返回方法时的ArrayExceptionOutOfBounds

Question

I am attempting to create a fillArray method that fills and array with 20 random values and sums every third value. I am getting an ArrayExceptionOutOfBounds at line 21 which is when the method is called. Through the debug I have watched the array fill with proper values and the sum properly calculated. I am wondering what the error is.

public static void fillArray(){
    //adding up A[0], A[3], A[6}, ...

    double[] A = new double[20];
    for(int i = 0; i < A.length; i++)
        A[i] = Math.random();

    double sum = 0;
    int k = 0;
    do{
        k += 3;
        sum += A[k];
    }while(k < 20);

    System.out.println("sum = " + sum);
}

Again I am looking to determine the reason for the error not necessarily a way to fix it.

Answer 1

Here is your problem:

do{
    k += 3;
    sum += A[k];
}while(k < 20);

K will be equal to 0, then 3, then 6, etc, up until it reaches 21 and then you try and access A[21] which is out of bounds.

This is because when k = 18 on the 6th iteration of the while loop, (k < 20) is true and therefore the while loop continues and adds another 3 to k making it 21. After that, the while loop stops as k is not less than 20 anymore leaving k with a value of 21.

Answer 2

You're getting the error because you're hitting 21 on a array with the size of 20. To fix:

 do{
    k += 3;
    if(k <= 20){
       sum += A[k];
   }
}while(k < 20);

Answer 3

Your second loop, where you calculate the sum, is a do/while loop, meaning the condition will always get checked after the loop body is executed. You count k up in steps of 3, meaning it will reach 21 at some point before the while (k < 20) condition returns false, resulting in your error.

Answer 4

I think the problem is that k is incrementing and being used as an array index, before the <20 test.

something like this might work better:

for (int k = 0; k < 20; k = k + 3) {
        sum += A[k];
    }

In general, i think the do while construct here is a bit of unnecessary complexity. The version above is easier to read and is a more common pattern.

Answer 5

With the current logic, k will be 3,6,9,12,15,18 and 21. The last value is responsible for the out of bounds exception. Your loop will not stop at 18 as it is smaller than 20. You could resolve the bug by changing your code to this:

do{
    k += 3;
    sum += A[k];
}while(k < 18);

Answer 6

You increment the array index before using it. So you are not only going beyond the array index in last pass, you are also not adding the element at 0 index.