How does Linear Recursion work?

Question

I have written a java program to add elements in an array using Linear Recursion. The output obtained is not as expected. Can anyone point what is wrong with this program?

public class TestSum {

    public int count = 0;


    public int sum(int[] a){

        count++;

        if(a.length == count){
            return a[count -1];
        }

        return sum(a) + a[count -1] ;
    }

    public static void main(String[] args) {

        int[] a = {1,2,3};

        int val = new TestSum().sum(a);
        System.out.println(val);
    }

}

I am expecting the output as 6 but obtained is 9 . What is wrong?

Strangely if I change the order of addition ie return a[count -1] + sum(a); then it gives output as 6 .

Answer 1

Generally, recursive programs that are not re-entrant (ie relying on external state) are suspicious. In your particular case count will change between invocations of sum , making the behavior hard to trace, and ultimately resulting in the error that you observe.

You should pass the index along with the array to make it work:

// The actual implementation passes the starting index
private static int sum(int[] a, int start){
    if(a.length == start){
        return 0;
    }
    return sum(a, start+1) + a[start];
}
// Make sure the method can be called with an array argument alone
public static int sum(int[] a) {
    return sum(a, 0);
}

Unlike an implementation that increments the count external to the method, this implementation can be called concurrently on multiple threads without breaking.