Simple recursion logic

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

Code :

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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);
    }
}

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output

Code :

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10

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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)?

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.

A recursive call is just like any other method call:

Code :

---

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

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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:

Code :

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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

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ahh so thats how recursion process works...

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

so thats it... tnx guys...