Reputation: 537

How to create a random integer array generator

I have created a method which takes generate random numbers with the condition that the next random number doesn't match with the previous one inside the array here's the code

// some code   

int k=0;

//some code....

randomgenerator(k); // method call

public void randomgenerator(int j)
{

    for(j=0; j<=99; j++){
        if(j >= 1){
            if (randomset.get(j) == randomset.get(j-1)){
                randomset.add(0 + ( j  ,  int)(Math.random() * ((99 - 0) + 1)));
            }
            else{
                randomset.add(0 + (int)(Math.random() * ((99 - 0) + 1)));
            }
        }       
    }
}

The error I get is java.lang.IndexOutOfBoundsException: Invalid index 1, size is 1

Upvotes: 2

Answers (5)

Ionut

Reputation: 486

What is with you're function? You receive a parameter named j and then you reassigned it?

randomset.get(j) == randomset.get(j-1) <- at this line you have a java.lang.IndexOutOfBoundsException because you call for the value from position 1 but in you're list you have only a value on position 0; so an error is thrown

and, what is this ? ((99 - 0) + 1)) ,you could white 100, it is easy, and more readable

by the way, at this line you have an error, randomset.add(0 + ( j , (int)(Math.random() * ((99 - 0) + 1)));

You should write a cleaner code.

I've prepared a solution for you: a function which generates a List with random numbers and respects you condition: two consecutive numbers are not the same.

You must call this method generateRandomList with the number of elements you want to generate.

public static final Integer MAX_RANDOM_NUMBER = 100;


public static List<Integer> generateRandomList(int randomNumbers) {
    return generateRandomList(randomNumbers, -1);
}

private static List<Integer> generateRandomList(final int randomNumbers, final int previousNumber) {
    if (randomNumbers == 1) {
        return new ArrayList<Integer>() {
            {
                add(getNextNumber(previousNumber));
            }

        };
    } else {

        return new ArrayList<Integer>() {
            {
                int value = getNextNumber(previousNumber);
                add(value);
                addAll(generateRandomList(randomNumbers - 1, value));
            }

        };
    }
}

private static int getNextNumber(int previousNumber) {
    boolean generateNewValue = true;
    int currentValue = 0;
    while (generateNewValue) {
        currentValue = (int) (Math.random() * MAX_RANDOM_NUMBER);
        generateNewValue = currentValue == previousNumber;
    }
    return currentValue;
}

Upvotes: 0

andr

Reputation: 16054

You can't reference an element of ArrayList whose index is not in bounds [0, size() - 1]. Creating ArrayList via ArrayList() creates a list of size 0. To add elements to this array you must call one of the methods that adds an element, e.g. add(). Your first call is to get(), but the list has size 0, so even get(0) will cause an IndexOutOfBoundsException.

What to do depends on the expected contents of the list. In your case, I would recommend writing a helper function that generates a random number in range excluding specified number. You could use that function in a simple loop to generate the whole list, passing previous element to mentioned helper function.

Example:

public static int randomInRange(int a, int b) {
    return (int)(Math.random() * (b - a + 1));
}

public static int randomInRangeExcluding(int a, int b, int excluding) {
    int result = (int)(Math.random() * (b - a));
    if (result == excluding) {
        result++;
    }
    return result;
}

public static List<Integer> generateRandomList(int size) {
    ArrayList<Integer> result = new ArrayList<Integer>();
    for (int j = 0; j <= size; j++) {
        if (j > 0) {
            result.add(randomInRangeExcluding(0, size - 1, result.get(j - 1)));
        } else {
            result.add(randomInRange(0, size - 1));
        }    
    }
    return result;
}

and get the value using:

generateRandomList(100);

Calling this results in a list of random integers having no two consecutive elements equal:

[27, 34, 53, 92, 56, 93, 21, 22, 45, 95, 48, 25, 18, 26, 54, 1, 82, 26, 5, 62, 84, 23, 8, 84, 25, 0, 36, 37, 54, 95, 4, 26, 65, 53, 81, 16, 47, 56, 73, 46, 60, 50, 37, 89, 61, 84, 23, 79, 47, 87, 68, 49, 15, 17, 55, 71, 17, 55, 71, 51, 67, 33, 80, 47, 81, 24, 10, 41, 76, 60, 12, 17, 96, 43, 57, 55, 41, 56, 21, 85, 98, 40, 9, 39, 53, 28, 93, 70, 89, 80, 40, 41, 30, 81, 33, 53, 73, 28, 38, 87, 29]

Upvotes: 0

hmatar

Reputation: 2419

Because initially randomset is empty therefore its size is 0 and returns exception at index 1. The best way to add randomset.add(0 + (int)(Math.random() * ((99 - 0) + 1))); if j < 1 (not >=1).

Correct code:

public void randomgenerator(int j)
{
for(j=0; j<=99; j++){
    if(j >= 1){
        if (randomset.get(j) == randomset.get(j-1)){
            randomset.add(0 + ( j  ,  int)(Math.random() * ((99 - 0) + 1)));
        }
        else{
            randomset.add(0 + (int)(Math.random() * ((99 - 0) + 1)));
        }
    }
    else {
           randomset.add(0 + (int)(Math.random() * ((99 - 0) + 1)));
    }       
}

}

Upvotes: 1