循环遍历For循环，ArrayIndexOutOfBounds

Question

Tried changing around the for loop condition several times, still get ArrayIndexOutOfBounds when I pass zero as a parameter. Every other number works fine, I am trying to account for zero by setting it equal to zero automatically, am I doing that part incorrectly? Everything compiles and runs fine except for zero.

private static int iterativeCalculation(int userEntry)
    {

        int iterativeArray[] = new int[userEntry + 1];
        iterativeArray[0] = 0;
        iterativeArray[1] = 1;

        for (int i = 2; i <= userEntry; i++)
        {
          iterativeArray[i] = (3 * iterativeArray[i - 1]) - (2 * iterativeArray[i - 2]);
          iterativeEfficiencyCounter++;  
        } 
          return iterativeArray[userEntry];

    }

public static void main(String[] args) {

        System.out.println(iterativeCalculation(0));
}

Tried debugging my way through the code, still not understanding what is going wrong. Would appreciate any help! Thanks!

Answer 1

When you pass zero as parameter, userEntry + 1 = 1.

But here:

    iterativeArray[1] = 1;

You are trying to set the second element's value. Remember that length of array is one less than its actual size. So removing this line will fix it. Or use userEntry + 2 instead and alter your loop accordingly.

EDIT:

If you really want to fix first and second element, then use this instead:

int iterativeArray[] = new int[userEntry + 2];
iterativeArray[0] = 0;
iterativeArray[1] = 1;

This will create an array of adequate base size.

And remember, length you enter in [...] while creating array has to be one more than the actual length you want . Because actual array starts counting from 0.

In your case, you were setting length as 1 (minimum). That would create an array which can store only one element; that is iterativeArray[0] = //something . Anything above that is OutOfBounds .

Answer 2

You are setting iterativeArray[1] = 1; regardless of whether or not there are actually 2 or more items in the array. That will be out of bounds with one element.

Answer 3

I think you should step through the code in debugger to best understand what the problem is. You'll see exactly where it's got a problem if you single-step through the code. This is a fundamental technique and tool.