Martin Johnsrud Martin Johnsrud - 3 months ago 18
C++ Question

Why does my code work? Simple arithmetics

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
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;
}