Using Random Class to populate array and then print it's values

Question

I'm relatively new to programming in Java.

I've got a task for class in which I have to populate an array with random numbers from 0 to 100. However the user needs to be able to define the number of random numbers which are inserted within this array. For this I've created the variable 'n' for now.

I think I've got most of the code nailed. I wanted to experiment using the Random class to create the random numbers, as opposed to math.random*100.

I'm getting an error:

Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException: 10 at CreateArray.main(CreateArray.java:11)

Here's my code:

import java.util.Scanner;
import java.util.Random;

public class CreateArray{
    public static void main(String [] args){ 
        int n = 10;
        Random myrandom = new Random(); 
        int[] engine = new int[n]; // Initialising my array and giving it the name engine, I'm also passing value 'n' to my array length
        engine[n] = myrandom.nextInt(100-0); //This exclues 100 and 0, to include those I would say +100 or +0

        for(int i=0; i < engine.length; i++){ //Using a for loop to cycle through my array and print out its elements
            System.out.println(engine[i]); //Printing out my array 
        }
    }
}

`myrandom.nextInt(100-0)` would not exclude 100 and 0. It would give a number with (100) possible outcomes, starting at 0, giving you 0 to 99. To exclude 0 and 100, you would want `myrandom.nextInt(99) + 1`. — Clark Kent, Oct 15 '15 at 18:30

YoungHobbit · Answer 1 · 2015-10-15T18:45:21.993

1

engine[n] = myrandom.nextInt(100-0);

This should be like this:

engine[n-1] = myrandom.nextInt(99)+1; // To exclude 0 and 100.

Java array range is 0 to n-1. The last element of the array is n-1 not n. You are accessing the n index, which not even in the range.

For adding the elements to the array. If add them using the loop then you don't need to add the value for last element separately. It will be taken care in the loop.

for(int i=0; i < engine.length; i++) { 
    engine[i] = myrandom.nextInt(99)+1; // To exclude 0 and 100.
}

edited Oct 15 '15 at 18:45

answered Oct 15 '15 at 18:23

YoungHobbit

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omg yes! thank you, only problem now is I'm getting 9 zero values and then a random number... 0 0 0 0 0 0 0 0 0 32 – Anthony J Oct 15 '15 at 18:25
That is because you are probably setting only one value which is `n-1`. set the value for each element in the for loop, similar to how you are printing them. – YoungHobbit Oct 15 '15 at 18:26
Got it, will give it a go – Anthony J Oct 15 '15 at 18:28

score 1 · Accepted Answer · edited May 23 '17 at 12:03

As for your error, the size of your array is n, so the indexes in your array will be from 0 to n-1. The code you have shown in your post tries to put a random variable in an index past the end of the array that does not exist (it is 'out of the bounds' of the array, thus the ArrayIndexOutOfBoundsException). When n=10, your array will have indexes 0 through 9, and you are trying to put a value at index 10.

Instead, you want to initialize each index in your array with a random integer, so you will want to loop through your array, like you are doing for printing the array, and create a new random variable for each index.

For generating random numbers, if you are using Java 1.7 or greater, you can use ThreadLocalRandom. See this great answer to a similar question for the 'proper' way to generate a random number in Java (whether you're using Java 1.7+ or not).

If using Java < 1.7, the 'standard' way to generate a random number in a range looks something like this:

public static int randInt(int min, int max) {
    //you can initialize your Random in a number of different ways. This is just one.
    Random rand = new Random();

    int randomNum = rand.nextInt((max - min) + 1) + min;

    return randomNum;
}

Note that in your code you mention that myrandom.nextInt(100-0); will give you numbers between 0 and 100, excluding 0 and 100. That is not entirely correct. That will give you numbers between 0 and 99, including 0 and 99.

Since you know you only want numbers from 1 to 99, you can plug those in the method above to get this.

public static int randInt() {
    Random rand = new Random();
    return rand.nextInt(99) + 1;
}

Since this is basically a one-liner, it doesn't really necessitate its own method, so you could generate your random numbers and store them in your engine array without the method, like this.

Random myrandom = new Random();
for (int i=0; i < engine.length; i++) {
    engine[i] = myrandom.nextInt(99) + 1;
}

Finally, using Java 1.7+, you can use ThreadLocalRandom, which you can use without initializing a Random instance. Here is what using that would look like.

for (int i=0; i < engine.length; i++) {
    engine[i] = ThreadLocalRandom.current().nextInt(1, 100);
}

Using Random Class to populate array and then print it's values

2 Answers2