Question

I have been trying to learn divide and conquer algorithms and I have come up with what I thought would work using java. The algorithm is supposed to take an array of size n that is a base 2. It should divide the array to a base case of 4 then add those for indexes together. Then it will add all those together to find the sum of the entire array. Here is what I have done so far in java and my error. Am I at least on the right track for the divide and conquer algorithms?

Exception raised:

Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException: 8
at getSum.sumArray(getSum.java:17)
at getSum.sumArray(getSum.java:21)
at getSum.main(getSum.java:7)

Here is the code:

public class getSum {
    static int sum = 0;
    public static void main(String[] args) {
            int[] numbers = {2,2,2,2,2,2,2,2};
            int amount = 0;
           amount = sumArray(0,numbers.length,numbers);
           System.out.print(amount);
    }

    public static int sumArray(int first, int last, int[] A){
        int index = last - first;
        if(index == 1){
            return sum;
        }else if(index <= 4 && index > 1){
            for(int i = first; first < last; i++){
                sum += A[i];
            }
            return sum;
        }
        return (sumArray(first, last / 2, A) + sumArray(last / 2, A.length, A));
    }
}

Answer 1

You need to change:

for(int i = first; first < last; i++){

to:

for(int i = first; i < last; i++){

You keep comparing first and last, when you increment only i

And as @Some1.Kill.The.DJ pointed out,

sum should be part of method sumArray