Simple recursion question - c++

Question

Let's say we have a simple recursion like.
int x(int a){
if(a<10)
x(a+1);
else
!STOP!
b++;
return b;
}
Globaly:
int b=0;
In main we could have something like this:
int p=x(1);
Is there any way to stop the recursion so that the p will be 0, this means that "b++" will never be executed.
I'll be grateful if you could tell me some expresion to put instead of the !STOP!
But, I don't want anything like this, I just want to stop the recursion, like break; does in a while() loop...:
int ok=0;
int x(int a){
if(a<10)
x(a+1);
else
ok=1;
if(ok==0)
b++;
return b;
}
If there's anything unclear about the question, just ask.

Answer 1

Why wouldn't you do this?
int x(int a){
if(a<10) {
x(a+1);
b++;
}
return b;
}
The thing is, though, you're modifying a global in a recursive routine, which is not especially threadsafe and pretty sloppy. You're returning a value that is always ignored except by the top level caller. You're also doing something that is better off being done in a loop (but I assume that your actual case is bigger than this, or you're a student).
You can't really "break" the recursion - returning unwinds well enough. In oldey-timey C you might use setjmp/longjmp (and all its perils - in other words, DON'T), and in C++ you might use try/catch/throw, which will unwind the stack as well.

Answer 2

How about like this?
int x(int a){
if(a>0 && a<10)
x(a+1);
b++;
return b;
}

Answer 3

The only thing in C++ that will unwind the stack like that is an exception. There's also setjmp()/longjmp(), but those should never be used in a C++ program. Any other construct can at most return from the current function.

Answer 4

How about returning?
int x(int a){
if(a<10)
x(a+1);
else
return b;
b++;
return b;
}
I think this looks a bit better
int x(int a){
if(a<10)
x(a+1);
else
return b;
return ++b;
}
EDIT:
I think You could use exception mechanism to unwind the stack and get to the point of first invocation, but it's safe after entering main(). Referencing b in x, given the code:
int b = 0;
int p = x(1);
suggests that x is used for initialization of some global variable and may be executed before main(). How about using some helper function that wraps invocation of x in a try - catch block and throwing an exception in the place of |STOP|?

Answer 5

If you're trying to declare b in main(), and use b in x() then there's something wrong already to begin with. Instead, make b into a local variable by passing it as a parameter to x, and returning a modified version of b.
int x(int a, int b){
if(a<10)
return x(a+1,b+1);
else
return b;
}

Answer 6

I'm not a big fan of using an Exception for control. I don't expect you'll save many cycles by using Exceptions instead of if/return statements. You're going to have to test your boundary conditions anyway before throwing an Exception.
You can however simplify the problem a bit by changing the return type of the function.
bool x(int a){
if(ok) //Exit early before next call up?
return true;
if(a<10){
if(x(a+1)) //Have we been told to exit early?
return true; //Yes
b++; //Do some work
if(ok) //Exit early in the next call down?
return true;
}
return false; //Normal Exit
}