Recursion inside while loop, How does it work?

问题

Can you please tell me how does this java code work? :

public class Main {
    public static void main (String[] args)  {
        Strangemethod(5);
    }
    public static void Strangemethod(int len) {
        while(len > 1){
            System.out.println(len-1);
            Strangemethod(len - 1);
        }
}
}

I tried to debug it and follow the code step by step but I didn't understand it.

update: sorry I didn't mention that I know the result of this code but just want to know the steps of the execution..

回答1:

That'll print 4 3 2 1 1 1 1 1 1...

And get stuck in a loop because nothing ever modifies len in the scope of the while loop. The first calls (with len=5, 4, then 3) go through one loop iteration, and are left waiting for Strangemethod to return. When when len=2, the while loop calls strangemethod(1), and since len is not greater than 1, the while loop finishes and that call returns. But len is still 2 in the bottom-most remaining whle loop, so it calls strangemethod(2) again. And again. And again.

if() would've been more appropriate than while().

回答2:

If I'm not mistaken, isn't this causing an infinite loop?

Once strangemethod(1) returns the strangemethod(2) would print 1 again and then call strangemethod(1) again.

Are you forgetting to decrement len after the strangemethod call?

回答3:

EDIT :SORRY For the first reply didnt realise it..it will cause an infinite loop

Here is a simple flow - for e.g len =5

 public static void Strangemethod(5) {
            while(5 > 1){
                System.out.println(5-1);
                Strangemethod(5 - 1);
            }
public static void Strangemethod(4) {
            while(4 > 1){
                System.out.println(4-1);
                Strangemethod(4 - 1);
            }
public static void Strangemethod(3) {
            while(3 > 1){
                System.out.println(3-1);
                Strangemethod(3 - 1);
            }
    public static void Strangemethod(2) {
            while(2 > 1){
                System.out.println(2-1);
                Strangemethod(2 - 1);
            }
    public static void Strangemethod(1) {
            while(1 > 1){//goes back to original(above) call and then an infinite loop since len was never  decremented

            }

Prints 4 3 2 1 1.....

回答4:

You don't say what you expect the code to do. However, the obvious point of note that the len variable does not change value within the Strangemethod method - it could have been declared final. Possibly what you wanted to do was decrement it with --len; (equivalent to len = len - 1;).

回答5:

Try adding len--; after Strangemethod(len - 1);. It won't send you into an infinite loop then. Alternatively, you could do

System.out.println(--len);
Strangemethod(len);

回答6:

This code will loop forever.

The result of len - 1 is never stored in the while loop so it can't exit and when len = 2 it'll just sit there outputting 1s.

It's unusual to use a while in recursive functions. I'd typically expect to see an if in its place, which would give you the output:

If you really do need the while then I'd rewrite the loop like this:

while(len > 1)
{
  len--;
  System.out.println(len);
  Strangemethod(len);
}

This will output:

回答7:

Also, I think someone should point out that len is never decremented so you get an infinite loop. I see that only 7 people have mentioned that so far.

来源：https://stackoverflow.com/questions/3070693/recursion-inside-while-loop-how-does-it-work

标签

java

recursion

while-loop