在这里无法理解递归

Question

Can someone please help to explain to me why this ends up in an infinite recursion loop?

The variable length reaches the value 1 but for some reason the loop is still entered even though the loops condition is while (length>1).

I've tried printing values and running it over and over again, maybe I'm missing something more obvious or someone can explain this more simply. Thanks.

public static void main(String[] args) {
    xMethod(5);
}

public static void xMethod(int length) { 
    while (length > 1) {
        System.out.print((length - 1) + " ");
        xMethod(length - 1);
    }
}

Additional info.

When I dubugged this code :

public static void main(String[] args) {
    xMethod(5);
}

public static void xMethod(int length) { 
    while (length > 1) {
        System.out.print((length - 1) + " ");
        xMethod(length - 1);
    }
    System.out.println("Coming out of while");
}

Below is the output :

4 3 2 1 Coming out of while
1 Coming out of while
1 Coming out of while
1 Coming out of while
1 Coming out of while
1 Coming out of while
1 Coming out of while
//repeated Infinite times

After coming out of while loop why it is going back in the same while loop with length as 2?

Edit: I appreciate all of your responses and understand that if I wanted to code something like this I would probably us an if statement as most recursive methods do but this is simply a question of me perhaps not understanding how the scope or call stack works. If I'm correct the while loop block holds on to the value of length as 2 no matter what happens outside of that block?

Answer 1

Because you are not updating the value of length in the current method. The value is just decremented when sending to the method.

public static void main(String[] args) {
    xMethod(5);
}

public static void xMethod(int length) {
    while (length > 1) {
        System.out.print((length) + " ");
        xMethod(length);
        length--;
    }
}

Answer 2

The variable length never reaches the value 1 in any loop, you mix two design and i think you need on of them, recursive method or loop.

first design:

public static void main(String[] args) {
    xMethod(5);
}
public static void xMethod(int length) { 
    System.out.print((length - 1) + " ");
    if(length > 1)
        xMethod(length - 1);
    }
}

another way:

public static void main(String[] args) {
    xMethod(5);
}
public static void xMethod(int length) {
    while (length > 1) {
        System.out.print((length--) + " ");
    }
}

you can select one of these, it depend on your design. if it's not your answer, please write your expect output.

Answer 3

You are doing 2 things here. When you are writing a recursive code, you always need to think when the when code will end. Your code does not have a end case.

public static void main(String[] args) {
             xMethod(5);
}

public static void xMethod(int length) { 

     System.out.println("Method Start "+ length);
        while (length > 1) {

            System.out.println("Inside while "+ length);

             xMethod(length - 1);
        }
        System.out.println("Method End "+ length);                 
    }
}

Now this code produces the following output:

Method Start 5
Inside while 5
Method Start 4
Inside while 4
Method Start 3
Inside while 3
Method Start 2
Inside while 2
Method Start 1
Method End 1
Inside while 2
Method Start 1
Method End 1
Inside while 2
Method Start 1
Method End 1
Inside while 2
Method Start 1
Method End 1
.
.

As you can clearly see,

Inside while 2
Method Start 1
Method End 1

is repeated again and again.

So what this means is, when the length is 2, the following will happen.

while (2 > 1) {
     System.out.println("Inside while "+ length);
     xMethod(1);
}

The output for this is

Inside while 2

Now, xMethod(1) doesn't even enter the while loop, so this will be printed.

Method Start 1
Method End 1

But you should now understand that while(2>1) is again execute because the length has not changed and it is still 2 .

while (2 > 1){
    System.out.println("Inside while "+ length);
    xMethod(1);
}

goes on and the loop continues.

Answer 4

because when length reaches 2 xMethod(2) is called and therefore xMethod(1) follows, when xMethod(1) ends, since length is still 2 it again calls xMethod(2) and the this calls xMethod(1) it repeates..

to fix it, use return after xMethod(length - 1);

public static void main(String[] args){
        xMethod(5);
    }

    public static void xMethod(int length) { 

        while (length > 1) {

            System.out.print((length - 1) + " ");

             xMethod(length - 1);
             return;
        }
        System.out.println("Coming out of while");
    }