Assistance writing a recursive function that sums a list of arrays in Java

Question

I'm attempting to write a recursive program that adds up the sum of a list of arrays. It requires use of a helper function, as reflected below.

I am failing to see what I am doing wrong, but I think it has to do with a failure to get the program to remove items from the list after they are added up. Could anyone help me?

EDIT: Let me explain my logic in pseudocode: I want to get the program to recognize an item in an array list and store that in double sum. Then through recursive passing I want to keep repeating this process by moving through the items in the list of arrays and add them to double sum.

public static double sum (double[] a) {
    double sum = 0;
    return sumHelper (a, a.length-1, sum); // TODO
}

public static double sumHelper (double[] a, int i, double sum) 
{
    if (a.length == 0) {
        return sumHelper (a, a-1, sum);
    }
    else {
        sum = sum + a[i];
        i = i-1;
        a.length -1; // THIS LINE DOESN'T WORK
        return sumHelper (a, a-1, sum);
    }   
    return sum;
}

public static void main (String[] args) {
    double[] list0 = new double[] {};
    double[] list1 = new double[] { 5 };
    double[] list2 = new double[] { -3, 5 };
    double[] list3 = new double[] { 2, -3, 5 };
    double[] list4 = new double[] { -1, 2, -3, 5 };
    double[] list5 = new double[] { 33, 44, 55 };

    System.out.println("Display the sum of the array contents");
    System.out.println ("list5: " +sum (list5));
    System.out.println ("list0: " +sum (list0));
    System.out.println ("list1: " +sum (list1));
    System.out.println ("list2: " +sum (list2));
    System.out.println ("list3: " +sum (list3));
    System.out.println ("list4: " +sum (list4));

You never call sumHelper in the else block of sumHelper so there isn't any recursion happening — Aaron Davis
– Aaron Davis, Commented Jan 23, 2017 at 18:34
Sorry forgot to include the recursive lines, which I temporarily removed to test shortening the length of the arrays by other means. It is edited to include them again. — MarkCoder
– MarkCoder, Commented Jan 23, 2017 at 18:36
Also you are passing the index to the helper so you don't need to modify the array at all, just update the index and sum — Aaron Davis
– Aaron Davis, Commented Jan 23, 2017 at 18:36
a.length -1; // THIS LINE DOESN'T WORK Of course it doesn't. This syntax is invalid. — Christopher Schneider
– Christopher Schneider, Commented Jan 23, 2017 at 18:39

Andy · Accepted Answer · 2017-01-23 19:06:36Z

1

Is this is what you are trying to do?

public static double sum (double[] a) {
    double sum = 0;
    return sumHelper (a, a.length-1, sum); // TODO
}

public static double sumHelper (double[] a, int i, double sum) 
{
    if (i < 0) { 
        return sum;
    }
    else {
        sum = sum + a[i];
        i = i-1;
        return sumHelper (a, i, sum);
    }   
}

public static void main(String[] args) {
    double[] list0 = new double[] {};
    double[] list1 = new double[] { 5 };
    double[] list2 = new double[] { -3, 5 };
    double[] list3 = new double[] { 2, -3, 5 };
    double[] list4 = new double[] { -1, 2, -3, 5 };
    double[] list5 = new double[] { 33, 44, 55 };

    System.out.println("Display the sum of the array contents");
    System.out.println ("list5: " +sum (list5));
    System.out.println ("list0: " +sum (list0));
    System.out.println ("list1: " +sum (list1));
    System.out.println ("list2: " +sum (list2));
    System.out.println ("list3: " +sum (list3));
    System.out.println ("list4: " +sum (list4));

}

Results:

Display the sum of the array contents list5: 132.0 list0: 0.0 list1: 5.0 list2: 2.0 list3: 4.0 list4: 3.0

answered Jan 23, 2017 at 19:06

Andy

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2 Comments

MarkCoder Over a year ago

That was pretty much it. Clearly I confused my death to death by trying to adjust i both recursively and within main, and then on top of that needlessly trying to adjust the length of a in three different places. This is tremendously helpful! Sorry for the newcomer question!

Andy Over a year ago

Anyways Java arrays length are immutable. (You can't change the length).

ravip · Accepted Answer · 2017-01-23 19:07:51Z

You misplaced the variables, a-1 instead of i. And array will start from 0 index, So you need to add the elements till the 0 index.

static double[] list0 = new double[] {};
        static double[] list1 = new double[] { 5 };
        static double[] list2 = new double[] { -3, 5 };
        static double[] list3 = new double[] { 2, -3, 5 };
        static double[] list4 = new double[] { -1, 2, -3, 5 };
        static double[] list5 = new double[] { 33, 44, 55 };
        public static void main(String[] args) {
            // TOsuDO Auto-generated method stub
            System.out.println("Display the sum of the array contents");
            System.out.println ("list5: " +sum (list5));
            System.out.println ("list0: " +sum (list0));
            System.out.println ("list1: " +sum (list1));
            System.out.println ("list2: " +sum (list2));
            System.out.println ("list3: " +sum (list3));
            System.out.println ("list4: " +sum (list4));
        }
        public static double sum (double[] a) {
            double sum = 0;
            return sumHelper (a, a.length-1, sum); // TODO
        }
        public static double sumHelper (double[] a, int i, double sum) 
        {
            if (i == -1) {
                {
                    System.out.println("Added all the elements");
                    return sum;
                }
            }
            else {
                sum = sum + a[i];
                i = i-1;
                //a.length -1; // THIS LINE DOESN'T WORK
                return sumHelper (a, i, sum);
            }
        }

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