java Simple recursion

Question

So there is a recursive method (can not use any loop), that has one parameter n, and the program will print out 2^n "*", for example, if n was 2, the output is ****, and if n was 3, the output is ********.

I'm stuck with this problem because I ran into some infinite recursion issues.

First I had this: but I soon realize that n will never reach the check.

↑↑↑ That was supposed to be n+1 (even though it doesn't work)

Then I tried this:

public class test {

    public static void main(String[] args) {
        printPowerOfTwoStars(3);

    }

    public static void printPowerOfTwoStars(int n){
        n = (int)Math.pow(2, n);
        if(n == 0){
            return;
        }
        else{
            System.out.print("*");
            printPowerOfTwoStars(n-1);
        }

    }

}

Ran into some infinite recursion again.

This seems like a simple program with simple logic, but I'm having trouble with the condition check, because what's being compared with n keeps changing.

How should I fix this problem?

Without using any helper method or creating any static variables.

Thank you

Dawood ibn Kareem · Accepted Answer · 2016-11-20 01:24:09Z

4

You need to use the fact that 2 to the power of n is just 2 to the power of n-1, doubled. Your base case is that 2 to the power of 0 is 1.

public static void printPowerOfTwoStars(int n){
    if(n <= 0){
        System.out.print("*");
    }
    else{
        printPowerOfTwoStars(n-1);
        printPowerOfTwoStars(n-1);
    }
}

answered Nov 20, 2016 at 1:24

Dawood ibn Kareem

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4 Comments

yyin Over a year ago

That's brilliant! Now I'm just curious if there is a way to use Math.pow in this case. Thank you!

yyin Over a year ago

If the question was to print 3^n times, how would I implement this problem? Using the same logic, 3 ^n is just 3^n-1, tripled (3^n-1) x 3 = 3^n... how would you do that? Is it just printPowerOfTwoStars(n-1); three times?

Dawood ibn Kareem Over a year ago

Well, you could just have printPowerOfThreeStars(n-1) three times inside the else clause. Or put a loop inside the else clause. Or use two dimensional recursion - which is probably a whole other question.

yyin Over a year ago

Thank you very much

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