng: &mut R) -> RandoSpinner {
        RandoSpinner(match rng.gen_range(0..=2) {
            0 => another_crate::Spinner::One,
            1 => another_crate::Spinner::Two,
            _ => another_crate::Spinner::Three,
        })
    }
}

fn main() {
    let RandoSpinner(spinner) = rand::random();
    println!("{:?}", spinner);
}

See also:

• How do I implement a trait I don't own for a type I don't own?

Is it possible to get a random element from an enum class in Kotlin?

You can get a random enum value by doing:

val randomAnswer = Answer.values().toList().shuffled().first().text

Keep in mind that it goes for convenience over performance.

---

Remember to expose the text property with val. For now, it's just a constructor param:

enum class Answer(val text: String)

How do I select a random value from an enumeration?

Array values = Enum.GetValues(typeof(Bar));
Random random = new Random();
Bar randomBar = (Bar)values.GetValue(random.Next(values.Length));

How to efficiently pick a random element from an enumeration in scala?

Less than a minute with the documentation shows

final def maxId: Int

The one higher than the highest integer amongst those used to identify values in this enumeration.

and

final def apply(x: Int): Value

The value of this enumeration with given id x

So

Animals(scala.util.Random.nextInt(Animals.maxId))
//> res0: recursion.recursion.Animals.Value = Monkey

(assuming all values are used, and you didn't pass in an initial value to the constructor)

Or you could enumerate the values with Animals.values and then refer to this question

---

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