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Thread: Simple recursion logic

  1. #1
    Java kindergarten chronoz13's Avatar
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    Default Simple recursion logic

    can any one help me figure out how this recursion done?

    public class RecursiveMethodSum {
     
        public static int sum(int n) {
     
            if (n == 1) {
     
                return 1;
            }
            else {
     
                return n + sum(n - 1);
            }
        }
     
        public static void main(String[] args) {
     
            int x = sum(4);
     
            System.out.println(x);
        }
    }

    output
    10



    one question is... during the process of the recursion... (4 + (4 -1)) its == 7? ryt?
    but where does the number seven goes? i can only see the the process is just adding
    the value that has been passed to the parameter... where are the sum(s)?


  2. #2
    Super Moderator helloworld922's Avatar
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    Default Re: Simple recursion logic

    Remember, you have to traverse down to the lowest level before back-tracing in a recursive algorithm.

    So, the first recursive call would be:
    sum(4) = 4 + sum(3-1)
    sum(3) = 3 + sum(3-1)
    sum(2) = 2 + sum(2-1)
    sum(1) = 1

    So, the end result is:

    sum(4) = 4 + 3 + 2 + 1

    This doesn't really illustrate the order operations are done, though. Instead, the computer would evaluate it this way:

    sum(4) = 4 + (3 + (2+ (1)))

    So, the number 7 never shows up.

  3. The Following User Says Thank You to helloworld922 For This Useful Post:

    chronoz13 (December 24th, 2009)

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    Default Re: Simple recursion logic

    A recursive call is just like any other method call:

    public int ReturnOne() {
         return 1;
    }
     
    public static void main(String[] args) {
     
        int two = ReturnOne() + ReturnOne();
     
        System.out.println(two);
    }

    In the above the result of the two is initially null then it's 1 after the first method call, then it's two after the second method call. While the method calls are made the intermediate value of two is stored on a call stack by the java virtual machine.

    The same applies for recursion. In your case this is what happens:
    the method sum(int n) is called with value 4
    return value stored on stack is 4
       method sum(int n) is called with value 3
       return value stored on stack is 3
          method sum(int n) is called with value 2
          return value stored on stack is 2   
             method sum(int n) is called with value 1
             return value is 1
          return value is (stack value) 2 + (returned value) 1 = 3
       return value is (stack value) 3 + (returned value) 3 = 6
    return value is (stack value) 4 + (returned value) 6 = 10
    Last edited by myCoffee; December 24th, 2009 at 02:56 PM.

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    chronoz13 (December 24th, 2009)

  6. #4
    Java kindergarten chronoz13's Avatar
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    Default Re: Simple recursion logic

    ahh so thats how recursion process works...

    im just confuse where does the value go every time it evaluates....

    so thats it... tnx guys...

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