CODEWITHSUNDEEP
javarecursion
golden_kiwi
golden_kiwi

Reputation: 35

having a variable in recursive method not redefine itself after each call

I am going to post my code below because this is kind of hard to describe. The code below works, but it is using Math.pow in the main method rather than in the helper, so if someone could show me a way to move the power to the helper method without messing up the program that would be much appreciated.

Main method: 

  Scanner keyboard = new Scanner(System.in);

  System.out.println("Please enter an integer: ");
  double input = keyboard.nextInt();

  double x = Math.pow(2.0, input);
  int n = (int)x;

  System.out.println(starStr(n));

Helper method: 

  public static String starStr(int n)
  {     
     if (n >= 1) {
        return ("*" + starStr(n-1));
     }
     else {
        return "";
     }
  }

EDIT:

 if(n == 0) {
     return "*";
  }   
  else {
     return starStr(n - 1) + "**";
  }

Upvotes: 2

Views: 98

Answers (2)

OneCricketeer
OneCricketeer

Reputation: 191743

I think this is what you are looking for. Not sure if you have learned the tree data-structure, but that's the purpose of my variable names. 

public class Main {
    public static void main(String[] args) {

        for (int i = 0; i < 5; i++) {
            // 1 + (2^n)-1 = 2^n 
            System.out.println("*" + doStars(i));
        }
    }

    public static String doStars(int n)
    {
        if (n == 0) {
            return "";
        }
        else {
            String subTree = doStars(n - 1); 
            return subTree + "*" + subTree; // length = (2^n)-1
        }
    }
}

Output

*
**
****
********
****************

Visualization - read clockwise in triangles from little to big

                        "*"
                         +
                     doStars(2)
                        "*"
      doStars(1)         +        doStars(1)
          "*"                         "*"
doStars(0) + doStars(0)     doStars(0) + doStars(0)
   ""           ""              ""          ""

Upvotes: 0

Jeffersah
Jeffersah

Reputation: 26

Something like this would work. You don't really need to use a power function at all. Just start with 1 star and double the number of stars in every step of the recursion.

public static void main(String[] args) {

    Scanner keyboard = new Scanner(System.in);

    System.out.println("Please enter an integer for the number of stars: ");
    int input = keyboard.nextInt();

    System.out.println(doStars(input));
}

public static String doStars(int n)
{
    //If n == 0 the recursion is done
    //Otherwise, reduce n by 1 and double the number of stars
    if(n == 0)
        return "*";
    else
    {
        String output = doStars(n - 1);
        return output + output;
    }
}

Upvotes: 1

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