need explanation

[jsmith1]

Can someone explain why the two functions act differently? From my point of view they are the same but give different outputs.

Thanks

#include <iostream>

using namespace std;

/*int getFibNumber(int fibIndex){ // works fine
    if (fibIndex < 2)
        return fibIndex;
    else
        return getFibNumber(fibIndex - 1) + getFibNumber(fibIndex - 2);
}*/

int main()
{
    int index = 0;
    cout << "Enter the desired number: ";
    cin >> index;
    index = (index - 1) + (index - 2); // does not work

    cout << "Fibonacci number is: "  << index << endl;
    return 0;
}
  

[keskiverto]

getFibNumber is a recursive function. If you call it with 4, f(4), that function will call f(3) and f(2), which in turn call f(2), f(1), f(1), f(0), and first of those calls f(1) and f(0). That is 9 function calls.

The simple equation:
index = index - 1 + index - 2
Set index=4 =>
index = 4-1 + 4-2
= 3 + 2
= 5
There is nothing recursive in that.

[ne555]

> why the two functions act differently?
¿two functions? ¿which functions?


`does not work' is not an error message

[jsmith1]

The two functions are getFibNumber and the equation in the main. I'm trying to figure out what the difference between the two. Its confusing getFibNumber isn't a loop.

[Bourgond Aries]

getFibNumber unrolls:

getFibNumber(4); calls itself
getFibNumber(4 - 1) + getFibNumber(4 - 2)
...
getFibNumber(1) + getFibNumber(0) + getFibNumber(1) + getFibNumber(1) + getFibNumber(0)
1 + 0 + 1 + 1 + 0 = 3

[Daleth]

keskiverto just explained to you that getFibonacci is a recursive function. It calls itself from within the block, so it is essentially a loop.
getFibonacci calls itself, passing lower values of fibIndex until fibIndex is less than 2.

The real difference between the two methods you show is that getFibonacci can calculate as many iterations as needed while main will not be able to calculate the correct Fibonacci numbers that require more than 3 iterations. In addition, there is no check if index is less than 2, so if the user puts in 1, a negative number is returned.

[Bourgond Aries]

@Daleth

A positive integer is returned from getFibNumber(1);

[Daleth]

I was talking about his main function. I guess I could have made that more clear.

cin >> index; //user puts in 1
//No check if index < 2 as in getFibonacci
index = (index-1) + (index-2); // index = 0 + -1
cout << "Fibonacci number is: "  << index << endl; // "Fibonacci number is: -1 

[Bourgond Aries]

Sorry about that :)