Basic recursion with if else statement (java)

Question

Reviewing my notes and was wondering if anyone has a concise explanation for what is occurring below:

public class HelloWorld{

public static void main(String[]args){
    helloWorld(10);

}

public static void helloWorld(int n){
      if(n > 0){
        System.out.println("HelloWorld! " + n);
        helloWorld(n-1);
        }
        System.out.println("GoodBye from " + n);
      }
    }

Output:

HelloWorld! 10
HelloWorld! 9
HelloWorld! 8
HelloWorld! 7
HelloWorld! 6
HelloWorld! 5
HelloWorld! 4
HelloWorld! 3
HelloWorld! 2
HelloWorld! 1
GoodBye from 0
GoodBye from 1
GoodBye from 2
GoodBye from 3
GoodBye from 4
GoodBye from 5
GoodBye from 6
GoodBye from 7
GoodBye from 8
GoodBye from 9
GoodBye from 10

Just to be clear, I totally understand what's happening above with the recursion and the call stack unwinding. My question is about what happens when I include an else statement below:

public static void helloWorld(int n){

    if(n > 0){
        System.out.println("HelloWorld! " + n);
        helloWorld(n-1);

        }else{
        System.out.println("GoodBye from " + n);

        }
    }

Output:

HelloWorld! 10
HelloWorld! 9
HelloWorld! 8
HelloWorld! 7
HelloWorld! 6
HelloWorld! 5
HelloWorld! 4
HelloWorld! 3
HelloWorld! 2
HelloWorld! 1
GoodBye from 0

Why does the call stack appear to not unwind when it hits the else statement?

Why would Goodbye 1-10 get printed if that only happens when n is 0 (or less)? — Bernhard Barker
– Bernhard Barker, Commented May 25, 2017 at 9:19
When it enters in if statement, Why do you expect it to execute else statement as well? — Sanket Makani
– Sanket Makani, Commented May 25, 2017 at 9:19
0 is only reached once, hence only once the else branch is executed. The stack unwinding happens 11 times in both versions, only without else 10 extra prints are added. — Joop Eggen
– Joop Eggen, Commented May 25, 2017 at 9:22

Eran · Accepted Answer · 2017-05-25 09:21:30Z

4

The call stack unwinds in both cases. It just doesn't print anything when it unwinds in the second snippet, since you only print "GoodBye from " + n when n is 0, which happens exactly once.

answered May 25, 2017 at 9:21

Eran

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1 Comment

Harry Over a year ago

Thank you! Much appreciated :)

kk. · Accepted Answer · 2017-05-25 09:21:52Z

1

In 2nd case the else block will be executed only for n = 0 as it is going in the if block. However in the first case, after each recursion completes System.out.println("GoodBye from " + n); will be executed as it is not bound with any condition.

answered May 25, 2017 at 9:21

kk.

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