C - recursive function for sum of n natural numbers

Question

Below is the recursive function for sum of natural numbers up to n.

int calculateSum(int n) {
    if (n == 0){
        return 0;
    }else{
        return n + calculateSum(--n); //Replacing --n with n-1 works
    }
}

Input: 5

If I use --n then the output is 10, but when I replace --n with n - 1 then the correct result is returned (15). Why the difference?

Answer 1

As surprising as it might be, the behaviour of the expression

n + calculateSum(--n);

is undefined, because --n not only evaluates to the value of n after being decremented by 1, it also changes n to the new value. The change (a side effect), however, is unsequenced in relation to the other evaluation of n (a value computation) on the left. And according to C11 Appendix J.2., the behaviour is undefined, when

A side effect on a scalar object is unsequenced relative to either a different side effect on the same scalar object or a value computation using the value of the same scalar object (6.5).

More of the same class of errors in this question.

---

You can consider yourself lucky when you got a wrong value, because your compiler could have also generated code that would return the "correct value"... given the phase of the moon, and the optimization settings, or the number of lines in the calling function...

Answer 2

--n also modifies the local variable n; the expression n-1 just returns the decremented value to pass it on to the recursive call, but does not modify your input-parameter.

In detail for the first iteration with input 5:

• Correct is return 5 + calculateSum(5-1);
• With --n you would also decrement from n and end up with 4 + calculateSum(4)

@user3386109 & @Yunnosch worded it this way: With --n you calculate the sum 0...(5-1) instead of 0...5.

See @Antti answer, it explains that it's actually undefined behaviour.