List not displaying .System.ListException: List index out of bounds

Question

When I run the below code, I get this error:

System.ListException: List index out of bounds

Why?

public with Sharing class EngageDeltaController {

    public static Engage__c[] getDelta() {

        Engage__c[] x = [SELECT Name, ID,Score__c,CreatedDate,Account_ID__c,Delta__c FROM    Engage__c ];
        if(!x.isEmpty()){
            for(Integer i=0,j=0; i<= x.size();i++)
            {
                if(x[i].Account_ID__c == x[i+1].Account_ID__c)
                {
                    x[i].Delta__c = x[i+1].Score__c-x[i].Score__c;        

                    j++;
                }
                else system.debug('nothing found');
            }
        }
        return x;

    }
}

Answer 1

Not sure why you incorporated j at all... You're getting this error because, when i == x.size(), and you try to get x[i], your index is indeed out of bounds. Even when i == x.size() - 1, you are referencing x[i+1] with the same issue. A more typical loop might be:

for (i = 1; i < x.size(); i++)
{
    Engage__c record1 = x[i-1];
    Engage__c record2 = x[i];
    // logic here
}

Or alternatively:

for (i = 0; i < x.size() - 1; i++)
{
    Engage__c record1 = x[i];
    Engage__c record2 = x[i + 1];
    // logic here
}

---

Here's how the indices work:

List - [ 'a', 'b', 'c', 'd', 'e' ]
Index -   0    1    2    3    4

Notice that the size of the list is 5, but the highest index is 4. If you're using i+1 as an index, the highest value you can use for i is 3.

If you're trying to get each pair, you can either use [i, i+1] and values of i 0-3, or [i-1, i] and values of i 1-4. I prefer the latter because it allows you to use a more concise loop syntax.

Answer 2

try using i< x.size() instead of i<= x.size().

List element counting always starts with 0.