Question

I am having a lot of trouble with this basic recursion problem in java; any pointers would be great.

“Write a static recursive method to print out the nth term of the
geometric sequence: 2, 6, 18, 54.”

From what I can gather, somewhere in the code I should be recursively multiplying something by 3, but I’m struggling to figure out how to do this. I know I need a termination statement, but when does that occur? Do I need a helper method?

Answer 1

A Recursive Function is a function whose implementation references itself. Below is some funny example:

public class Inception {
   public void dream() {
      boolean enoughDreaming = false;
      //Some code logic below to check if it's high time to stop dreaming recursively
      ...
      ...

      if(!enoughDreaming) {
           dream(); //Dream inside a Dream
      }
   }
}

And the solution for your problem:

public class GeometricSequence {
    public static void main(String[] args) {
        //Below method parameters - 5 = n, 1 = count (counter), res = result (Nth number in the GP.
        System.out.println(findNthNumber(5, 1, 2));

    }

    public static int findNthNumber(int n, int count, int res) {
        return ((count == n)) ? res : findNthNumber(n, count+1, res *3);
    }
}

EDIT:

The above class uses “int”, which is good only for small numbers (because of Integer Overflow problem). The below class is better for all types/numbers:

public class GeometricSequence {
    public static void main(String[] args) {
        //Below method parameters - 5 = n, 1 = count (counter), res = result (Nth number in the GP.
        System.out.println(findNthNumber(2000, 1, new BigInteger("2")));

    }

    public static BigInteger findNthNumber(int n, int count, BigInteger res) {
        return ((count == n)) ? res : findNthNumber(n, count+1, res.multiply(new BigInteger("3")));
    }
}