Как найти случайное число в массиве

I am looking for solution to pick number randomly from an integer array.

For example I have an array new int[]{1,2,3}, how can I pick a number randomly?

You can use the Random generator to generate a random index and return the element at that index:

//initialization
Random generator = new Random();
int randomIndex = generator.nextInt(myArray.length);
return myArray[randomIndex];

answered Nov 9, 2011 at 13:17

If you are going to be getting a random element multiple times, you want to make sure your random number generator is initialized only once.

import java.util.Random;

public class RandArray {
    private int[] items = new int[]{1,2,3};

    private Random rand = new Random();

    public int getRandArrayElement(){
        return items[rand.nextInt(items.length)];
    }
}

If you are picking random array elements that need to be unpredictable, you should use java.security.SecureRandom rather than Random. That ensures that if somebody knows the last few picks, they won’t have an advantage in guessing the next one.

If you are looking to pick a random number from an Object array using generics, you could define a method for doing so (Source Avinash R in Random element from string array):

import java.util.Random;

public class RandArray {
    private static Random rand = new Random();

    private static <T> T randomFrom(T... items) { 
         return items[rand.nextInt(items.length)]; 
    }
}

With Java 7, one can use ThreadLocalRandom.

A random number generator isolated to the current thread. Like the global Random generator used by the Math class, a ThreadLocalRandom is initialized with an internally generated seed that may not otherwise be modified. When applicable, use of ThreadLocalRandom rather than shared Random objects in concurrent programs will typically encounter much less overhead and contention. Use of ThreadLocalRandom is particularly appropriate when multiple tasks (for example, each a ForkJoinTask) use random numbers in parallel in thread pools.

public static int getRandomElement(int[] arr){
    return arr[ThreadLocalRandom.current().nextInt(arr.length)];
}

//Example Usage:
int[] nums = {1, 2, 3, 4};
int randNum = getRandomElement(nums);
System.out.println(randNum);

A generic version can also be written, but it will not work for primitive arrays.

public static <T> T getRandomElement(T[] arr){
   return arr[ThreadLocalRandom.current().nextInt(arr.length)];
}

//Example Usage:
String[] strs = {"aa", "bb", "cc"};
String randStr = getRandomElement(strs);
System.out.println(randStr);

answered Jan 26, 2021 at 20:01

Use the Random class:

int getRandomNumber(int[] arr)
{
  return arr[(new Random()).nextInt(arr.length)];
}

answered Nov 9, 2011 at 13:17

answered Nov 9, 2011 at 13:17

Java has a Random class in the java.util package. Using it you can do the following:

Random rnd = new Random();
int randomNumberFromArray = array[rnd.nextInt(3)];

Hope this helps!

answered Nov 9, 2011 at 13:18

Since you have java 8, another solution is to use Stream API.

new Random().ints(1, 500).limit(500).forEach(p -> System.out.println(list[p]));

Where 1 is the lowest int generated (inclusive) and 500 is the highest (exclusive). limit means that your stream will have a length of 500.

 int[] list = new int[] {1,2,3,4,5,6};
 new Random().ints(0, list.length).limit(10).forEach(p -> System.out.println(list[p])); 

Random is from java.util package.

answered Sep 21, 2015 at 11:24

package workouts;

import java.util.Random;

/**
 *
 * @author Muthu
 */
public class RandomGenerator {
    public static void main(String[] args) {
     for(int i=0;i<5;i++){
         rndFunc();
     } 
    }
     public static void rndFunc(){
           int[]a= new int[]{1,2,3};
           Random rnd= new Random();
           System.out.println(a[rnd.nextInt(a.length)]);
       }
}

answered Mar 6, 2016 at 13:56

package io.github.baijifeilong.tmp;

import java.util.concurrent.ThreadLocalRandom;
import java.util.stream.Stream;

/**
 * Created by BaiJiFeiLong@gmail.com at 2019/1/3 下午7:34
 */
public class Bar {
    public static void main(String[] args) {
        Stream.generate(() -> null).limit(10).forEach($ -> {
            System.out.println(new String[]{"hello", "world"}[ThreadLocalRandom.current().nextInt(2)]);
        });
    }
}

answered Jan 3, 2019 at 11:38

It’s a simple one-liner:

const randomElement = array[Math.floor(Math.random() * array.length)];

For example:

const months = ["January", "February", "March", "April", "May", "June", "July"];

const random = Math.floor(Math.random() * months.length);
console.log(random, months[random]);

If you’ve already got underscore or lodash included in your project you can use _.sample.

// will return one item randomly from the array
_.sample(['January', 'February', 'March']);

If you need to get more than one item randomly, you can pass that as a second argument in underscore:

// will return two items randomly from the array using underscore
_.sample(['January', 'February', 'March'], 2);

or use the _.sampleSize method in lodash:

// will return two items randomly from the array using lodash
_.sampleSize(['January', 'February', 'March'], 2);

answered Jun 19, 2015 at 18:40

You may consider defining a function on the Array prototype, in order to create a method [].sample() which returns a random element.

First, to define the prototype function, place this snippet in your code:

Array.prototype.sample = function(){
  return this[Math.floor(Math.random()*this.length)];
}

Later, to sample a random element from the array, just call .sample():

[1,2,3,4].sample() //=> a random element

_{I’m releasing these code snippets into the public domain, under the terms of the CC0 1.0 license.}

answered Nov 24, 2015 at 23:52

~~ is much faster than Math.Floor(), so when it comes to performance optimization while producing output using UI elements, ~~ wins the game. MORE INFO

var rand = myArray[~~(Math.random() * myArray.length)];

But if you know that the array is going to have millions of elements then you might want to reconsider between Bitwise Operator and Math.Floor(), as bitwise operators behave weirdly with large numbers. See below example explained with the output.

var number = Math.floor(14444323231.2); // => 14444323231
var number = 14444323231.2 | 0; // => 1559421343

answered May 5, 2018 at 12:25

The shortest version:

var myArray = ['January', 'February', 'March']; 
var rand = myArray[(Math.random() * myArray.length) | 0]
console.log(rand)

Say you want to choose a random item that is different from the last time (not really random, but still a common requirement)…

/**
 * Return a random element from an array that is
 * different than `last` (as long as the array has > 1 items). 
 * Return null if the array is empty.
*/
function getRandomDifferent(arr, last = undefined) {
  if (arr.length === 0) {
    return null;
  } else if (arr.length === 1) {
    return arr[0];
  } else {
    let num = 0;
    do {
      num = Math.floor(Math.random() * arr.length);
    } while (arr[num] === last);
    return arr[num];
  }
}

Implement like this:

const arr = [1,2,3];
const r1 = getRandomDifferent(arr);
const r2 = getRandomDifferent(arr, r1); // r2 is different than r1.

answered Aug 14, 2012 at 1:57

If you have fixed values (like a month name list) and want a one-line solution

var result = ['January', 'February', 'March'][Math.floor(Math.random() * 3)]

The second part of the array is an access operation as described in Why does [5,6,8,7][1,2] = 8 in JavaScript?

If you want to write it on one line, like Pascual’s solution, another solution would be to write it using ES6’s find function (based on the fact, that the probability of randomly selecting one out of n items is 1/n):

var item = ['A', 'B', 'C', 'D'].find((_, i, ar) => Math.random() < 1 / (ar.length - i));
console.log(item);

Use that approach for testing purposes and if there is a good reason to not save the array in a seperate variable only. Otherwise the other answers (floor(random()*length and using a seperate function) are your way to go.

answered Sep 11, 2017 at 18:01

answered Aug 5, 2022 at 5:26

SeagullSeagull

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ManngoManngo

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Faker.js has many utility functions for generating random test data. It is a good option in the context of a test suite:

const faker = require('faker');
faker.helpers.arrayElement(['January', 'February', 'March']);

As commenters have mentioned, you generally should not use this library in production code.

let rndItem = a=> a[rnd()*a.length|0];
let rnd = ()=> crypto.getRandomValues(new Uint32Array(1))[0]/2**32;

var myArray = ['January', 'February', 'March'];

console.log( rndItem(myArray) )

Kamil KiełczewskiKamil Kiełczewski

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dwiltdwilt

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Max HeiberMax Heiber

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var myArray = ['January', 'February', 'March'];    
var item = myArray[(Math.random()*myArray.length)|0];
console.log('item:', item);

Pavel PPavel P

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Simple Function :

var myArray = ['January', 'February', 'March'];
function random(array) {
     return array[Math.floor(Math.random() * array.length)]
}
random(myArray);

OR

var myArray = ['January', 'February', 'March'];
function random() {
     return myArray[Math.floor(Math.random() * myArray.length)]
}
random();

OR

var myArray = ['January', 'February', 'March'];
function random() {
     return myArray[Math.floor(Math.random() * myArray.length)]
}
random();

answered Dec 17, 2017 at 5:42

In my opinion, better than messing around with prototypes , or declaring it just in time, I prefer exposing it to window:

window.choice = function() {
  if (!this.length || this.length == 0) return;
  if (this.length == 1) return this[0];
  return this[Math.floor(Math.random()*this.length)];
}

Now anywhere on your app you call it like:

var rand = window.choice.call(array)

This way you can still use for(x in array) loop properly

answered May 13, 2014 at 10:27

I’ve found a way around the top answer’s complications, just by concatenating the variable rand to another variable that allows that number to be displayed inside the calling of myArray[];. By deleting the new array created and toying around with it’s complications, I’ve come up with a working solution:

<!DOCTYPE html>
<html>
<body>

<p id="demo"></p>

<script>

var myArray = ['January', 'February', 'March', 'April', 'May'];    

var rand = Math.floor(Math.random() * myArray.length);

var concat = myArray[rand];

function random() {
   document.getElementById("demo").innerHTML = (concat);
}
</script>

<button onClick="random();">
Working Random Array generator
</button>

</body>
</html>

answered Apr 27, 2016 at 22:01

static generateMonth() { 
const theDate = ['January', 'February', 'March']; 
const randomNumber = Math.floor(Math.random()*3);
return theDate[randomNumber];
};

Neil MeyerNeil Meyer

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blagusblagus

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Array.prototype.random = function(n){
  if(n&&n>1){
    const a = [];
    for(let i = 0;i<n;i++){
      a.push(this[Math.floor(Math.random()*this.length)]);
    }
    return a;
  } else {
    return this[Math.floor(Math.random()*this.length)];
  }
}

const mySampleArray =  ['a','b','c','d','e','f','g','h'];

mySampleArray.random(); // return any random value etc. 'a', 'b'
mySampleArray.random(3); //retun an array with random values etc: ['b','f','a'] , ['d','b','d']

alert(mySampleArray.random());
alert(mySampleArray.random(3));

Kamuran SönecekKamuran Sönecek

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let some_array = ['Jan', 'Feb', 'Mar', 'Apr', 'May'];
let months = random_elems(some_array, 3);

console.log(months);

function random_elems(arr, count) {
  let len = arr.length;
  let lookup = {};
  let tmp = [];

  if (count > len)
    count = len;

  for (let i = 0; i < count; i++) {
    let index;
    do {
      index = ~~(Math.random() * len);
    } while (index in lookup);
    lookup[index] = null;
    tmp.push(arr[index]);
  }

  return tmp;
}

RafaelRafael

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Here is an example of how to do it:

$scope.ctx.skills = data.result.skills;
    $scope.praiseTextArray = [
    "Hooray",
    "You're ready to move to a new skill", 
    "Yahoo! You completed a problem", 
    "You're doing great",  
    "You succeeded", 
    "That was a brave effort trying new problems", 
    "Your brain was working hard",
    "All your hard work is paying off",
    "Very nice job!, Let's see what you can do next",
    "Well done",
    "That was excellent work",
    "Awesome job",
    "You must feel good about doing such a great job",
    "Right on",
    "Great thinking",
    "Wonderful work",
    "You were right on top of that one",
    "Beautiful job",
    "Way to go",
    "Sensational effort"
  ];

  $scope.praiseTextWord = $scope.praiseTextArray[Math.floor(Math.random()*$scope.praiseTextArray.length)];

Create one random value and pass to array

Please try following code..

//For Search textbox random value
var myPlaceHolderArray = ['Hotels in New York...', 'Hotels in San Francisco...', 'Hotels Near Disney World...', 'Hotels in Atlanta...'];
var rand = Math.floor(Math.random() * myPlaceHolderArray.length);
var Placeholdervalue = myPlaceHolderArray[rand];

alert(Placeholdervalue);

answered Feb 3, 2017 at 12:49

randojs makes this a little more simple and readable:

console.log( rando(['January', 'February', 'March']).value );

<script src="https://randojs.com/1.0.0.js"></script>

answered Dec 2, 2019 at 2:47

I am really surprised nobody tried to use native random values:

array[Date.now()%array.length]

It will not work for array length over 160000000000, but I am sure you will never create arrays like this

UPD

As far as you question is how to pick random value from array called myArray (with len=3), the solution should be:

myArray[Date.now()%myArray.length]

answered Nov 3, 2021 at 5:38