finding the sum of positive elements in an array using recursion in java

Question

Hi I'm just learning recursion and I am trying to write a recursive program in Java to find the sum of all positive elements in an array. Using : here are the expected outputs:
0 5 7 3 0 17

import java.util.Arrays;
public static void main(String[] args) {
    int[] list0 = new int[] {};
    int[] list1 = new int[] { 5 };
    int[] list2 = new int[] { 3, 4 };
    int[] list3 = new int[] { -2, 3, -4 };
    int[] list4 = new int[] { -1, -2, -4, -5 };
    int[] list5 = new int[] { 6, 1, 2, -3, 8 };
}
public static int sumOfPositivesRecursive (int[] a) {
    return sumOfPositivesHelper(a, a.length);
}

public static int sumOfPositivesHelper(int[] a, int n) {
    if(n == 0) {
        return 0;
    }
    System.out.println(n);
    int total = 0;
    if(a[n-1] > 0) {
        total += a[n-1];
        sumOfPositivesHelper(a, n-1);
    }
    return total;
}

My output: 0 5 4 0 0 8, only seems to check the last element the first time and never loops through again. Please help I know I am doing something wrong with the recursive call. Thanks all.

Your sumOfPositivesHelper should call itself, which it is not doing at the moment. — Sweeper
– Sweeper, Commented Feb 8, 2020 at 19:57

PurpleKiwi · Accepted Answer · 2020-02-09 10:27:34Z

import java.util.Arrays;

public class HelloWorld{

public static void main(String[] args) {
    int[] list0 = new int[] {};
    int[] list1 = new int[] { 5 };
    int[] list2 = new int[] { 3, 4 };
    int[] list3 = new int[] { -2, 3, -4 };
    int[] list4 = new int[] { -1, -2, -4, -5 };
    int[] list5 = new int[] { 6, 1, 2, -3, 8 };

    int sum = sumOfPositivesRecursive(list5);
    System.out.println("the sum of all positive number is " + sum);
}

public static int sumOfPositivesRecursive (int[] a) {
    int i =0;
    return sumOfPositivesHelper(a, i, 0);
}

public static int sumOfPositivesHelper(int[] a, int i, int sum) {
    if(i == a.length) {
        return sum;
    }
    if(a[i] > 0) {
        sum+= a[i];
        return sumOfPositivesHelper(a, ++i, sum);
    }else{
        return sumOfPositivesHelper(a, ++i, sum);
    }
  }
}

Just made it work but the concept remain the same, were just missing a few returns ;)

Black0ut · Accepted Answer · 2020-02-08 20:15:59Z

1

You code doesn't use recursion properly as the total variable is not passed between the sumOfPositivesHelper calls and not handled correctly, a valid solution will be something like that:

package test;

public class Main {

    public static void main(String[] args) {
        System.out.println(sumOfPositives(new int[] {}));
        System.out.println(sumOfPositives(new int[] { 5 }));
        System.out.println(sumOfPositives(new int[] { 3, 4 }));
        System.out.println(sumOfPositives(new int[] { -2, 3, -4 }));
        System.out.println(sumOfPositives(new int[] { -1, -2, -4, -5 }));
        System.out.println(sumOfPositives(new int[] { 6, 1, 2, -3, 8 }));
    }

    private static int sumOfPositives(int[] list) {
        return list.length == 0 ? 0 : sumOfPositives(list, 0, 0);
    }

    private static int sumOfPositives(int[] list, int index, int total) {
        if (list[index] > 0) {
            total++;
        }
        if (index + 1 < list.length) {
            return sumOfPositives(list, index + 1, total);
        }
        return total;
    }

}

answered Feb 8, 2020 at 20:15

Black0ut

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1 Comment

ziggz234 Over a year ago

thank you for this insight. makes a lot more sense now.

Arvind Kumar Avinash · Accepted Answer · 2020-02-08 20:28:16Z

Do it as follows:

public class Main {
    public static void main(String[] args) {
        int[] list0 = new int[] {};
        int[] list1 = new int[] { 5 };
        int[] list2 = new int[] { 3, 4 };
        int[] list3 = new int[] { -2, 3, -4 };
        int[] list4 = new int[] { -1, -2, -4, -5 };
        int[] list5 = new int[] { 6, 1, 2, -3, 8 };

        System.out.println("Sum of positive elements in list0: " + sumOfPositivesRecursive(list0));
        System.out.println("Sum of positive elements in list1: " + sumOfPositivesRecursive(list1));
        System.out.println("Sum of positive elements in list2: " + sumOfPositivesRecursive(list2));
        System.out.println("Sum of positive elements in list3: " + sumOfPositivesRecursive(list3));
        System.out.println("Sum of positive elements in list4: " + sumOfPositivesRecursive(list4));
        System.out.println("Sum of positive elements in list5: " + sumOfPositivesRecursive(list5));
    }

    public static int sumOfPositivesRecursive(int[] a) {
        return sumOfPositivesHelper(a, a.length);
    }

    public static int sumOfPositivesHelper(int[] a, int n) {
        if (n == 0) {
            return 0;
        }
        return a[n - 1] > 0 ? a[n - 1] + sumOfPositivesHelper(a, n - 1) : sumOfPositivesHelper(a, n - 1);
    }
}

Output:

Sum of positive elements in list0: 0
Sum of positive elements in list1: 5
Sum of positive elements in list2: 7
Sum of positive elements in list3: 3
Sum of positive elements in list4: 0
Sum of positive elements in list5: 17

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