Fill an array with random numbers

Question

I need to create an array using a constructor, add a method to print the array as a sequence and a method to fill the array with random numbers of the type double.

Here's what I've done so far:

import java.util.Random;

public class NumberList {
    private static double[] anArray;

    public static double[] list() {
        return new double[10];
    }
    
    public static void print(){
        System.out.println(String.join(" ", anArray);
    }

    public static double randomFill() {
        return (new Random()).nextInt();
    }
    
    public static void main(String args[]) {
        // TODO
    }
}

I'm struggling to figure out how to fill the array with the random numbers I have generated in the randomFill method. Thanks!

If your array is supposed to hold `double`s, why are you generating random `int`s? — GriffeyDog, Dec 04 '13 at 16:45
You're almost there, the steps you describe now need to go into `main()`. — DeltaLima, Dec 04 '13 at 16:47
Please use google first http://mathbits.com/MathBits/Java/arrays/Initialize.htm — aalku, Dec 04 '13 at 16:47

score 16 · Answer 1 · edited Nov 23 '19 at 17:37

16

You can use IntStream ints() or DoubleStream doubles() available as of java 8 in Random class. something like this will work, depends if you want double or ints etc.

Random random = new Random();

int[] array = random.ints(100000, 10,100000).toArray();

you can print the array and you'll get 100000 random integers.

edited Nov 23 '19 at 17:37

idmean

14,540
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answered Jan 18 '18 at 05:45

Samir Ouldsaadi

1,404
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score 9 · Accepted Answer · answered Dec 04 '13 at 16:49

You need to add logic to assign random values to double[] array using randomFill method.

Change

 public static double[] list(){
    anArray = new double[10];   
    return anArray;
 }

To

 public static double[] list() {
    anArray = new double[10];
    for(int i=0;i<anArray.length;i++)
    {
        anArray[i] = randomFill();
    }
    return anArray;
}

Then you can call methods, including list() and print() in main method to generate random double values and print the double[] array in console.

 public static void main(String args[]) {

list();
print();
 }

One result is as follows:

-2.89783865E8 
1.605018025E9 
-1.55668528E9 
-1.589135498E9 
-6.33159518E8 
-1.038278095E9 
-4.2632203E8 
1.310182951E9 
1.350639892E9 
6.7543543E7

score 3 · Answer 3 · answered Dec 04 '13 at 16:45

This seems a little bit like homework. So I'll give you some hints. The good news is that you're almost there! You've done most of the hard work already!

Think about a construct that can help you iterate over the array. Is there some sort of construct (a loop perhaps?) that you can use to iterate over each location in the array?
Within this construct, for each iteration of the loop, you will assign the value returned by randomFill() to the current location of the array.

Note: Your array is double, but you are returning ints from randomFill. So there's something you need to fix there.

Arvind Kumar Avinash · Answer 4 · 2021-03-30T23:29:37.170

`Arrays#setAll`

import java.util.Arrays;

public class Main {
    public static void main(String[] args) {
        double[] anArray = new double[10];
        Arrays.setAll(anArray, i -> Math.random());
        System.out.println(Arrays.toString(anArray));
    }
}

Output of a sample run:

[0.1182317850027168, 0.9437573020895418, 0.2690505524813662, 0.6771923722130754, 0.04893586074165357, 0.42483010937765653, 0.16310798731469023, 0.2541941051963008, 0.9838342001474454, 0.8595732419005068]

Alternatively,

import java.util.Arrays;
import java.util.stream.IntStream;

public class Main {
    public static void main(String[] args) {
        double[] anArray = new double[10];
        
        anArray = IntStream.range(0, anArray.length)
                    .mapToDouble(i -> Math.random())
                    .toArray();
        
        System.out.println(Arrays.toString(anArray));
    }
}

Alternatively,

import java.util.Arrays;
import java.util.Random;

public class Main {
    public static void main(String[] args) {
        double[] anArray = new Random().doubles(10).toArray();
        System.out.println(Arrays.toString(anArray));
    }
}

score 2 · Answer 5 · edited Oct 08 '20 at 04:27

2

Try this:

public void double randomFill() {
    Random rand = new Random();
    for (int i = 0; i < this.anArray.length(); i++)
        this.anArray[i] = rand.nextInt();
}

edited Oct 08 '20 at 04:27

ib.

27,830
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100

answered Dec 04 '13 at 16:45

hawks

114
1
4

score 2 · Answer 6 · edited Oct 08 '20 at 13:53

You could loop through the array and in each iteration call the randomFill method. Check it out:

import java.util.Random;

public class NumberList {
    private static double[] anArray;

    public static double[] list() {  
        return new double[10];
    }

    public static void print() {
        System.out.println(String.join(" ", anArray));
    }


    public static double randomFill() {
        return (new Random()).nextInt();
    }

    public static void main(String args[]) {
        list();
        for(int i = 0; i < anArray.length; i++)
            anArray[i] = randomFill();

        print();
    }
}

score 2 · Answer 7 · answered Aug 04 '18 at 11:26

2

People don't see the nice cool Stream producers all over the Java libs.

public static double[] list(){
    return new Random().ints().asDoubleStream().toArray();
}

answered Aug 04 '18 at 11:26

Johannes Kuhn

14,778
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Add a `.limit(0)`. – Johannes Kuhn Mar 31 '21 at 02:29

Rahul · Answer 8 · 2020-10-13T10:00:21.167

You can call the randomFill() method in a loop and fill your array in the main() method like this.

public static void main(String args[]) {
    for (int i = 0; i < anArray.length; i++) {
        anArray[i] = randomFill();
    }
}

You would then need to use rand.nextDouble() in your randomFill() method the array to be filled is a double array. The below snippet should help you get random double values to be filled into your array.

double randomDoubleValue = rand.nextDouble();
return randomDoubleValue;

score 1 · Answer 9 · answered Aug 03 '18 at 18:00

1

Probably the cleanest way to do it in Java 8:

private int[] randomIntArray() {
   Random rand = new Random();
   return IntStream.range(0, 23).map(i -> rand.nextInt()).toArray();
}

answered Aug 03 '18 at 18:00

Jeff smith

147
7

2

Nope. `new Random().ints().limit(22).toArray();`. – Johannes Kuhn Aug 04 '18 at 11:22

score 1 · Answer 10 · answered Jan 28 '20 at 22:11

I tried to make this as simple as possible in Java. This makes an integer array of 100 variables and fill it with integers between 0 and 10 using only three lines of code. You can easily change the bounds of the random number too!

int RandNumbers[]=new int[100];
for (int i=0;i<RandNumbers.length;i++)
    RandNumbers[i]=ThreadLocalRandom.current().nextInt(0,10);

score 1 · Answer 11 · answered Feb 06 '20 at 07:52

1

Fast and Easy

double[] anArray = new Random().doubles(10).toArray();

answered Feb 06 '20 at 07:52

Julian

334
4
18

score 0 · Answer 12 · answered Dec 04 '13 at 16:55

You can simply solve it with a for-loop

private static double[] anArray;

public static void main(String args[]) {
    anArray = new double[10]; // create the Array with 10 slots
    Random rand = new Random(); // create a local variable for Random
    for (int i = 0; i < 10; i++) // create a loop that executes 10 times
    {
        anArray[i] = rand.nextInt(); // each execution through the loop
                                        // generates a new random number and
                                        // stores it in the array at the
                                        // position i of the for-loop
    }
    printArray(); // print the result
}

private static void printArray() {
    for (int i = 0; i < 10; i++) {
        System.out.println(anArray[i]);
    }
}

Next time, please use the search of this site, because this is a very common question on this site, and you can find a lot of solutions on google as well.

Vibhatha Abeykoon · Answer 13 · 2016-10-30T19:34:55.003

If the randomness is controlled like this there can always generate any number of data points of a given range. If the random method in java is only used, we cannot guarantee that all the numbers will be unique.

package edu.iu.common;

import java.util.ArrayList;
import java.util.Random;

public class RandomCentroidLocator {

public static void main(String [] args){
 
 int min =0;
 int max = 10;
 int numCentroids = 10;
 ArrayList<Integer> values = randomCentroids(min, max, numCentroids); 
 for(Integer i : values){
  System.out.print(i+" ");
 }
 
}

private static boolean unique(ArrayList<Integer> arr, int num, int numCentroids) {

 boolean status = true;
 int count=0;
 for(Integer i : arr){
  if(i==num){
   count++;
  }
 }
 if(count==1){
  status = true;
 }else if(count>1){
  status =false;
 }
 

 return status;
}

// generate random centroid Ids -> these Ids can be used to retrieve data
// from the data Points generated
// simply we are picking up random items from the data points
// in this case all the random numbers are unique
// it provides the necessary number of unique and random centroids
private static ArrayList<Integer> randomCentroids(int min, int max, int numCentroids) {

 Random random = new Random();
 ArrayList<Integer> values = new ArrayList<Integer>();
 int num = -1;
 int count = 0;
 do {
  num = random.nextInt(max - min + 1) + min;
  values.add(num);
  int index =values.size()-1;
  if(unique(values, num, numCentroids)){    
   count++;
  }else{
   values.remove(index);
  }
  
 } while (!( count == numCentroids));

 return values;

}

}

score 0 · Answer 14 · answered Nov 17 '17 at 20:22

This will give you an array with 50 random numbers and display the smallest number in the array. I did it for an assignment in my programming class.

public static void main(String args[]) {
    // TODO Auto-generated method stub

    int i;
    int[] array = new int[50];
    for(i = 0; i <  array.length; i++) {
        array[i] = (int)(Math.random() * 100);
        System.out.print(array[i] + "  ");

        int smallest = array[0];

        for (i=1; i<array.length; i++)
        {
        if (array[i]<smallest)
        smallest = array[i];
        }



    }

}

}`

score 0 · Answer 15 · answered Apr 06 '21 at 17:13

Java 8+

You can use ThreadLocalRandom with the Random#doubles method to generate a certain number of random doubles.

import java.util.concurrent.ThreadLocalRandom;
//...
public static double[] list() {
    return anArray = ThreadLocalRandom.current().doubles(10).toArray();
    //Generate 10 random doubles
}

To generate random doubles within a certain range, you can provide two more arguments to Random#doubles.

import java.util.concurrent.ThreadLocalRandom;
//...
private static final double MIN = 0, MAX = 10;
public static double[] list() {
    return anArray = ThreadLocalRandom.current().doubles(10, MIN, MAX).toArray();
    //Generate 10 random doubles between MIN (inclusive) and MAX (exclusive)
}

Fill an array with random numbers

15 Answers15

`Arrays#setAll`

Alternatively,

Alternatively,

Java 8+

Linked

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