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Authors:
Stuart Reges, Marty Stepp
Chapter:
Recursion
Exercise:
Exercises
Question:1 | ISBN:9780136091813 | Edition: 2

Question

Write a recursive method called starString that accepts an integer as a parameter and prints to the console a string of stars (asterisks) that is 2n (i.e., 2 to the nth power) long. For example,
• starString(0) should print * (because 20 == 1)
• starString(1) should print ** (because 21 == 2)
• starString(2) should print **** (because 22 == 4)
• starString(3) should print ******** (because 23 == 8)
• starString(4) should print **************** (because 24 == 16)
The method should throw an IllegalArgumentException if passed a value less than 0.

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Answer

Implementation of starString method:

public static String starString(int n) {
    if (n < 0) {
        throw new IllegalArgumentException();
    } else if (n == 0) {
        return "*";
    } else {
        return starString(n - 1) + starString(n - 1);
    }
}

Test Program for starString method:

public class Ch12Ex01 {
	public static void main(String[] args) {
		System.out.println("starString(0): " + starString(0));
		System.out.println("starString(1): " + starString(1));
		System.out.println("starString(2): " + starString(2));
		System.out.println("starString(3): " + starString(3));
		System.out.println("starString(4): " + starString(4));
	}
	
	public static String starString(int n) {
	    if (n < 0) {
	        throw new IllegalArgumentException();
	    } else if (n == 0) {
	        return "*";
	    } else {
	        return starString(n - 1) + starString(n - 1);
	    }
	}
}

Output:

starString(0): *
starString(1): **
starString(2): ****
starString(3): ********
starString(4): ****************

 

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