Getting bizarre issue with variables outside the main method? - c++

Question

I have some global variables defined just above my main method. When I try to call the function "fib()" while using those variables, I get incorrect results.
When I move the variables into the local scope of the function, I get correct results.
Why does this happen? Here is the correct code:
using namespace std;
#include <iostream>
#include <vector>
vector <int> fibVals;
int fibInput;
int fib(int n);
int main()
{
cout << "Please enter 3 Fibonacci numbers you wish to find:: \n";
for(int i = 0; i < 3; i++)
{
cin >> fibInput;
fibVals.push_back(fibInput);
}
cout << "The values at those Fibonacci numbers are(in order)::\n";
for(int i = 0; i < 3; i++)
{
cout << fib( fibVals[i] ) << "\n";
}
return 0;
}
int fib(int n)
{
int sum = 0;
int fib1 = 1;
int fib2 = 0;
if (n == 0 || n == 1)
return n;
for(int i = 2; i <= n; i++)
{
sum = fib1 + fib2;
fib2 = fib1;
fib1 = sum;
}
return sum;
}
The incorrect code has the three variables "sum", "fib1", and "fib2" above the main method. Why does this not work, but declaring the variables inside the function does?
Edit: below is the incorrect code. Note that the only difference is the placement of the variables.
using namespace std;
#include <iostream>
#include <vector>
vector <int> fibVals;
int fibInput;
int fib(int n);
int sum = 0;
int fib1 = 1;
int fib2 = 0;
int main()
{
cout << "Please enter 3 Fibonacci numbers you wish to find:: \n";
for(int i = 0; i < 3; i++)
{
cin >> fibInput;
fibVals.push_back(fibInput);
}
cout << "The values at those Fibonacci numbers are(in order)::\n";
for(int i = 0; i < 3; i++)
{
cout << fib( fibVals[i] ) << "\n";
}
return 0;
}
int fib(int n)
{
if (n == 0 || n == 1)
return n;
for(int i = 2; i <= n; i++)
{
sum = fib1 + fib2;
fib2 = fib1;
fib1 = sum;
}
return sum;
}
I understand the concept of scope. I thought that making those three variables global(outside main) would make no difference than if they were made local(inside fib() ).

Answer 1

You should try to make your question clearer. Nevertheless I will take a shot in the dark and try to respond to what I believe your question is. Think what happens if you move sum, fib1, and fib2 outside the function fib(), in other words you make them global. They will keep their value across function invocations (technically, they have static storage duration). Hence, when you invoke fib() the second time, sum will be equal to the previous value returned by fib(). However, when you put them inside the function, they are local to that scope, and at each function invocation they start "fresh" and are re-initialized (technically they have automatic storage duration).