Value returned when printed is different in C (Recursion Problem)

Question

So I got this question in my end-sem:

//The value returned by sam(3,0) of the below function is:
#include <stdio.h>
int sam(int n,int a)
{

    if(n==0)
         return a;
    else
    {
        a+=n;
        sam(--n, a);
        a+=n;
    }
}

I calculate using Tree-method and got the answer as 6. But if I compile and directly print return I get 2.

int main()
{
    printf("%ld",sam(3,0));
    return 0;
}

OUTPUT: 2

I checked again and again with what is wrong and couldn't understand. The mystery is if I print(a) just before it returns. It returns 6 (what I calculated).

#include <stdio.h>
int sam(int n,int a)
{
    if(n==0){
        printf("%d",a); //Check the change here
        return a;
    }
    else{
        a+=n;
        sam(--n,a);
        a+=n;
    }
}

int main()
{
    sam(3,0);
    return 0;
}

OUTPUT: 6

If I explain this I get 4 marks. Enough to change my grade

I genuinely solved it using pen and paper. This time atlesat. — Bhaskar Mishra, Aug 30 '22 at 18:55
Am I reading this correctly? Are you saying that was the definition of the function given on your exam? Push back; that is undefined behavior. (Unless there's a copy-paste error.) — William Pursell, Aug 30 '22 at 18:55
`sam` when `n!=0` returns the else statement running the recursion — Bhaskar Mishra, Aug 30 '22 at 18:58
[Implicit int return value of C function](https://stackoverflow.com/a/10079437) It's ok to not return a value, but UB if you try to use the result of such a function. So the answer is Undefined Behavior. — 001, Aug 30 '22 at 18:59
@BhaskarMishra Look more closely. When `n != 0`, the function is called again and the recursion happens but no value is returned. — William Pursell, Aug 30 '22 at 19:09

score 0 · Accepted Answer · answered Aug 30 '22 at 19:02

With a bit of formatting assistance, please note that the below function only explicitly returns when n is 0. Otherwise it performs recursion, but uselessly since it never returns any value created by the recursion.

As a result, you find yourself in the realm of undefined behavior.

#include <stdio.h>

int sam(int n, int a) {
    if (n == 0)
        return a;
    else {
        a += n;
        sam(--n, a);
        a += n;
    } 
}

Value returned when printed is different in C (Recursion Problem)

1 Answers1