CODEWITHSUNDEEP
javafunctionloopsrecursion
user
user

Reputation: 543

Simple Recursion

I was playing around with this function:

public class x {

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

public static int recurse(int theNumber) {

    if(theNumber == 0) {
        return 0;
    }

    else
    {
        System.out.println(theNumber);
        theNumber--;
        recurse(theNumber);
        System.out.println(theNumber);
    }

    return -1;
}
}

and I got this output:

10 9 8 7 6 5 4 3 2 1 0 1 2 3 4 5 6 7 8 9 Press any key to continue . . .

How is this possible? I understand where the countdown from 10 to 0 comes from.. but how on earth does it count back up?? I'm quite certain I'm missing a fundamental concept about recursion. Can someone fill in the blanks??

Upvotes: 2

Views: 234

Answers (7)

bennidi
bennidi

Reputation: 2112

Habib explained it quite well actually. Just make sure you understand that the recursion is followed until the parameter theNumber is zero, hence the recursion terminates, and that this happens before any of the the last Sysout calls is done. Also keep in mind that the variable is not shared but local to each invocation. Visually, it might look like this

recurse(5)
print(5)
recurse(4)
  print(4)
  recurse(3)
    print(3)
    recurse(2)
      print(2)
      recurse(1)
         print(1)
         recurse(0)
         print(0)
      print(1)
    print(2)
  print(3)
print(4)

Calls that are in the same column belong to the same stackframe, i.e. same method invocation. Notice, how you decent in the call hierarchy before you actually reach the first print statement after the recursive invocation. This should make it clear, to you.

Upvotes: 3

Abhijeet
Abhijeet

Reputation: 1719

Before your last syso statement you are calling recurse function so it is going on adding your calls on stack..and after reaching value to 0 it is returning to current function back by pulling values from stack.. and after that it is printing values by your last syso statement...

Upvotes: 0

croyd
croyd

Reputation: 1105

For brevity, let's say you call recurse(2) in your main program. Then you have:

theNumber = 2 prints 2, decrements, calls recurse(1)

--> theNumber = 1, prints 1, decrements, calls recurse(0)

----> theNumber = 0, returns without printing

--> theNumber = 1 prints 0 (decremented from 1), returns

theNumber = 2 prints 1 (decremented from 2), returns

Upvotes: 2

Sudhanshu Umalkar
Sudhanshu Umalkar

Reputation: 4202

This is what you need-

public static int recurse(int theNumber) {

    System.out.println(theNumber);
    if (theNumber > 0) {
        theNumber--;
        recurse(theNumber);
    }

    return 0;
}

Upvotes: 1

Sachin Gorade
Sachin Gorade

Reputation: 1467

As Habib said flow goes into recursion and then after completion of executing recursive function your last System.out.println(theNumber) gets called.

---->  public static int recurse(int theNumber) {
|   
|       if(theNumber == 0) {
|           return 0;
|       }
|   
|       else
|       {
|           System.out.println(theNumber);
|           theNumber--;
----------< recurse(theNumber);
            System.out.println(theNumber);
        }

        return -1;
    }

So the time when function returns from recursion theNumber contains the value which was was decremented by 1 only.

Upvotes: 0

gkiko
gkiko

Reputation: 2289

Your last System.out.println(theNumber) is the reason of it. As you were asking about the Flow. First the recursive call is completed and then it's making BackTracking. Check this Link BackTracking

Upvotes: 0

Habib
Habib

Reputation: 223267

That is because of the last System.out.println(theNumber); in your method. After recursively calling the method, and upcon completion of the method it is printing those values.

Upvotes: 6

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