Java : Recursion -While Loop Vs If Loop

Question

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?

Answer 1

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
    }  

Answer 2

输入while循环数大于零且其值不变。

Answer 3

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.
    }   

Answer 4

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

Answer 5

   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--;
            } 

Answer 6

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) .

Answer 7

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.

Answer 8

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);
    }
}