Which value of static variable is returned if it is present in return statement of a recursive function?

Question

#include <stdio.h>

int f(int n)
{ 
    static int r = 0;
    if (n <= 0) return 1;
    r=n;
    return f(n-1) + r;
}

int main() {
    printf("output is %d", f(5));
    return 0;
}

Output obtained was 

output is 6

So does this mean that always return statement is executed left to right and value of static variable r will be the latest value after it has been updated by all recursive calls f(4), f(3), f(2), f(1)? 

However, output is same if I change the return statement as return r + f(n-1); which implies that it always takes latest updated value irrespective of left to right order. Is this assumption correct?

Comments

As per my knowledge, it should be undefined behaviour.

Addition has two arguments, and there's no standard rule to evaluate either left first or right first.

---

Yes it is undefined

Think about assembly code.

Compiler may choose to evaluate any of the operand first. As order of evaluation in C is not fix. https://stackoverflow.com/a/2934909/3699192

Answer 1

Static variables have a property of preserving their value even after they are out of their scope! Hence, static variables preserve their previous value in their previous scope and are not initialized again in the new scope.

Value of return is same for f(n)+r and r+f(n) both, as the value of “r” remains same ie “r=1”.
Top to Down function call:

n	r
5	5
4	4
3	3
2	2
1	1

Final Value of “r” is “1”
Return Value:

N	r	f(n-1)	f(n-1)+r
0			1
1	1	1	2
2	1	2	3
3	1	3	4
4	1	4	5
5	1	5	6

This may help:

Answer 2

The last but one call i.e f(1) is updating the value of 'r' to 1 and since it's static it's not stored on stack and hence all invocations of the function f have the value of r as 1. Hence

f(1) = f(0) + r // r == 1

       = 2

f(2) = f(1) + r = 2 + 1 = 3

continuing in this way

f(5) = 6

 

You will get 16 as output had a stack variable been there eg. int temp = 0; and set temp = r after r=n;

then write return f(n - 1) + temp;

Answer 3

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https://pythontutor.com/c.html#mode=edit