Thread: Need help using Arrays!

I have been working on this code for awhile now and I almost have it except I cant figure out how to get rid of the "java.lang.ArrayIndexOutOfBoundsException" at lines 21 & 43(shown In parentheses). I have to generate 100 random numbers between 0-9 and count how many times each number comes up. Can I not take this approach or is there something that needs tweaked?? My full code is below, thank-you!

public static void main(String[] args) {
// TODO code application logic here
int [] numbers = createArray();
System.out.println("The Numbers are:");
displayArray(numbers);
(21) int[] counts = countNumbers(numbers);[/COLOR]
System.out.println();
System.out.println("The occurence of each number are: ");
displayCounts(counts);
}
public static int[] createArray(){
int[] numbers = new int[100];
for(int i=0; i<numbers.length; i++)
numbers[i]= (int)(Math.random()*10);
return numbers;
}
public static void displayArray(int[] numbers){
for (int i = 0; i<numbers.length; i++){
if((i+1)%20 == 0)
System.out.println(numbers[i]);
else
System.out.print(numbers[i] + " ");
}
}
public static int[] countNumbers(int[] numbers){
int[] counts = new int[10];
for (int i =0; i <numbers.length; i++)
(43) counts[numbers[i] + '0']++;
return counts;
}
public static void displayCounts(int[] counts){
for (int i =0; i<counts.length; i++){
if ((i+1)%10 == 0)
System.out.println(counts[i] + " " + (int)(i + '0'));
else
System.out.println(counts[i] + " " + (int)(i + '0') + " ");
}
}
}

If you are getting an ArrayIndexOutOfBoundsException with "counts[numbers[i] + '0']++;" then it might be a good idea to check with System.out.println() which array index you are trying to access.

public static int[] countNumbers(int[] numbers){
    int[] counts = new int[10];
    for (int i =0; i <numbers.length; i++) {
        System.out.println("About to access counts array at index " + (numbers[i] + '0'));
        counts[numbers[i] + '0']++;
    }
    return counts;
}

The indexes producing the problem seem to change each time the program runs, but it always seems to be in the range of indexes 48-55. What does this tell me?

Originally Posted by asb23698

The indexes producing the problem seem to change each time the program runs, but it always seems to be in the range of indexes 48-55. What does this tell me?

Well, the first thing it tells you is what you already knew: you have a problem! Because the array itself only has 10 slots in it so index values in the range 48-55 will break things.

So basically your formula numbers[i]+'0' for calculating the index is wrong and has to be changed. You are wanting to calculate an index value in the correct range: zero to nine.

Think about what counts is supposed to represent, and what index you should use for a given element in the numbers array. Perhaps more System.out.println() might help. (Remember while you're debugging the program you don't actually need 100 numbers.)

public static int[] countNumbers(int[] numbers){
    int[] counts = new int[10];
    for (int i =0; i <numbers.length; i++) {
        System.out.println("Found " + numbers[i] + " in the numbers array")
        System.out.println("About to update counts array at index " + (numbers[i] + '0'));
        counts[numbers[i] + '0']++;
    }
    return counts;
}

What you are looking for is something more appropriate than numbers[i]+'0' to use as an array index.