StackOverflow Error on Java-Cannot see an infinite loop?

Question

I am attempting to create a program which finds values which are both "triangle numbers" and "star numbers". However I am slightly confused about when the program branches to the second function etc. Any help is appreciated!

public class Recursion {

        public static void main(String[] args) {
                // TODO Auto-generated method stub
                int count =0;
                int n = 1;
                int t=0;
                int triangularNumber =0;
                while (n<Integer.MAX_VALUE)
                {
                        t = isTriangularNumber(n,count,triangularNumber);  
                        triangularNumber=0;
                        int starNumber= ((6*n)*(n-1)) + 1;
                        if (starNumber ==t)
                        {
                                System.out.println(t);
                        }
                        n++;
                }      
                if (n==Integer.MAX_VALUE)
                {
                    System.exit(0);
                }
        }


        public static int isTriangularNumber(int n, int count, int triangularNumber)
        {
                triangularNumber =triangularNumber + (n-(n-count));
                if (count<=n)
                {      
                        return isTriangularNumber(n,(count++), triangularNumber);
                }      
                else return triangularNumber;
        }

}

Rohit Jain · Accepted Answer · 2012-12-03 21:00:58Z

4

return isTriangularNumber(n,(count++), triangularNumber);

In the above invocation, count++ is evaluated to count only. So, on every invocation, you are actually passing unchanged value of count. And hence the if condition: -

if (count<=n)

will always be evaluated to true, if it is true for the first invocation. Thus filling the stack with infinite method invocation.

Your invocation should be with ++count: -

return isTriangularNumber(n,(++count), triangularNumber);

answered Dec 3, 2012 at 21:00

Rohit Jain

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