This recursive function puzzles me, what is going on?

Question

I was playing around with recursion and did this simple function. I was assuming that it would print out 9-0 to stdout, but, it prints 0-9. I can't see how that happens at all.

int main()
{
        rec(10);
        return 0;
}

int rec(int n){
        if(n > 0)
                printf("%d\n", rec(n -1));
        return n;
}

Answer 1

The rec function on the printf line is evaluated before the printf itself. Thus the deepest instance of the rec function is printed first.

Answer 2

Think of it like this.

rec(10)
rec(9)
rec(8)
rec(7)
rec(6)
rec(5)
rec(4)
rec(3)
rec(2)
rec(1)
rec(0)

Start Unwinding

printf("%d\n", 0);
printf("%d\n", 1);
printf("%d\n", 2);
printf("%d\n", 3);
printf("%d\n", 4);
printf("%d\n", 5);
printf("%d\n", 6);
printf("%d\n", 7);
printf("%d\n", 8);
printf("%d\n", 9);

Answer 3

Let's rewrite your code like this:

int rec(int n){
        if(n > 0)
        {
                int retval = rec(n -1);
                printf("%d\n", retval);
        }
        return n;
}

Does it make it clear why 0 printed first before 9?