Array error — won't output list of randomly generated numbers

Question

I am taking a beginner Java course and working on arrays. My assignment is to generate a specified amount of random integers between two values inputted by the user.

This is my code:

    // Variables
    int amount,ur,lr,range;

    System.out.println("Enter the amount of random numbers to generate::");
    amount = Integer.parseInt(myInput.readLine());

    System.out.println("\nEenter the upper range:");
    ur = Integer.parseInt(myInput.readLine());

    System.out.println("\nEnter the lower range:");
    lr = Integer.parseInt(myInput.readLine());

    // Create a new array that holds up to user inputted amount of variables
    int[] generate = new int[amount];

    // Create a range of numbers the array can randomly select as its value,
    // given the user's input of the lowest and highest values available to be selected
    range = (int)(Math.random() * ur) +lr;

    System.out.println("\nGENERATED NUMBERS");
    System.out.println("=================");

    // Loop to print randomized numbers up to the amount the user inputted
    for (int n=0; n < generate.length; n++){
        // Give the array the value of the range
        generate[amount] = range;
        // Output [amount] variables
        System.out.println(generate[amount]);
    }

The error I get is

 Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException: 10
 at randomnumbers.RandomNumbers.main(RandomNumbers.java:42)
 Java Result: 1

If anyone could explain why this occurs/what it means, and how to fix, that would be helpful.

Answer 1

The exception is due to

generate[amount] = range;

System.out.println(generate[amount]);

This should be

generate[n] = range;

System.out.println(generate[n]);

This is because your array has a size amount so it goes from position 0 to amount-1, thus you cannot use or assign to generate[amount].

Moreover, you should generate the range number every time since this is only one random number. So the range = (int)(Math.random() * ur) +lr; should be inside the loop

Answer 2

You're indexing the array, generate by a variable, amount , whose value doesn't change once it has been initially set from the user's input. Think again about how to reference array elements and you'll probably see why this won't work.

Answer 3

3 things that will need corection:

1. Arrays are indexed from 0 till lengtOfArray - 1 , so for array of 10 elements last element will be under array[9] . You never want to use something like array[sizeOfArray] = ... because it will be to big number.
2. Math.random() will never return 1 (it will return values from 0 till almost 1), so think your randomizing formula again.
3. Right now you are filling your array with one number randomly generated before your for loop. I believe you want to move it into for loop to get more random numbers in your array.

Answer 4

Rather than using Math.Random() I'd suggest you create an actual Random object since you gonna generate the number more than once as

     Random randomNumber = new Random();

Then inside your loop you do what the other comments above suggest

    for(int n=0;n<generate.length;n++)
    {
      range = lr +randomNumber.nextInt(ur+1);
      generate[n] = range;
      System.out.println(generate[n]);
    }

I'm sure this will cover the base...