based on recursion using java

Question

public void length() {
    System.out.println(length(head, 0));
}
public int length(Node he, int count) {
    if(he!=null) {
        // System.out.println(he.data +"   "+count++);
        // count++;
        // return length(he.next, count);
        return length(he.next, count++);
    }
    return count;
}

In the code above, I have to find the length of linked list. If I run the same code, I am getting the length 0.But, when i use commented code and I am getting the correct length. Why is that happening?

`count++` actually passes `count`, [see this Question how post Increment works](http://stackoverflow.com/questions/2371118/how-do-the-post-increment-i-and-pre-increment-i-operators-work-in-java) — SomeJavaGuy, Aug 11 '16 at 07:12
What about removing the second parameter and simply `return (he == null) ? 0 : 1 + length(he.next);`? — fabian, Aug 11 '16 at 07:20
thanks @fabian .. I actually found other way through you. :) — Krishna krish, Aug 11 '16 at 09:00

score 5 · Answer 1 · answered Aug 11 '16 at 07:12

5

length(he.next, count++) passes the original value of count to the method call, since you are using the post increment operator. Therefore you are always passing 0.

length(he.next, ++count) would work, since here the incremented value of count will be passed.

In your commented code you are not passing the value of count++ to the method call, you are passing count after it was already incremented, which also works.

answered Aug 11 '16 at 07:12

Eran

387,369
54
702
768

but, the code is recursing and for the next recursio step.. it has to increment right? as like waht happens in iterative steps? why didn't that happen in recursion? – Krishna krish Aug 11 '16 at 07:38
@Krishnakrish I'm not sure what you mean. You are passing the un-incremented value of `count` to each recursive call. The only reason your recursion ends (instead of producing StackOverflowError) is that the stopping condition depends on the passed `Node` (which advances in each recursive call) and not on the passed `count` (which is always 0). – Eran Aug 11 '16 at 07:43
@Eran, looks like he is expecting the value to be incremented after the previous call stack completes, which is not the case. The variables passed as arguments to a recursive method are part of only that particular recursion call stack – JavaHopper Aug 11 '16 at 15:17

burakozgul · Answer 2 · 2016-08-11T08:04:34.743

0

Use

return length(he.next, ++count);

or

System.out.println(he.data +"   "+count);
count++;
return length(he.next, count);

I will try to simulate your and my code.

Your code :

//count = 0

length(he.next, count++) // length(he.next, 0) 

//after recursion call count increments , count = 1

My code :

//count = 0

// before recursion call, ++count increments count 

// count = 1 

length(he.next, ++count) // length(he.next, 1)

Guys im new here if im wrong, please edit me :)

edited Aug 11 '16 at 08:04

answered Aug 11 '16 at 07:16

burakozgul

775
6
11

but, the code is recursing and for the next recursio step.. it has to increment right? as like waht happens in iterative steps? why didn't that happen in recursion? – Krishna krish Aug 11 '16 at 07:38
Yes you need to increment. Actually you are incrementing count but incrementation happens after recursion call. Because of that you are passing old value of count which is 0. – burakozgul Aug 11 '16 at 07:44

based on recursion using java

2 Answers2

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