Why does my code work? Simple arithmetics

Question

I am writing a simple code to calculate Fabonacci numbers as an exercise. The code works, but i don't get why. I have some special cases for n=1 and n=2 which is the place of the number in the sequence (the numbers are 0 and 1). However after those, the number is calculated in this loop.

while(n>LoopCount)
{

    Fib_n=Fib_1+Fib_2;

    Fib_2=Fib_1;

    Fib_1=Fib_n;

    LoopCount++;
}

Going in to the loop, Fib_1=0, Fib_2=0, and Fib_n=1. Why does not this loop just spit out 0 no matter what? The whole code is below.

#include <iostream>

using namespace std;

int main()  
{
    cout <<"Which number of the Fibonacci sequence do you want to calculate?" <<endl;

    int n;
    cin >>n;
    cout <<endl;

    int Fib_n;
    int Fib_1;
    int Fib_2;
    int LoopCount=1;

    if(n>1)
    {
        Fib_n=1;
        LoopCount++;

        while(n>LoopCount)
        {

            Fib_n=Fib_1+Fib_2;

            Fib_2=Fib_1;

            Fib_1=Fib_n;

            LoopCount++;
        }
    }

    cout <<Fib_n;

    return 0;
}

Answer 1

int Fib_1;
int Fib_2;

were never initialized. Therefore, the first time you calculate Fib_n=Fib_1+Fib_2;, Fib_n will get the sum of two uninitialized variables.

I have modified your code so it would work.

#include <iostream>

using namespace std;

int main()  
{
    cout <<"Which number of the Fibonacci sequence do you want to calculate?" <<endl;

    int n;
    cin >> n;
    cout << endl;

    int Fib_1 = 1;
    int Fib_2 = 1;

    int count = 0;

    while(n > count)
    {
        Fib_1 = Fib_1 + Fib_2;
        Fib_2 = Fib_1 - Fib_2;
        count++;
    }

    cout << Fib_1;
    return 0;
}