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1

Code Design 1 : works perfectly

 public static void main (String[] args)
    {       
        recursion(2);       
    }

    public static void recursion(int num)
    {
        if (num > 0)
        {
            recursion( num - 1 );
            System.out.println(num);
        }   
    }

Code Design 2 : Infinite Loop. ? 

public static void main (String[] args)
    {       
        recursion(2);       
    }

    public static void recursion(int num)
    {
        if (num == 0) return;
        while (num > 0)
        {
            recursion( num - 1 );
            System.out.println(num);
        }   
    }
  1. Can someone plz help me in understanding why 2nd design is getting into infinite loop?
  2. I have already put return in 2nd design . So it should have worked fine. Also can you plz give me explanation in detail?
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Why are most of these answers being downvoted? There are 6 answers that say basically the same thing. – irrelephant Aug 7 '12 at 6:26
Not sure who is downvoting . – Manish Aug 7 '12 at 6:29
Thanks a lot everyone. I think I got the answer which is the combination of inputs i got from all these posts. 1. Using if loop - It calls recursion(2)-->Recursion(1) and then ends . 2) Using while Loop - It calls recursion 2 --> recursion 1. Now Recursion 1 runs system.out but does not run make recursive call and keeps on running while loop forever. – Manish Aug 7 '12 at 6:44
Good. I have added the explanation as well. And do accept an answer which helped you the most. – SiB Aug 7 '12 at 6:46
There is no such thing as an "if loop". – ThiefMaster♦ Aug 7 '12 at 13:23
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8 Answers

up vote 1 down vote accepted

1. First of all if is Not a Loop, we only have for loop, for-each, while loop and do-while loop.

2. The reason for the 2nd code to go in for a Infinite loop is that, you are never decrementing the value of num.

Do this....

while (num > 0)
    {
        recursion( num - 1 );
        System.out.println(num);
        num = num - 1;           // Decrementing the value of num by 1
    }
share|improve this answer
up vote 0 down vote 
   while (num > 0)
            {
                recursion( num - 1 );
                System.out.println(num);
            }

in your while loop you are passing 2 for num . 2 is always greater than 0 so it goes to infinite loop;

To avoid infinite loop you have change the value of num variable;

 while (num > 0)
            {
                recursion( num - 1 );
                System.out.println(num);
                num--;
            }
share|improve this answer
up vote -2 down vote

The reason is that num is copied by-value, not as a reference in the call to recursion( num - 1 );

Edit: Yeesh, people are happy to downvote, arent' they? True, reference vs. copy-by value wasn't the correct answer, but in my initial reading of his code example it looked like a misunderstanding of that particular problem. I stand corrected.

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2  
not exactly the reason. – JW 웃 Aug 7 '12 at 6:24
up vote 0 down vote

Whether use loop or recursion, not both of them at the same time.

Generally, if one can do his desired actions using loops, he does not consider recursion anymore as using loops are faster and has less overhead.

Back to your question:
1. Infinite loop is inevitable. As in the loop num is not decreasing.
2. Of course there are some returns, but not from the case where you enter the recursion with num = 2 and num = 1.
Here is what happens: You enter the recursion with num = 2. It calls recursion with num = 1. There, there is infinite loop and of course infinite returns. But there is no return back to recursion(2).

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1  
Can you plz explain me how stacks are being created in the backend . What values do they have in them and the flow of program ? – Manish Aug 7 '12 at 6:36
How stacks are being created, you ask. It's totally related to OS I believe. A quick search would give you some useful hints. I give you a brief explanation about its usage (as it's not the main point here) For every function call there are three parameters (at least) put in the stack. which might be considered as overheads. So we better avoid this if we have other options. – Matin Kh Aug 7 '12 at 6:43
up vote -2 down vote

I don't know why you are going through all this to do a function like that? I apologize if I'm not getting your question but here is a good code I would suggest..

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

public void countdown(int num){

    for(int i = num; i >= 0; i--){
        System.out.println(num);
    }
}
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1  
I don't get why I just got a -1? If you want to correct me just leave a comment and I'll reply? Thanks...... – 36redsoxfan Aug 7 '12 at 6:26
up vote 0 down vote

The value of num does not change inside the loop. So it keeps looping.

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up vote 0 down vote

the value of the num is not changed after it has been pass in the recursion method.

    if (num == 0) return;
    while (num > 0)
    {
        recursion( num - 1 );
        System.out.println(num);
        num--;                   // add this line.
    }
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up vote 1 down vote

When enter the while loop num is greater than zero and its value is not changed.

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