recursively sum the integers in an array

Question

I have a program that I'm trying to make for class that returns the sum of all the integers in an array using recursion. Here is my program thus far:

public class SumOfArray {

private int[] a;
private int n;
private int result;

    public int sumOfArray(int[] a) {

      this.a = a;
      n = a.length;

      if (n == 0)  // base case
      result = 0;
      else
          result = a[n] + sumOfArray(a[n-1]);

      return result;

   } // End SumOfArray method

} // End SumOfArray Class 

But I'm getting three error which are all related, I believe, but I can't figure out why it is finding a type of null:

SumOfArray.java:25: sumOfArray(int[]) in SumOfArray cannot be applied to (int)
    result = a[n] + sumOfArray(a[n-1]);
                    ^
SumOfArray.java:25: operator + cannot be applied to int,sumOfArray
    result = a[n] + sumOfArray(a[n-1]);
              ^
SumOfArray.java:25: incompatible types
found   : <nulltype>
required: int
    result = a[n] + sumOfArray(a[n-1]);
                  ^
3 errors

Solution 1

The solution is simpler than it looks, try this (assuming an array with non-zero length):

public int sumOfArray(int[] a, int n) {
    if (n == 0)
        return a[n];
    else
        return a[n] + sumOfArray(a, n-1);
}

Call it like this:

int[] a = { 1, 2, 3, 4, 5 };
int sum = sumOfArray(a, a.length-1);

Solution 2

The issue is that a[n-1] is an int, whereas sumOfArray expects an array of int.

Hint: you can simplify things by making sumOfArray take the array and the starting (or ending) index.

Solution 3

a[n-1] 

is getting the int at n-1, not the array from 0 to n-1.

try using

Arrays.copyOf(a, a.length-1);

instead

Solution 4

How about this recursive solution? You make a smaller sub-array which contains elements from the second to the end. This recursion continues until the array size becomes 1.

import java.util.Arrays;

public class Sum {
    public static void main(String[] args){
        int[] arr = {1,2,3,4,5};
        System.out.println(sum(arr)); // 15
    }

    public static int sum(int[] array){
        if(array.length == 1){
            return array[0];
        }

        int[] subArr = Arrays.copyOfRange(array, 1, array.length);
        return array[0] + sum(subArr);
    }
}

Solution 5

a is an int array. Thus a[n-1] is an int. You are passing an int to sumOfArray which expects an array and not an int.

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