Question

so I was playing around with recursion in C and I am not sure why is this happening:
code A

int foo(int x)
{
 if (x==0) return 0;
 else return foo(--x)+x;
}

int main() { printf("%d\n", foo(10));

code B

int foo(int x)
{
 if (x==0) return 0;
 else return foo(x--)+x;
}

int main() { printf("%d\n", foo(10));

Code A prints 45 instead of 55 and I figured that out. It is because the recursive call gets unwounded this way: 9+8+7+...+0 = 45

Code B on the other hand gets stuck and never returns to the prompt!! i have to ctrl+c it. Why does it get stuck? Is it because it never decrements past 10?

Answer 1

That’s because in snippet B x gets decremented after the recursive call so the function is always called with foo(10)

EDIT:
As James McNellis pointed out in its comment x gets decremented before calling the function but its original value is used in the call instead of the decremented one.
Basically the expression x– is evaluated before foo(x–) but its value is actually x, therefore the function is called with x = 10