斐波那契数的递归函数

Question

I have to write a simple program as follows: "Given a non-negative integer n, find the nth Fibonacci number using recursion". I think what this means is that, for any value entered by the user, I have to get the Fibonacci number. For example, if the user entered 4, I would have to get the 4th value in the list of Fibonacci numbers (which is 2). Below is what I have written, but there is something wrong with my recursion as it crashes when I run it. Appreciate any help...

int userValue;
int fibo;
int fib(int n);
int fibValue;


int main() {
    cout << "Please provide your value" << endl;
    cin >> userValue;

    while (userValue < 0) {
        cout << "Fibonacci numbers only start at 0, please try again: " << endl;
        cin >> userValue;
    }

    if (userValue == 1 || userValue == 0) {
        cout << "Fibonacci result is: " << userValue << endl;
        return 0;
    }
    else {
        fib(userValue);
        cout << "Fibonacci result is: " << fibValue << endl;
        //return 0;
    }
}

int fib(int n)
{
    fibValue = fib(n - 1) + fib(n - 2);
    return fibValue;
}

Answer 1

The problem lies in fib method.There is no termination condition provided. So, The recursion will happen in a loop without terminating.

First, try to debug any problem by giving multiple inputs and you will understand where the problem lies.

In your case,

For suppose n=3 ,

the trace would be like this

fib(3) -> which further invokes fib(2) and fib(1)

fib(2) -> which further invokes fib(1) and fib(0)

now since there is no termination condition

fib(0) will further invoke fib(-1) and fib(-2)

since fib of negative value does not exists termination condition should be provided so that recursion would stop and return the result.

For fibonacci number, termination condition would be like:

 if(n == 0){
  return 0;
 }else if (n == 1){
  return 1;
 }

Few reference

https://blog.hartleybrody.com/debugging-code-beginner/

https://www.codementor.io/mattgoldspink/how-to-debug-code-efficiently-and-effectively-du107u9jh%60

Hope this helps. Thanks.

Answer 2

Your recursion's stop condition is outside of it, in the following if:

if (userValue == 1 || userValue == 0) {...}

But it should instead be in the fib function.

What you currently have is an endless recursion which is the shortest path to a stack overflow.

Answer 3

Recursion in the above code never stops, it stops only when stack is full resulting in run-time termination of program. For you program:

int fib(int n){
    if(n<=1)
        return n;
    else
        return fib(n-1)+fib(n-2);
}