how to fix: Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException" for boolean array of random numbers in java

Question

I am making a program where I get how many random numbers they want and the highest number they want to go up to, but when I run the code it does not give me the amount of numbers they asked for. it either gives me 1 number or none. Then it says "Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException: 9 at practice4.practice4.main(practice4.java:38)" in red.

double randomNum;
        int highestNumber, numsInLottery, display;
        boolean[] numberUsed;

        System.out.println("Welcome to the Lottery Number Generator!");

        System.out.println("How many numbers are in your lottery?");
        numsInLottery = TextIO.getInt();

        System.out.println("What is the highest possible number?");
        highestNumber = TextIO.getInt();


        numberUsed= new boolean [numsInLottery];

        for (int index = 0; index < numsInLottery; index++)
        {
        numberUsed [index]= false;

        while(index < highestNumber)
        {
            do
            {

                randomNum = Math.random() * highestNumber + 1;
                display = (int) randomNum ;

            } while (numberUsed [display - 1 ] ); 

        System.out.print(display + " ");
        numberUsed [display + 1] = true;

        }   
        }
        }   
        }

before I had numberUsed= new boolean [numsInLottery]; I accidentally put highestNumber where numsInLottery is. when I had that it would give me all the numbers. but now that I have changed it it does not work any more this is what I get now

Welcome to the Lottery Number Generator!
How many numbers are in your lottery?
6
What is the highest possible number?
? 30
1 Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException: 21
    at practice4.practice4.main(practice4.java:35)

Answer 1

do
{
    randomNum = Math.random() * highestNumber + 1;
    display = (int) randomNum ;

} while (numberUsed [display - 1 ] );

In this code you are accessing some random index of array which might not even exist for example you have 25 as the random number while you have array of size 6 only so when you access numberUsed[25] you get this exception of IndexOutOfBounds

Answer 2

In your case it seems that you need to use a boolean array to be able to validate the generated number hasn't been generated before, in that case this is the core logic you will be needing.

int[] lotteryNumbers = new int[numsInLottery];
boolean[] lotteryNumberHasBeenGenerated = new boolean[highestNumber];

for(int i=0;i<highestNumber;i++){ //none of the numbers have been generated at this point
    lotteryNumberHasBeenGenerated[i]=false;
}

for (int i = 0; i < numsInLottery; i++) {
    int random;
    do{
        random=((Double)(Math.random() * highestNumber)).intValue();
    }while(lotteryNumberHasBeenGenerated[random]);

    lotteryNumbers[i] = random;
    lotteryNumberHasBeenGenerated[random]=true; //the number is maked as generated

}

This will make the trick and the final method should look like this:

public void lottery() {

    System.out.println("Welcome to the Lottery Number Generator!");
    System.out.println("How many numbers are in your lottery?");
    int numsInLottery = TextIO.getInt();
    System.out.println("What is the highest possible number?");
    int highestNumber = TextIO.getInt();

    int[] lotteryNumbers = new int[numsInLottery];
    boolean[] lotteryNumberHasBeenGenerated = new boolean[highestNumber];

    for(int i=0;i<highestNumber;i++){ //none of the numbers have been   generated at this point
        lotteryNumberHasBeenGenerated[i]=false;
    }

    for (int i = 0; i < numsInLottery; i++) {
        int random;
        do{
            random=((Double)(Math.random() * highestNumber)).intValue();
        }while(lotteryNumberHasBeenGenerated[random]);

        lotteryNumbers[i] = random;
        lotteryNumberHasBeenGenerated[random]=true; //the number is maked as    generated

}

    System.out.println("Lottery numbers:\n" + Arrays.toString(lotteryNumbers));
}

Although if this is the case you will be missing a lot of validations :S

Also, The problem you are having in the console logs is thrown in java when you try to access a position in the array that it does not have, for example if you created an array with size 5 and you are accessing the position number 9 or with index 8 Exception. Here is an explanation of how that works

Answer 3

Again, as I mentioned before, I am assuming your professor wants you to use a boolean array to check if a randomly generated number already exists.

One way to accomplish this is by creating an array of booleans (isLotteryNumberGenerated) where every element represents an indication of whether the index number of that element has been generated or not (For example if we generate the random number 3 we check if isLotteryNumberGenerated[3] == true, true meaning that the number 3 has already been generated (vice versa if false). Because of this, the boolean array must be of size highestNumber + 1 in order to create an index for every number that can possibly be generated (from 0 - highestNumber)

Every time you generate a random number for element i in the lotteryNumbers array you need to check if that number has already been generated for another lotteryNumbers element. In the code below, I use a do...while loop that iterates until it generates a number that has not been generated before.

Using some of White_King's code from above...

public void lottery() {

    System.out.println("How many numbers are in your lottery?");
    int numsInLottery = TextIO.getInt();
    System.out.println("What is the highest possible number?");
    int highestNumber = TextIO.getInt();

    int[] lotteryNumbers = new int[numsInLottery];
    boolean[] isLotteryNumberGenerated = new boolean[highestNumber + 1];

    for (int i = 0; i < numsInLottery; i++) { 
        do { 

            lotteryNumbers[i] = (int) (Math.random() * (highestNumber + 1)); 

        } while (isLotteryNumberGenerated[lotteryNumbers[i]]); 

        isLotteryNumberGenerated[lotteryNumbers[i]] = true; 

    }

   System.out.println("Lottery numbers:\n" + Arrays.toString(lotteryNumbers));

}